Viral load
Encyclopedia
Viral load is a measure of the severity of a viral
infection, and can be calculated by estimating the amount of virus
in an involved body fluid. For example, it can be given in RNA copies per milliliter of blood plasma
. Tracking viral load is used to monitor therapy during chronic viral infections, and in immunocompromised patients such as those recovering from bone marrow
or solid organ transplant
ation. Currently, routine testing is available for HIV
-1, cytomegalovirus
, hepatitis B virus, and hepatitis C
virus.
amplification based tests (NAT
s or NAATs) commercially available in the United States
with Food and Drug Administration
(FDA) approval, or on the market in the European Economic Area
(EEA) with the CE-mark; (2) "Home–brew" or in-house NATs; (3) non-nucleic acid-based test.
plasma is the best source of cell-free viral RNA for RNA-based viral load testing. Consideration of specimen
collection, storage and biosafety
measures is essential. Extraction of RNA from plasma requires specialized equipment, reagents and training, placing it out of reach for medium to small labs with limited resources. A large sample (> 1 mL of plasma) is needed for a linear range bottoming out at 50 copies/mL, requiring venipuncture
. This linear range is best for treatment monitoring. If a higher linear range of more than 1000 copies/mL is acceptable, a finger stick would supply a sufficient specimen for diagnosis of HIV infection during infancy.
EDTA plasma can be stored at room temperature for 30 hours, 14 days at 4oC and extended periods of time at -70oC without significant decreases in viral load signal. The RNA in smaller blood specimens, such as dried plasma spots (DPS) or dried blood spots
(DBS) from finger sticks is stable at room temperature for extended periods. Reports have varied from 4 weeks to 1 year. The virus is inactivated in DPS, reducing the danger from specimen handling. DBS and DPS were successfully evaluated for viral load testing, but their linear range is 3 log10 or 4 log10 copies/mL. This is not sufficient for treatment monitoring.
for a test is the amount of time from the initial infection event until the disease can be detected. Exposure to HIV, followed by replication of the virus, may take as long as six months to reach a level detectable in many testing methods. An HIV antibody test
usually detects the HIV antibodies
within two to eight weeks, but can have a valid negative result for a long as 2 to 6 months after initial infection.
Viral load tests can also be used to diagnose HIV infection, especially in children under 18 months born to mothers with HIV, where the presence of maternal antibodies prevents the use of antibody-based (ELISA
) diagnostic tests
. Pooled viral RNA testing shortens the window period to a median of 17 days (95% CI, 13-28 Days). Although it is not the standard of care to use this test for diagnosis, in communities with high HIV prevalence, this test has an increased sensitivity over 3rd and 4th generation tests for detecting acute HIV infections.
On June 15th 2010, the FDA approved the first diagnostic test capable of detecting HIV antigens and HIV antibodies. Use of the Abbott ARCHITECT HIV Ag/Ab combo test can reduce the window period to 20 days. This diagnostic test can be used during the acute phase, when the immune system is still developing antibodies and the virus is replicating unchecked. A person, who may be unaware of the infection, is highly infectious during this time yet may test negative for HIV using tests that detect anti-HIV antibodies only.
indicates a greater likelihood of developing AIDS
within five years. A viral load less than 10,000 copies/mL of blood in the early stages indicates a decreased risk of developing AIDS.
, an RNA virus that enters a host cell
and uses the host DNA replication machinery and the enzyme
reverse transcriptase
to produce DNA from the viral RNA genome. HIV also produces an integrase
enzyme which it uses to integrate its newly produced DNA into the host’s DNA. The virus is then replicated every time the host cell's DNA replicates.
Due to the nature of the virus the drugs used to treat HIV are called antiretroviral medicines, and the course of treatment is called antiretroviral therapy (ART). These potent medicines cannot cure an individual; they can however manage the virus and slow the progression of the HIV infection. Strict compliance with the prescribed ART regiment is vital to controlling the disease.
Highly active antiretroviral therapy (HAART) is the current recommended treatment for HIV. HAART entails taking a combination (regiment) of three or more ART medications from at least two different classes of drugs. There are six classes of ART medications:
Each class of medications uses a different mode of action to blocks the virus. Treatment is more effective in controlling the virus when a combination of medications from different classes is used. HAART also reduces the risk of developing drug resistance. Viral load tests are used to monitor the effects ART, to track viral suppression, and detect treatment failure. Successful combination ART should give a fall in viral load of 1.5 to 2 logs (30-100 fold) within six weeks, with the viral load falling below the limit of detection within four to six months.
Laboratory monitoring schedule for patients using ART:
and/or viral mutations, and often dictate changes in ART.
The viral load test only detects the amount of virus circulating in the blood (about 2%). Approximately 98% of the HIV is actually in the body tissues such as the lymph nodes, gut-associated lymphoid tissue
(GALT), spleen
, brain
, and other body tissues and fluids. Viral level fluctuations in these tissues parallel the levels in the blood but there is not an immediate correlation in time or rate.
Time of day, fatigue, and stress can also affect viral load values. Recent immunizations or infections can affect the viral load test. Testing should be postponed for at least four weeks after an immunization or infection.
cells are the primary target of HIV. A CD4 test quantifies helper T-cells and is often combined with viral load testing to monitor the progression of HIV. CD4 testing shows the strength of the immune system, but does not report viral activity. As established by the Centers for Disease Control and Prevention
(CDC), a person with HIV and a CD4 count below 200 or a CD4 percentage below 14% is considered to have AIDS
. An increased CD4 count can result from an immune response to an infection or a recent vaccination
. A decreased CD4 count, in combination with higher numbers on a viral load test, indicates an increased risk of getting sick from opportunistic diseases.
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
infection, and can be calculated by estimating the amount of virus
Virus Quantification
Virus quantification involves counting the number of viruses in a specific volume to determine the virus concentration. It is utilized in both research and development in commercial and academic laboratories as well as production situations where the quantity of virus at various steps is an...
in an involved body fluid. For example, it can be given in RNA copies per milliliter of blood plasma
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
. Tracking viral load is used to monitor therapy during chronic viral infections, and in immunocompromised patients such as those recovering from bone marrow
Bone marrow
Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...
or solid organ transplant
Organ transplant
Organ transplantation is the moving of an organ from one body to another or from a donor site on the patient's own body, for the purpose of replacing the recipient's damaged or absent organ. The emerging field of regenerative medicine is allowing scientists and engineers to create organs to be...
ation. Currently, routine testing is available for HIV
HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
-1, cytomegalovirus
Cytomegalovirus
Cytomegalovirus is a viral genus of the viral group known as Herpesviridae or herpesviruses. It is typically abbreviated as CMV: The species that infects humans is commonly known as human CMV or human herpesvirus-5 , and is the most studied of all cytomegaloviruses...
, hepatitis B virus, and hepatitis C
Hepatitis C
Hepatitis C is an infectious disease primarily affecting the liver, caused by the hepatitis C virus . The infection is often asymptomatic, but chronic infection can lead to scarring of the liver and ultimately to cirrhosis, which is generally apparent after many years...
virus.
Technologies for viral load testing
A recent review study by Puren et al categorizes viral load testing into three types: (1) nucleic acidNucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...
amplification based tests (NAT
Nucleic acid test
A nucleic acid test, often called a "NAT" test, is a biochemical technique used to detect a virus or a bacterium...
s or NAATs) commercially available in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
with Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...
(FDA) approval, or on the market in the European Economic Area
European Economic Area
The European Economic Area was established on 1 January 1994 following an agreement between the member states of the European Free Trade Association and the European Community, later the European Union . Specifically, it allows Iceland, Liechtenstein and Norway to participate in the EU's Internal...
(EEA) with the CE-mark; (2) "Home–brew" or in-house NATs; (3) non-nucleic acid-based test.
Nucleic acid-based tests (NATs)
There are many different molecular based test methods for quantifying the viral load using NATs. The starting material for amplification can be used to divide these molecular methods into three groups:- Target amplification which uses the nucleic acid itself. Just a few of the more common methods
- The Polymerase Chain Reaction ((PCR) method of in vitroIn vitroIn vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
DNADNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
synthesis uses a DNA template, polymerasePolymeraseA polymerase is an enzyme whose central function is associated with polymers of nucleic acids such as RNA and DNA.The primary function of a polymerase is the polymerization of new DNA or RNA against an existing DNA or RNA template in the processes of replication and transcription...
, buffers, primersPrimer (molecular biology)A primer is a strand of nucleic acid that serves as a starting point for DNA synthesis. They are required for DNA replication because the enzymes that catalyze this process, DNA polymerases, can only add new nucleotides to an existing strand of DNA...
, and nucleotides to multiply the HIV in the blood sample. Then a chemical reaction marks the virus. The markers are measured and used to calculate the amount of virus. PCR is used to quantify integratedIntegraseRetroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
DNA - Reverse Transcription Polymerase Chain Reaction (RT-PCR) is a variation of PCR that can be used to quantify viral RNA. RNA is used as the starting material for this method and converted to double-stranded DNA, using the enzyme reverse transcriptase (RTDNA polymeraseA DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....
) - The Nucleic Acid Sequence Based Amplification (NASBANASBA (molecular biology)Nucleic acid sequence based amplification is a method in molecular biology which is used to amplify RNA sequences.NASBA was developed by J Compton in 1991, who defined it as "a primer-dependent technology that can be used for the continuous amplification of nucleic acids in a single mixture at one...
) method is a transcription-based amplification system (TAS) variation of PCR. RNA is used as the target and a DNA copy is made. The DNA copy is then transcribed into RNA and amplified. Several TAS commercial variations are available including; transcription-mediated amplification (TMA), and self-sustaining sequence replication (3SR)
- The Polymerase Chain Reaction ((PCR) method of in vitro
- ProbeMolecular probeA molecular probe is a group of atoms or molecules attached to other molecules or cellular structures and used in studying the properties of these molecules and structures. Radioactive DNA or RNA sequences are used in molecular genetics to detect the presence of a complementary sequence by...
specific amplification uses synthetic probes that preferentially bind to a target sequence. The probes are then amplified - Signal amplification uses large amounts of signal bound to an unamplified target originally present in the sample. One commonly used method:
- The branched DNA (bDNABDNA testBranched DNA testing or bDNA testing is a test created by the Chiron company to measure the viral load of HIV in a sample of blood. A phosphorescent chemical that is known to bind to HIV RNA is added to the suspect DNA...
) method can use either DNA or RNA as the target nucleic acid. Short probes attached to a solid support and capture the target nucleic acid. Additional extender probes also bind to the target nucleic acid and to numerous reporter molecules which are used to increase the signal intensity, which is converted to a viral count.
- The branched DNA (bDNA
| Test name | Molecular method | Use | Approved | Manufacturer |
|---|---|---|---|---|
| Roche Amplicor HIV-1 Monitor Test | PCR | Viral load | 3/2/1999 | Roche Molecular Systems, Inc. Pleasanton, CA |
| NucliSens HIV-1 QT | NASBA NASBA (molecular biology) Nucleic acid sequence based amplification is a method in molecular biology which is used to amplify RNA sequences.NASBA was developed by J Compton in 1991, who defined it as "a primer-dependent technology that can be used for the continuous amplification of nucleic acids in a single mixture at one... |
Viral load | 11/19/2001 | bioMerieux, Inc Durhan, NC |
| Trugene HIV-1 Genotyping Kit and Open Gene DNA Sequencing System | HIV-1 Genotyping | Patient monitoring | 4/24/2002 | Siemens Medical Solutions Diagnostics Siemens Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG... Berkeley, CA |
| ViroSeq HIV-1 Genotyping System with the 3700 Genetic Analyzer | HIV-1 Genotyping | Patient monitoring | 6/11/2003 | Celera Diagnostics Alameda, CA |
| Versant HIV-1 RNA 3.0 | bDNA BDNA test Branched DNA testing or bDNA testing is a test created by the Chiron company to measure the viral load of HIV in a sample of blood. A phosphorescent chemical that is known to bind to HIV RNA is added to the suspect DNA... |
Patient monitoring | 9/11/2002 | Siemens Medical Solutions Diagnostics Siemens Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG... Berkeley, CA |
| Procleix Ultrio Assay | TMA (commercial variation of NASBA) The three steps to the Procleix Ultrio Assay are done in one tube. The first step is specimen preparation, the second is transcription-mediated amplification (TMA), and the third is a hybridization protection assay (HPA) using single-stranded complementary chemiluminescent labeled probes. A luminometer is used to measure the signal. The sensitivity Sensitivity and specificity Sensitivity and specificity are statistical measures of the performance of a binary classification test, also known in statistics as classification function. Sensitivity measures the proportion of actual positives which are correctly identified as such Sensitivity and specificity are statistical... of this assay Assay An assay is a procedure in molecular biology for testing or measuring the activity of a drug or biochemical in an organism or organic sample. A quantitative assay may also measure the amount of a substance in a sample. Bioassays and immunoassays are among the many varieties of specialized... is 98%. |
Qualitative detection of HIV-1 RNA and hepatitis C virus (HCV) RNA from volunteer donors of whole blood and blood components, screen of live organ donors, and test blood specimens to screen cadaveric donors. HBV screening of individual samples and pooled samples. | 10/3/2006 | Gen-Probe San Diego, CA US Licence 1592 Chiron Corporation Chiron Corporation Chiron Corporation was a multinational biotechnology firm based in Emeryville, California that was acquired by Novartis International AG on April 20, 2006. It had offices and facilities in eighteen countries on five continents. Chiron's business and research was in three main areas:... |
| Human Immunodeficiency Virus, Type 1 (HIV-1) Reverse Transcription (RT) Polymerase Chain Reaction (PCR) Assay | PCR | Qualitative detection of HIV-1 RNA in pools of human Source Plasma | 1/31/2007 | BioLife Plasma Services, L.P. Deerfield, IL US Licence 1640 |
| Abbott RealTime HIV-1 Amplification Kit | RT-PCR The sensitivity (linear range) for this assay is 40 copies/mL to 1010 copies/mL. Two probes are used. The HIV-1 probe is labeled with a fluorescent molecule Fluorescent labelling Fluorescent labelling is the process of covalently attaching a fluorophore to another molecule, such as a protein or nucleic acid. This is generally accomplished using a reactive derivative of the fluorophore that selectively binds to a functional group contained in the target molecule... and covalently binds to the 5’ end. The second probe is a short oligonucleotide Oligonucleotide An oligonucleotide is a short nucleic acid polymer, typically with fifty or fewer bases. Although they can be formed by bond cleavage of longer segments, they are now more commonly synthesized, in a sequence-specific manner, from individual nucleoside phosphoramidites... with a 3’ end quencher molecule attached complementary to the 5’ end of the HIV-1 probe. If the HIV-1 probe finds and attaches to a HIV target the quencher molecule is released and the resulting fluorescent emission is measured. The fluorescence is proportional to the log of the amount of virus in the sample. |
Quantitation of HIV-1 | 5/11/2007 | ABBOTT Molecular, Inc Abbott Laboratories Abbott Laboratories is an American-based global, diversified pharmaceuticals and health care products company. It has 90,000 employees and operates in over 130 countries. The company headquarters are in Abbott Park, North Chicago, Illinois. The company was founded by Chicago physician, Dr.... Des Plaines, IL |
| COBAS AmpliPrep/COBAS TaqMan HIV-1 Test |
PCR Uses fluorescent resonance energy transfer (FRET) to enhance its automated RT-PCR. In the FRET reaction, a donor and acceptor probe exchange excitation energy Excited state Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state.... when within 1-5 base pairs (bp) on the target sequences. The energy is emitted in the form of heat or fluorescence. The probes are designed to bind 1-5 bp from each other. The energy emission is proportional to the concentration of viral particles. The linear range for this assay is 48 copies/mL of blood. |
Quantitation of HIV-1 | 5/11/2007 | Roche Molecular Systems, Inc Pleasanton, CA |
Non-nucleic acid-based tests
ExaVir™ Load Version 3 from Cavidi AB is a largely manual test, which has the European regulatory approval (CE-Mark) for clinical use and is also used for viral load monitoring. Virus-associated reverse transcriptase (RT) activity is measured and can therefore detect all types and subtypes of HIV. The technology does not require sophisticated laboratories.Specimens
EDTAEDTA
Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...
plasma is the best source of cell-free viral RNA for RNA-based viral load testing. Consideration of specimen
Laboratory specimen
In medicine, a laboratory specimen is a medical sample, that is, gathered matter of a medical patient's tissue, fluid, or other material derived from the patient used for laboratory analysis to assist differential diagnosis or staging of a disease process...
collection, storage and biosafety
Biocontainment
The concept of biocontainment, also called laboratory biosafety, pertains to microbiology laboratories in which the physical containment of highly pathogenic organisms or agents is required, usually by isolation in environmentally and biologically secure cabinets or rooms, to prevent accidental...
measures is essential. Extraction of RNA from plasma requires specialized equipment, reagents and training, placing it out of reach for medium to small labs with limited resources. A large sample (> 1 mL of plasma) is needed for a linear range bottoming out at 50 copies/mL, requiring venipuncture
Venipuncture
In medicine, venepuncture, venopuncture or venipuncture is the process of obtaining intravenous access for the purpose of intravenous therapy or for blood sampling of venous blood. This procedure is performed by medical laboratory scientists, medical practitioners, some EMTs, paramedics,...
. This linear range is best for treatment monitoring. If a higher linear range of more than 1000 copies/mL is acceptable, a finger stick would supply a sufficient specimen for diagnosis of HIV infection during infancy.
EDTA plasma can be stored at room temperature for 30 hours, 14 days at 4oC and extended periods of time at -70oC without significant decreases in viral load signal. The RNA in smaller blood specimens, such as dried plasma spots (DPS) or dried blood spots
Dried blood spot testing
Dried blood spot testing is a method of screening for HIV infection and other conditions using DNA amplification.-History:The concept that capillary blood, obtained from pricking the heel or finger and blotted onto filter paper, could be used to screen for metabolic diseases in large populations...
(DBS) from finger sticks is stable at room temperature for extended periods. Reports have varied from 4 weeks to 1 year. The virus is inactivated in DPS, reducing the danger from specimen handling. DBS and DPS were successfully evaluated for viral load testing, but their linear range is 3 log10 or 4 log10 copies/mL. This is not sufficient for treatment monitoring.
Measuring
Viral load is typically reported as copies of HIV in a milliliter (mL) of blood. Changes in viral load are usually reported as a log change (in powers of 10). For example, a three log increase in viral load (3 Log10) is an increase of 103 or 1000 times the previously reported level, while a drop from 500,000 to 500 copies would be a three-log-drop (also 3 Log10).Diagnosis
The window periodWindow period
In medicine, the window period for a test designed to detect a specific disease is the time between first infection and when the test can reliably detect that infection...
for a test is the amount of time from the initial infection event until the disease can be detected. Exposure to HIV, followed by replication of the virus, may take as long as six months to reach a level detectable in many testing methods. An HIV antibody test
HIV test
HIV tests are used to detect the presence of the human immunodeficiency virus , the virus that causes acquired immunodeficiency syndrome , in serum, saliva, or urine. Such tests may detect antibodies, antigens, or RNA.- Terminology :...
usually detects the HIV antibodies
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
within two to eight weeks, but can have a valid negative result for a long as 2 to 6 months after initial infection.
Viral load tests can also be used to diagnose HIV infection, especially in children under 18 months born to mothers with HIV, where the presence of maternal antibodies prevents the use of antibody-based (ELISA
ELISA
Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
) diagnostic tests
HIV test
HIV tests are used to detect the presence of the human immunodeficiency virus , the virus that causes acquired immunodeficiency syndrome , in serum, saliva, or urine. Such tests may detect antibodies, antigens, or RNA.- Terminology :...
. Pooled viral RNA testing shortens the window period to a median of 17 days (95% CI, 13-28 Days). Although it is not the standard of care to use this test for diagnosis, in communities with high HIV prevalence, this test has an increased sensitivity over 3rd and 4th generation tests for detecting acute HIV infections.
On June 15th 2010, the FDA approved the first diagnostic test capable of detecting HIV antigens and HIV antibodies. Use of the Abbott ARCHITECT HIV Ag/Ab combo test can reduce the window period to 20 days. This diagnostic test can be used during the acute phase, when the immune system is still developing antibodies and the virus is replicating unchecked. A person, who may be unaware of the infection, is highly infectious during this time yet may test negative for HIV using tests that detect anti-HIV antibodies only.
Prognosis
Viral load is used to predict how long an individual will remain healthy, or how quickly the disease will progress. A viral load greater than 100,000 copies/mL of blood within six months of seroconversionSeroconversion
Seroconversion is the development of detectable specific antibodies to microorganisms in the blood serum as a result of infection or immunization. Serology is used to determine antibody positivity...
indicates a greater likelihood of developing AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...
within five years. A viral load less than 10,000 copies/mL of blood in the early stages indicates a decreased risk of developing AIDS.
Therapy
Treatment guidelines recommend that anyone with a viral load greater than 100,000 copies/mL of blood should begin treatment. HIV is a retrovirusRetrovirus
A retrovirus is an RNA virus that is duplicated in a host cell using the reverse transcriptase enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an integrase enzyme. The virus thereafter replicates as part of the host cell's DNA...
, an RNA virus that enters a host cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
and uses the host DNA replication machinery and the enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
reverse transcriptase
Reverse transcriptase
In the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
to produce DNA from the viral RNA genome. HIV also produces an integrase
Integrase
Retroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
enzyme which it uses to integrate its newly produced DNA into the host’s DNA. The virus is then replicated every time the host cell's DNA replicates.
Due to the nature of the virus the drugs used to treat HIV are called antiretroviral medicines, and the course of treatment is called antiretroviral therapy (ART). These potent medicines cannot cure an individual; they can however manage the virus and slow the progression of the HIV infection. Strict compliance with the prescribed ART regiment is vital to controlling the disease.
Highly active antiretroviral therapy (HAART) is the current recommended treatment for HIV. HAART entails taking a combination (regiment) of three or more ART medications from at least two different classes of drugs. There are six classes of ART medications:
- non-nucleoside reverse transcriptase inhibitorsReverse transcriptase inhibitorReverse-transcriptase inhibitors are a class of antiretroviral drug used to treat HIV infection, tumors, and cancer. RTIs inhibit activity of reverse transcriptase, a viral DNA polymerase enzyme that retroviruses need to reproduce.-Mechanism:...
(NNRTIs) - Nucleoside reverse transcriptase inhibitors (NRTIs)
- protease inhibitorProtease inhibitorProtease inhibitor can refer to:* Protease inhibitor : a class of medication that inhibits viral protease* Protease inhibitor : molecules that inhibit proteases...
s (PIs) - entry inhibitors
- Fusion inhibitors
- Integrase inhibitorIntegrase inhibitorIntegrase inhibitors are a class of antiretroviral drug designed to block the action of integrase, a viral enzyme that inserts the viral genome into the DNA of the host cell. Since integration is a vital step in retroviral replication, blocking it can halt further spread of the virus...
s
Each class of medications uses a different mode of action to blocks the virus. Treatment is more effective in controlling the virus when a combination of medications from different classes is used. HAART also reduces the risk of developing drug resistance. Viral load tests are used to monitor the effects ART, to track viral suppression, and detect treatment failure. Successful combination ART should give a fall in viral load of 1.5 to 2 logs (30-100 fold) within six weeks, with the viral load falling below the limit of detection within four to six months.
Laboratory monitoring schedule for patients using ART:
| Initial Physician visit | Prior to beginning ART | Beginning or modifying ART | 2 - 8 weeks after beginning or modifying ART | Every 3 - 6 months | Every 6 - 12 months | Other | |
|---|---|---|---|---|---|---|---|
| Viral load | When entering into care | Every 3 - 6 months | Baseline for comparison | Repeat every 4 - 8 weeks until viral load is suppressed to <200 copies/mL blood, then every 3 - 6 months | Individuals with a suppressed viral load, who are clinically & immunologically stable for more than 2 - 3 years, may go to every 6 months | Treatment failure or when clinically indicated | |
| CD4 | When entering into care | Every 3 - 6 months | Baseline for comparison | Routine monitoring | Individuals with a suppressed viral load, who are clinically stable, CD4 count monitoring may bo to every 6 - 12 months | Treatment failure or when clinically incicated |
Blips
While receiving ART some patients with undetectable viral load measurements may experience an increase in viral load, to a low level (usually less than 500 copies/mL blood), and then returned to an undetectable level. These transient blips do not indicate that the virus is developing resistance to drug therapy. Blips appear to be more common in the winter, suggesting a connection with illness such as colds and influenza. Viral load blips are partially explained by various patient related factors, and thought to be relatively common. High level and sustained increases in viral load are frequently related to the development of drug resistanceDrug resistance
Drug resistance is the reduction in effectiveness of a drug such as an antimicrobial or an antineoplastic in curing a disease or condition. When the drug is not intended to kill or inhibit a pathogen, then the term is equivalent to dosage failure or drug tolerance. More commonly, the term is used...
and/or viral mutations, and often dictate changes in ART.
Contagiousness
The viral load in the blood is a good predictor of the likelihood of transmitting HIV to another. The higher the viral load value, the more viral elements there are in tissues and in circulating blood and other body fluids. Individuals with HIV are most contagious during the earliest (acute) stages of the infection. At this phase the immune response is still developing and antibody levels against the virus are often too low to be detected. Antibody testing at this stage often yields a negative result. This scenario increases the importance of HIV education and early detection and diagnosis. Widespread testing could provide significant public health benefits. There is no safe period for an individual infected with HIV. HIV can be passed to another individual even when the viral load has dropped to undetectable.The viral load test only detects the amount of virus circulating in the blood (about 2%). Approximately 98% of the HIV is actually in the body tissues such as the lymph nodes, gut-associated lymphoid tissue
Gut-associated lymphoid tissue
The digestive tract's immune system is often referred to as gut-associated lymphoid tissue and works to protect the body from invasion. GALT is an example of mucosa-associated lymphoid tissue.-Function:...
(GALT), spleen
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...
, brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
, and other body tissues and fluids. Viral level fluctuations in these tissues parallel the levels in the blood but there is not an immediate correlation in time or rate.
Other factors that affect viral load
Different test methods often give different results for the same patient sample. To be comparable the same test method (Target amplification, probe specific amplification, or signal amplification) should be used each time a patient specimen is run. Ideally patient testing should be conducted at the same medical laboratory, using the same viral load test and analyzer.Time of day, fatigue, and stress can also affect viral load values. Recent immunizations or infections can affect the viral load test. Testing should be postponed for at least four weeks after an immunization or infection.
CD4 white blood cells
CD4CD4
CD4 is a glycoprotein expressed on the surface of T helper cells, monocytes, macrophages, and dendritic cells. It was discovered in the late 1970s and was originally known as leu-3 and T4 before being named CD4 in 1984...
cells are the primary target of HIV. A CD4 test quantifies helper T-cells and is often combined with viral load testing to monitor the progression of HIV. CD4 testing shows the strength of the immune system, but does not report viral activity. As established by the Centers for Disease Control and Prevention
Centers for Disease Control and Prevention
The Centers for Disease Control and Prevention are a United States federal agency under the Department of Health and Human Services headquartered in Druid Hills, unincorporated DeKalb County, Georgia, in Greater Atlanta...
(CDC), a person with HIV and a CD4 count below 200 or a CD4 percentage below 14% is considered to have AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...
. An increased CD4 count can result from an immune response to an infection or a recent vaccination
Vaccination
Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...
. A decreased CD4 count, in combination with higher numbers on a viral load test, indicates an increased risk of getting sick from opportunistic diseases.