Editing Viral load

{{short description|Amount of virus found in host tissue}}
{{Expert needed|1=Medicine|reason=Uncited "how to" content should be reviewed & article would generally benefit from expansion|date=January 2020}}
'''Viral load''', also known as '''viral burden''', is a numerical expression of the quantity of virus in a given volume of fluid, including biological and environmental specimens. It is not to be confused with '''viral titre''' or '''viral titer''', which depends on the assay. When an assay for measuring the infective virus particle is done (Plaque assay, Focus assay), '''viral titre''' often refers to the ''concentration'' of infectious viral particles, which is different from the ''total'' viral particles. Viral load is measured using body fluids [[sputum]]<ref name="wolfel20">{{cite journal |doi=10.1038/s41586-020-2196-x|title=Virological assessment of hospitalized patients with COVID-2019|year=2020|last1=Wölfel|first1=Roman|last2=Corman|first2=Victor M.|last3=Guggemos|first3=Wolfgang|last4=Seilmaier|first4=Michael|last5=Zange|first5=Sabine|last6=Müller|first6=Marcel A.|last7=Niemeyer|first7=Daniela|last8=Jones|first8=Terry C.|last9=Vollmar|first9=Patrick|last10=Rothe|first10=Camilla|author-link10=Camilla Rothe |last11=Hoelscher |first11=Michael |last12=Bleicker |first12=Tobias |last13=Brünink |first13=Sebastian |last14=Schneider |first14=Julia |last15=Ehmann |first15=Rosina |last16=Zwirglmaier |first16=Katrin |last17=Drosten |first17=Christian |last18=Wendtner |first18=Clemens |journal=Nature|volume=581|issue=7809|pages=465–469|pmid=32235945|bibcode=2020Natur.581..465W|doi-access=free}}</ref> and [[blood plasma]].<ref name=puren10/> As an example of environmental specimens, the viral load of [[norovirus]] can be determined from run-off water on garden produce.<ref name="shaheen19">{{cite journal |doi=10.1007/s11356-019-05435-0|title=Quantitative PCR-based identification of enteric viruses contaminating fresh produce and surface water used for irrigation in Egypt|year=2019|last1=Shaheen|first1=Mohamed N. F.|last2=Elmahdy|first2=Elmahdy M.|last3=Chawla-Sarkar|first3=Mamta|journal=Environmental Science and Pollution Research|volume=26|issue=21|pages=21619–21628|pmid=31129895|bibcode=2019ESPR...2621619S |s2cid=167210903}}</ref> [[Norovirus]] has not only prolonged [[viral shedding]] and has the ability to survive in the environment but a minuscule [[infectious dose]] is required to produce infection in humans: less than 100 viral particles.<ref name="robilotti15">{{cite journal |doi=10.1128/CMR.00075-14|title=Norovirus|year=2015|last1=Robilotti|first1=Elizabeth|last2=Deresinski|first2=Stan|last3=Pinsky|first3=Benjamin A.|journal=Clinical Microbiology Reviews|volume=28|issue=1|pages=134–164|pmid=25567225|pmc=4284304}}</ref>

Viral load is often expressed as viral particles, (virions) or infectious particles per mL depending on the type of assay. A higher viral burden, titre, or viral load often correlates with the severity of an active [[virus|viral]] infection. The quantity of virus per mL can be calculated by estimating the live [[Virus quantification|amount of virus]] in an involved fluid. For example, it can be given in [[RNA]] copies per millilitre 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 transplantation]]. Currently, routine testing is available for [[HIV]]-1, [[cytomegalovirus]], [[hepatitis B]] virus, and [[hepatitis C]] virus. [[Viral load monitoring for HIV]] is of particular interest in the treatment of [[people with HIV]], as this is continually discussed in the context of [[management of HIV/AIDS]]. An undetectable viral load does not implicate a lack of infection. HIV positive patients on long-term combination antiretroviral therapy may present with an undetectable viral load on most clinical assays since the concentration of virus particles is below the [[limit of detection]] (LOD).

==Technologies for viral load testing==

A 2010 review study by Puren ''et al.''<ref name=puren10>{{cite journal |pages=S27–36 |doi=10.1086/650390 |title=Laboratory Operations, Specimen Processing, and Handling for Viral Load Testing and Surveillance |year=2010 |last1=Puren |first1=Adrian |last2=Gerlach |first2=Jay L. |last3=Weigl |first3=Bernhard H. |last4=Kelso |first4=David M. |last5=Domingo |first5=Gonzalo J. |journal=The Journal of Infectious Diseases |volume=201 |pmid=20225943|doi-access=free }}</ref> categorizes viral load testing into three types: (1) [[nucleic acid]] amplification based tests ([[Nucleic acid test|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 marking]]; (2) "Home–brew" or in-house NATs; (3) non-nucleic acid-based test.{{cn|date=January 2021}}

===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:<ref>{{cite book|last1=Buckingham|first1=L.|last2=Flaws|first2=M.L.|date=2007|title=Molecular Diagnostics Fundamentals, Methods, & Clinical Applications|url=http://www.justmed.eu/files/MolecularDiagnosticsFundamentalsMethodsandClinicalApplications.pdf|publisher=F.A. Davis Company|pages=121–154|isbn= 978-0-8036-1659-2|access-date=7 September 2020}}</ref>
# Target amplification which uses the nucleic acid itself. Just a few of the more common methods
#* The [[polymerase chain reaction]] (PCR) method of [[in vitro]] DNA synthesis uses a [[DNA]] template, [[polymerase]], buffers, [[Primer (molecular biology)|primers]], 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 [[Integrase|integrated]] 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]] (RT) for PCR.
#* The [[NASBA (molecular biology)|nucleic acid sequence-based amplification]] (NASBA) 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).
# [[Molecular probe|Probe]] 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 assay|branched DNA]] (bDNA) 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.

==Plasma specimens==
[[EDTA]] Plasma, from and EDTA blood sample is a good source of cell-free viral RNA for RNA-based viral load testing. Extraction of RNA from plasma requires specialized equipment, [[reagents]] and training, which might out of reach for medium to small laboratories. A large sample (> 1 mL of plasma) is needed requiring [[venipuncture]].
===Storage===
EDTA blood can be stored at room temperature for 30 hours, and  separated plasma for extended periods of time at -70&nbsp;°C without significant decreases in viral load.  

===Measuring===
Viral load is reported as copies of HIV RNA in a millilitre (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 10<sup>3</sup> or 1,000 times the previously reported level, while a drop from 500,000 to 500 copies would be a three-log-drop (also 3 log10).{{cn|date=January 2021}}

===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.

== See also ==

* [[Minimal infective dose]]
* [[Plaque-forming unit|Plaque forming unit]]
* [[Virus quantification]]

==References==

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{{Virus topics}}

{{DEFAULTSORT:Viral Load}}
[[Category:Virology]]
[[Category:HIV/AIDS]]
[[Category:Medical tests]]
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[[it:Carica virale]]
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