Viral Diagnostics: A Quick Recap
The two major methods for diagnosing viral infection are the polymerase chain reaction (PCR), and immunoassays:
PCR The polymerase chain reaction is a routine laboratory technique used to amplify small samples of DNA into larger quantities that can be detected and analysed. For patient diagnosis, a viral RNA or DNA sample is taken by swab or blood draw, before being sent to a specialist laboratory for analysis.
For RNA viruses, such as SARS-CoV-2, the polymerase chain reaction is preceded by an additional step to produce a complementary DNA template (cDNA) from RNA, by the addition of a reverse transcriptase enzyme (hence, Reverse Transcription-PCR (RT-PCR)).
PCR then begins by the addition of short DNA sequences known as primers, which bind the viral DNA strands. The double-stranded section of DNA is then recognised and bound by a thermostable polymerase enzyme which acts as a molecular photocopier to extend the sequence and produce a full, complementary strand.
By controlling the annealing, extension and denaturation steps with changes in temperature, the initial sample of viral DNA can be exponentially amplified, followed by the addition of specific DNA probes that produce a detectable signal (often fluorescent) to confirm the etiological agent.
Immunoassays Unlike molecular techniques, immunoassays detect the presence of specific immune proteins. These assays take on a wide range of different formats, but essentially consist of an antigen or antibody, immobilised on a surface (most often a titre plate or paper strip), which binds virus-specific antigens or antibodies from a patient sample (i.e. sputum or blood sera). By adding a further reporter protein, it is then possible to detect a virus-specific immune signal to confirm the presence of ongoing or past viral infection
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