Viruses require a host to replicate. The virus hijacks the host’s cells to produce more viral copies of itself. Diagnostic tests look for evidence of live virus within a sample of bodily substance, either looking for evidence of viral replication or for the proteins on the outer capsid of the virus to diagnose an active infection of SARS-CoV-2, the virus that causes COVID-19.
Diagnostic tests can be antigen based (“rapid antigen” tests), which look for protein markers on the outside of the virus, or they can be molecular based (including PCR, LAMP, CRISPR), which look for viral genomic material specific to SARS-CoV-2. Molecular-based tests that amplify genetic material are also called nucleic acid amplification tests (NAAT).
What types of diagnostic tests are on the market? What are the differences between them?
Five common types of antigen and molecular tests are currently available:
Type of Test
Time to Results
What It Tells Us
Limitations
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Rapid antigen test (RAT)
15-30 minutes
The presence of an active infection, by detecting specific viral proteins present in a patient sample. This is typically coupled with lateral flow assays to display results that can be read by eye.
Requires very careful design of synthetic antibodies, deep knowledge of viral proteins produced in various tissue environments, and may yield false negatives if the viral protein production is low.
The presence of an active infection, by targeting specific gene sequences of SARS-CoV-2. This can be quantitative but is usually qualitative (yes/no). It typically has a very low limit of detection, around 100 viruses/mL.
The time needed to complete the test; trained personnel and special equipment to analyze the results
The presence of an active infection, by targeting specific gene sequences of SARS-CoV-2. This is typically qualitative (yes/no). It relies on specially designed primers that help create the looped structures needed for amplification. It is very rapid and does not always require special equipment (can be measured by eye in some cases). It has a very low limit of detection of 125 viruses/mL.
Designing the primers needed can be complex, and debris can interfere with the reaction. It is also difficult to quantify the results (level of viral infection).
The presence of an active infection, by targeting specific gene sequences of SARS-CoV-2. This is typically qualitative (yes/no). It relies on the recombinase enzyme. It is very rapid and does not always require special equipment. It has a very low limit of detection of 125 viruses/mL.
Designing the necessary primers can be complex, and debris can interfere with the reaction. It is also difficult to quantitfy the results (level of viral infection).
The presence of an active infection, by targeting specific gene sequences of SARS-CoV-2. This is typically coupled with LAMP, but this is not always necessary. Results are typically visible to the eye, not requiring special equipment.
Requires expert, specific design of components (enzymes, primers, reporters).
