COVID-19 Tests

Also written as: SARS-CoV-2 test, Antigen test, PCR test, NAAT

Antigen and PCR tests for the COVID-19 virus, trading speed against sensitivity.

What it measures

Both types look for the SARS-CoV-2 virus, in different ways. Antigen tests detect viral surface proteins and give a result in about 15 minutes; PCR tests, a type of molecular test, amplify and detect the virus's genetic material, usually in a lab, and can find far smaller amounts of virus.

Why a doctor orders it

Testing is used for symptoms, after a known exposure, before certain procedures, and to meet some facility or travel requirements. The choice between antigen and PCR usually comes down to how fast an answer is needed versus how much sensitivity the situation calls for.

What is generally associated with a higher result

A positive result on either test generally indicates the virus is present. PCR can stay positive for weeks after an infection has resolved because it detects genetic fragments rather than intact virus, so a lingering positive PCR does not necessarily mean ongoing contagiousness.

What is generally associated with a lower result

A negative antigen test taken early - soon after symptoms start or after an exposure - is generally treated as provisional, and the FDA advises repeating antigen tests about 48 hours apart for this reason, since they often turn positive later than PCR. A negative PCR is stronger evidence, though even PCR can miss an infection if sampled very early.

Everyday things that move this number

Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:

Timing relative to exposure and symptom onset
Swab technique
Which test type was used - antigen versus PCR
Expired or heat-damaged home test kits
A recent prior infection, which can keep PCR positive for weeks

Questions to take to your appointment

Was this an antigen test or a PCR?
If the antigen test is negative, should it be repeated after about 48 hours?
How does a recent prior infection affect how this result reads?

Build a printable questionnaire with your own results →

Where to read more

MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .

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