As we are preparing for another wave of COVID-19, it’s useful to know the various roles of the rapid antigen test versus the traditional PCR test.

The PCR test is based on detecting viral genetic material that has been amplified in the lab;  the antigen test picks up actual structural viral proteins.

  • The PCR remains the most sensitive test (highest pick-up rate) but is more expensive and has a longer turn-around time and remains positive for several weeks after infection
  • The antigen test picks up COVID when patients are symptomatic and at their most infectious, has a very quick turn-around time but can miss some infections in the very early or late stages.

PLEASE NOTE THAT IN BOTH CASES A GOOD QUALITY NASOPHARYNGEAL SWAB PERFORMED BY A HEALTH CARE PROFESSIONAL IS BEST

For more detail, I’ve prepared the summary table below:

TEST INDICATIONS BEST TIMING ADVANTAGES DISADVANTAGES
PCR Any person with symptoms in which COVID-19 is considered a possibility.

All hospitalized patients or patients due to be hospitalized eg for surgery

Sometimes needed for individuals who are close contacts with infectious cases  and need to know their status to be able to return to work after isolation eg doctors/teachers

Will pick up SARS CoV2 virus from 1-2 days before symptom onset, throughout symptomatic stage, and even in asymptomatic cases; for up to 2- 3 months after infection. Very sensitive

Able to pick up even low loads of virus eg in asymptomatic people

Long window in which it will pick up the virus

Long turn-around time (usually 24 hours or more, though in exceptional cases can be completed in < 12 hours.)

More expensive

Can pick “dead” SARS COV2 particles many months after infection when the person may no longer be infectious

Rapid Antigen Test For identification of COVID in people who have symptoms suggestive of possible COVID

Good for a quick differentiation between COVID and a common cold/allergies if there has not been a particular exposure risk.

Role in port of entry screening- eg if large amounts of people need screening for travelling/ sports matches etc and it is impractical to wait 24 hours for each result.

When a person is actually symptomatic

Can pick up some COVID in the pre-symptomatic stage 1-3 days before onset of symptoms.

Quick turnaround time (15-30 minutes)

Lower cost

Fast identification of cases and contacts for quarantine/isolation

Very few false positives- if it is positive then you actually have COVID now

Some false negative results

If negative, and symptoms persist, best followed up by a PCR test.

Info source: www.nicd.co.za

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