Infectious Disease
SARS-CoV-2 antigen
SARS-CoV-2 antigen testing detects the viral nucleocapsid (nucleoprotein) protein from a nasal or nasopharyngeal swab, giving a rapid qualitative result that supports the diagnosis of acute COVID-19 at or near the patient. It is a triage and screening aid rather than a quantitative measurement, so a negative result does not fully exclude infection, particularly at low viral loads.
Why it is measured
A positive antigen result is a fast, actionable indicator of active SARS-CoV-2 infection, supporting prompt isolation and clinical decisions without waiting for a laboratory PCR. Sensitivity falls as viral load falls, so the result is most reliable early in symptomatic illness.
| Typical range | Qualitative, not numeric, so no reference interval or measurement units apply. The result is reported as Not detected (Negative) or Detected (Positive); some platforms use Reactive or Non-reactive. The expected result in an uninfected adult is Not detected. Instrument-read assays apply an internal signal cut-off rather than a reference interval, and clinical sensitivity varies with viral load (a lower PCR cycle threshold, meaning a higher viral load, gives higher antigen detection) and by method and manufacturer. |
|---|---|
| Sample | Anterior nasal or nasopharyngeal swab; some assays are also validated for oropharyngeal or combined swabs. Collect and process strictly per the assay instructions and within the stated specimen stability window. |
| Turnaround | Approximately 10 to 20 minutes, commonly about 15 minutes, for both instrument-read and visually read devices. |
Point of care devices that report it
- BD Veritor Plus System (SARS-CoV-2 and SARS-CoV-2 and Flu A+B assays), reader-based digital immunoassay
- Quidel Sofia 2 SARS Antigen FIA, reader-based fluorescent immunoassay
- LumiraDx SARS-CoV-2 Ag, portable instrument-read microfluidic immunoassay
- Abbott Panbio COVID-19 Ag Rapid Test Device, visually read lateral flow
- SD Biosensor STANDARD Q COVID-19 Ag (distributed by Roche), visually read lateral flow
Questions, answered
Does a negative antigen test rule out COVID-19?
No. Antigen tests are less sensitive than PCR, especially at low viral loads and very early or late in infection. In a symptomatic person or a close contact, a negative result may warrant repeat antigen testing or confirmatory molecular (PCR) testing in line with local protocols. This is general guidance and not advice for any individual patient.
What is the difference between an antigen test and a PCR test?
An antigen test detects viral protein directly and gives a fast qualitative result at the point of care, whereas PCR detects viral genetic material in a laboratory with higher analytical sensitivity. Antigen testing is quicker and lower cost, but it is more likely to miss low-viral-load infections.
Why do some antigen tests need a reader instrument rather than just a visible line?
Reader-based analysers such as the BD Veritor or Sofia 2 interpret the result objectively against a fixed signal cut-off, which removes subjective line-reading and supports consistent, auditable documentation. The underlying chemistry is similar to a visually read lateral flow strip.