Differences Between COVID-19 Tests: PCR, Antigen, and Antibody Tests

The different types of COVID-19 tests can often be confusing. Many people wonder which test they should take, when it should be performed, how reliable it is, and what exactly it detects.

The most appropriate test depends largely on what you want to determine. If you need to know whether you are currently infected, one type of test is appropriate. If you want to know whether you have previously been infected and developed antibodies, a different test should be used.

PCR Test

The Polymerase Chain Reaction (PCR) test detects the genetic material (RNA) of the SARS-CoV-2 virus from a biological sample, usually collected from the nose or throat.

PCR remains the reference standard for diagnosing an active COVID-19 infection due to its high analytical accuracy. Results are generally available within approximately 24 hours, as samples must be processed in a laboratory.

When should a PCR test be performed?

PCR testing is recommended when a person has symptoms consistent with COVID-19 or when there is a strong suspicion of recent infection. It can also detect infections in individuals who do not yet have symptoms.

How are PCR results interpreted?

• Positive result: Viral genetic material has been detected, confirming an active infection.

• Negative result: No viral genetic material has been detected in the sample. However, if clinical suspicion remains high—for example, because symptoms strongly suggest COVID-19—a repeat test may be recommended, as the timing or quality of the sample can influence the result.

Antigen Test

Rapid antigen tests also detect an active COVID-19 infection, but instead of identifying viral genetic material, they detect specific proteins (antigens) found on the virus. Samples are usually collected using a nasal swab.

The main advantage of antigen testing is speed: results are typically available in 15–30 minutes, making it a valuable tool for rapid diagnosis.

How reliable are antigen tests?

Rapid antigen tests are generally most accurate when the viral load is highest, which is usually during the first few days after symptoms begin.

Modern antigen tests have improved significantly and now provide excellent diagnostic performance when used appropriately, although PCR testing generally remains more sensitive, particularly when viral levels are low.

When should an antigen test be performed?

Antigen tests are commonly used for:

• Rapid diagnosis of symptomatic individuals.

• Screening programmes.

• Situations where immediate results are required.

• Complementing PCR testing in specific clinical circumstances.

Antibody Tests

Antibody (serological) tests determine whether a person has previously been infected with SARS-CoV-2 by detecting antibodies produced by the immune system.

Unlike PCR or antigen tests, antibody tests do not diagnose an active infection on their own.

Their primary purpose is to identify previous exposure to the virus or to assist healthcare professionals in monitoring a patient's immune response.

There are two main types of antibody tests:

• Rapid antibody tests (point-of-care).

• Laboratory-based serological tests.

What do antibody tests measure?

These tests typically detect two classes of antibodies:

• IgM antibodies, which generally appear during the early stages of the immune response.

• IgG antibodies, which usually develop later and may remain detectable for longer after infection.

Results are generally interpreted as follows:

• IgM positive: May indicate a recent or ongoing infection.

• IgG positive: Suggests previous exposure to the virus and development of an immune response.

• IgM and IgG positive: May indicate that the infection is progressing and antibodies are being produced.

• Both negative: No detectable antibodies are present. However, this does not always exclude previous infection, as some individuals may develop immunity primarily through cellular immune responses rather than measurable antibodies.

Rapid Antibody Self-Tests

Rapid antibody self-tests require only a small finger-prick blood sample and provide results within a few minutes.

In many countries, these tests are available under specific regulatory conditions and should be used according to local healthcare guidance.

Anyone performing a rapid antibody test should discuss the results with the healthcare professional who recommended or prescribed the test.

These tests are intended to indicate whether a person has previously developed antibodies and are not suitable for diagnosing an active COVID-19 infection.

If you have symptoms suggestive of COVID-19 or have recently been in close contact with someone who has tested positive, a PCR or rapid antigen test is generally more appropriate.

Laboratory Antibody Tests (ELISA)

Laboratory-based serological testing is performed using a blood sample collected from a vein and analysed in a clinical laboratory, commonly using ELISA (Enzyme-Linked Immunosorbent Assay) techniques.

Results are usually available within approximately 48 hours.

Compared with rapid antibody tests, laboratory testing generally provides greater analytical accuracy and may also estimate the concentration of antibodies present in the bloodstream.

Choosing the Right COVID-19 Test

Different COVID-19 tests answer different clinical questions:

• PCR Test: Detects an active infection with very high sensitivity.

• Rapid Antigen Test: Detects an active infection and provides fast results, particularly when viral load is high.

• Antibody Test: Indicates previous exposure to the virus and the body's immune response, but is not intended to diagnose a current infection.

If you require further information or would like to arrange a COVID-19 test, please contact Semesur Assistance on +34 91 10 10 900. Our healthcare professionals are available 24 hours a day, 7 days a week to help you choose the most appropriate test for your circumstances.