What Causes a False Positive Rapid Covid Test?

When it comes to testing for COVID-19, rapid antigen tests (RATs) have become the most simplified, convenient, and common method of testing in Australia and internationally. Since entering the public arena, there has been much speculation about the accuracy of rapid antigen tests, how common false positive test results are, and what happens when a RAT test provides an inaccurate result.

Rapid antigen tests work by detecting the presence of specific proteins of SARS-CoV-2 (the virus that causes COVID-19). Rapid antigen tests are designed to be the first step to determine whether someone may be COVID positive or not. As such, it’s important to understand when you should take results at face value, and when you should seek further testing through a PCR COVID test.

What is a false negative test result?

Any rapid antigen test approved in Australia by the TGA must have an accuracy rate, which is referred to as a sensitivity, of at least 80%. That means that approximately 80% of test results can be expected to be accurate in detecting infection in someone who has COVID. It also means that up to 20% of rapid antigen tests, depending on the tests used, may be a false negative.

When you test negative with a COVID-19 RAT test, it is possible that you’re getting a false negative test result. False negative test results can be caused by a number of factors including:

The amount of antigen in swab sample may change over the duration of the infection (RAT tests are most accurate in the first 7 days of testing)
The test was performed after the first 7 days of symptom onset
The test may be negative before you develop symptoms
The test was not performed per the instructions
Specimen collection, extraction or transport was not performed correctly

If you have COVID-19 symptoms, it is recommended that you either re-test with a different or get a PCR test to confirm your results.

What is a false positive rapid antigen test?

A false positive test refers to when infection is detected by the rapid antigen test, but the person who is completing the test does not actually have COVID.

Like other antigen test, there is a higher chance of a false negative than a false positive – meaning it’s more likely that your test will indicate that you do not have COVID-19 when you do have it, rather than an indicating that you do have it, and you don’t (false positive).

The TGA sets a specifity rate at 98%, which means that only 2% of tests may incur a false positive test result. Specifity relates to how well a test can confirm the absence of COVID-19 – which means that it indicates the percentage of true negative results in patients who do not have the infection. This specifity rate means that 98% of COVID-19 tests will provide an accurate positive reading. So, if you receive a positive test result from a rapid antigen test, you can be fairly confident that you’re infectious and should stay home and isolate for 7 days.

What can cause a false positive rapid COVID test?

Specifity tells us that there is a 2% chance that a rapid antigen COVID test may present a false positive result. However, incorrect results can occur if the test is not performed correctly, or an adequate sample is obtained. The result can be incorrect if inadequate sampling volume, test is compromised or contaminated, or if the number of extraction buffer drops are less than 3 or more than 4. Furthermore, research suggests that rapid COVID-19 tests are more accurate when used in the first week after symptoms start. It is very important to follow the instructions for use for both sample collection and performance of the test.

What should I do if I suspect a false positive rapid antigen test?

If you test positive for COVID-19 with a rapid antigen test, it’s important to follow your state or territory government regulations on what to do next. If you do not have any symptoms, no known close contacts, and believe that the test was conducted accurately, it may be worth completing a second rapid antigen test or seeking a PCR test to confirm the results.

How to avoid a false positive rapid COVID test

False positive rapid antigen tests are rare but can still occur. You can reduce the chances of a false positive or inaccurate test result by ensuring your test is conducted correctly in line with the relevant instructions for your rapid antigen test. For SureScreen’s COVID-19 Rapid Antigen testing, our product has a very high sensitivity (96.1%), and a specifity at 99.0%.

Here are the instructions for completing a SureScreen Diagnostics SARS-CoV-2 Antigen Rapid Test:

Take the test cassette out of the sealed pouch and place it onto a clean flat surface. Once opened, use within 1 hour, or the test will be void.
Place the extraction tube in the extraction tube holder (perforated circular area on lid of the box) to avoid spilling the liquid.
Carefully twist to snap open the liquid vial. Open it away from your face and be careful not to spill any of the liquid.
Open the extraction tube and gently squeeze all of the liquid from the vial into the tube. Take care and discard the empty liquid vial.
Gently blow your nose into a tissue and throw the tissue away in a closed bin.
Clean your hands thoroughly again for 20 seconds using soap and warm water or use sanitiser.
Pick up the swab and identify the fabric tip.
Peel open the wrap from the end opposite the fabric tip. Do not touch the tip and you should discard if this has happened.
Gently wipe the swab around the inside of your nostril in a circular motion 5 times for at least 5 seconds. Slowly remove it. Immediately sample the second nostril at
Using the same swab, place the fabric tip just inside about 1.5cm inside your nose. Gently wipe the swab around the inside of your nostril in a circular motion 5 times for at least 5 seconds.
Pick up the extraction tube and place the swab’s fabric tip in the extraction tube, so it is in the liquid. Press the swab head firmly against the inside of the tube body by pinching the bottom of the tube with your thumb and fingers. Whilst pinching, rotate the swab for at least 10 seconds.
Slightly lift the nasal swab then break the swab on the “breakpoint” position. Leave the swab head in the extraction tube. Place the swab handle in the plastic waste bag provided.
Fit the dropper cap tightly onto the top of the extraction tube.
Avoid touching the test kit and results window area where the sample is added. Ensure the test cassette is on a clean and flat surface.
Gently squeeze the extraction tube to place 3 drops of the liquid into the sample well (S) on the test strip. Make sure that you are dropping liquid and not an air bubble. Discard the extraction tube into the waste bag.
Start a timer and wait 10 minutes before reading the result. Do not move the test once it is running. Interpret the result after 10 minutes. Do not read after 20 minutes. You must wait the full 10 minutes’ of development time before you read your test result.