Understanding rapid antigen test lingo

With the use of rapid antigen tests on the increase, consumers presently have 15 different home tests listed on the ARTG to decide between.  

The comparisons between the tests ultimately use terminology which may not be familiar to most people. 

The technical data sheets supplied inside the rapid antigen test packages may contain similar terms which are difficult to understand.

To help you, there are two main technical measures for rapid antigen tests:

1. Specificity (or True Negative Rate) – The specificity of a test is the proportion of negative test results out of all truly negative samples. In other words, a test’s specificity is its ability to correctly identify those without the disease (the true negatives) while minimizing false positive results.  

2. Sensitivity – The sensitivity of a test is the proportion of positive test results out of all truly positive samples. In other words, a test’s sensitivity is its ability to correctly identify those with the disease (the true positives) while minimizing the number of false negative results.   

In Australia, the TGA has classified various rapid antigen tests based on a measure described as the PPA (positive percentage agreement). The PPA is determined by the proportion of individuals that produced a positive test result using a COVID-19 rapid antigen self-test in comparison with a positive result that was obtained using a more sensitive laboratory PCR test.  Only tests that have a PPA that is equal to or exceeds 80% is registered on the ARTG.  In this sense, the measure is a form of sensitivity analysis.

The World Health Organisation’s discussion of rapid antigen tests in its Interim Guidance (October 2021) refers only to the “sensitivity” of the tests. The WHO also recommends a minimum “sensitivity” of equal to or above 80%.

Finally, another measure of a test’s effectiveness relates to its ‘Limit of detection’.  This is a form of sensitivity analysis which describes the lowest detectable number of virus copies that will allow the test to return a positive result at least 95% of the time. A lower limit of detection indicates a more sensitive test, with fewer viral copies per sample necessary to produce a positive test result. 

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