Author: Dr Mark Ali MBBS BSc FRCS CTh, Medical Director 

To discuss how accurate covid-19 tests are, we need to understand what "accuracy" refers to. 

Test accuracy refers to two separate concepts: sensitivity and specificity.

Sensitivity concerns the false negative rate and is expressed as a percentage: out of 100 truely positive patients, how many are detected as positive by the test? The higher the sensitivity, the fewer the numbers of false negatives. In the USA, sensitivity is often called positive percent agreement (PPA). 

Specificity concerns the false positive rate and is expressed as a percentage: out of 100 truely negative patients, how many are detected as negative by the test? The higher the specificity, the fewer the numbers of false positives. An example here would be if the test detects an antibody response to a different but related coronavirus. The individual would be informed that they have antibodies to covid-19 when in fact it would reflect infection with a different but similar coronavirus. In the USA, specificity is often called negative percent agreement (NPA). 

We also need to interpret the test results in context. The likelihood of a true positive or true negative will vary depending on the prevalence-how common the condition is within a population. For covid-19, we can estimate but these are not firm yet. This is explained in a great article here.

Using these concepts, we can now review the accuracy of antigen and antibody tests.