Testing for COVID-19: Test Performance of a Rapid Antigen Detection Test

Positive Predictive Value

PPV answers the question, ‘If the test result is positive, what is the chance that the person actually has the disease?’, and is given by the formula:

Substituting, we get

Interpretation: If a person obtains a positive test result on the ‘Standard Q COVID-19 Ag detection kit’, and the prevalence of COVID-19 in the population is 25%, then the chance that the person actually has COVID-19 is 94.3%.

Note: Although the value 94.33% seems impressive, one must exercise caution. India has recently crossed 1,000,000 cases. If the prevalence of COVID-19 were 25% across the country, and the test were applied to 1,000,000 cases, the number of false positive patients would be 56,700.

[False +ve %= (100-94.33) = 5.67% = 5.67/100= 0.0567

1,000,000*0.0567 = 56,700]

Considering India’s total population is in excess of 1.3 billion, the number of false positives reaches alarming levels. Please note that given the higher value of specificity is 100%, we could theoretically have zero false positives as well.


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