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.