A screaning test has 99% sensitivity and 99% specificity prevalence of target disease is 5 per 1000 population. The PPV of the test -: (A) 10, (B) 70, (C) 33, (D) 99
Answer is C. Ans. is 'c' i.e., 33 o Positive predictive value is related to sensitivity specificity and prevalenceo This relationship is represented by Baye's theorem : -PPV =Sensitivity * Prevalence--------------------------------------------------(sensitivity x prevalence) + (100-spcificity) (100-prevalence)X 100o One thing should be remembered that prevalence is in percentage (per 100 population) not per 1000. o So, in this question, prevalence = 5 per 1000 = 0.5 per 100 or 0.5%PPV =99 x 0.5--------------------------(99x 0.5) + (100-99) (100-0.5)x 100 - 33.2