Fagan's nomogram for the calculation of post-test probabilities.

A pre-test probability of 37% for dengue disease was fixed, which was estimated by the number of symptomatic cases in selected studies. (A) Panbio had a post-test probability of 98%. For Platelia kits (B) post-test probability was 99%, ie, with an estimated prevalence of 37%, if this patient tests positive, the post-test probability that she truly has dengue would be 99% (solid line in red). On the other hand, if patient tests negative, the post-test probability that she truly has dengue would be 17% (A) or 13% (B) (blue dotted line). The results were obtained by the following calculations: pretest odds = prevalence/1-prevalence; post-test odds = pretest odds x LR- (LR+); post-test probability = post-test odds/1+post-test odds. LR, likelihood ratio.