Figure 3.

The Bayesian (Fagan's) nomogram demonstrating the pretest and posttest probability of having a diagnosis of immune-mediated AA based on PNH flow cytometry results. PNH clones have a near 100% positive predictive value for the diagnosis of immune-mediated AA, with a positive LR of approximately 92 for immune-mediated AA.⁸ Shown is an example corresponding to Case 2 in the text. To apply the nomogram tool, one first has to estimate the pretest probability of AA based on the clinical presentation. This previously healthy patient developed pancytopenia in young adulthood without any physical examination findings or family history suggestive of a congenital marrow failure disorder. Based on this presentation, her pretest probability of immune-mediated acquired AA is estimated as moderate to high. The green line shows the results for a conservative estimate of a moderate (approximately 60%) pretest probability of AA, crossing at the positive LR of 92. Detection of a PNH clone increases the posttest probability of AA to 99.3%, allowing to confidently establish the diagnosis of immune-mediated AA and initiate appropriate therapy. Importantly, a negative PNH test would not significantly change the pretest probability. Starting with a pretest probability of 60%, a negative PNH test (shown by the red line crossing at the negative LR of 0.54) will result in a posttest probability of 44.9%. The calculations for the Bayesian nomogram plot are based on Shah et al.⁸