PPV and NPV (solid lines) plotted as functions of the prevalence together with a 95% lower confidence bound (LCB, dashed lines).

<p>The one-test procedure is shown in turquoise and the two-test procedure in red. The two procedures are virtually indistinguishable for PPV, but differ markedly for NPV when prevalence is high and all patients are included. The top two plots (<b>A</b> and <b>B</b>) are shown for the 1044 patients without known sIgAD and the bottom (<b>C</b> and <b>D</b>) two for all 1071 patients. Disease prevalence between 9% and 60% is shown by grey shading. PPV and NPV of better than 90% and 95%, respectively, are shown by dashed black lines as is the prevalence of 5%.