TABLE 14 Computation of Sensitivity Specificity Positive Predicted Value PPVNegative Predictive Value NPV and Likelihood Ratios

Colon Polyps (per Patient Analysis)

Present Absent

Positive test (polyp on CT colonography) 42 (TP) 8 (FP)

Sensitivity = TP/(TP + FN) = 42/(42 + 9) = 42/51 = 84% Specificity = TN/(FP + TN) = 41/(41 + 8) = 41/49 = 82% PPV = TP/(TP + FP) = 42/(42 + 8) = 42/50 = 84% NPV = TN/(FN + TN) = 41/(9 + 41) = 41/50 = 82%

Likelihood ratio (positive test)

= Sensitivity/(1 - Specificity) = TP/(TP + FN)/(1 — TN)/(FP + TN) = 0.84/(1—0.82) = 84%/18% = 5

Likelihood ratio (negative test)

= (1 — Sensitivity)/Specificity = ([1 — TP]/[TP + FN])/ TN/(FP + TN) = (1 — 0.84)/0.82 = 16%/82% = 0.2

Adapted from Fenlon et al, 1999.

CT = computed tomography; FN = false negative; FP = false positive; TN = true negative; TP = true positive.

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