New concepts and methods of standardizing predictive value, accuracy and incorrect diagnostic rate

To express the value of a diagnostic test under standardized and comparable conditions.

Four new concepts of standardizing positive predictive value (SPPV), standardizing negative predictive value (SNPV), standardizing accuracy (SAc) and standardizing an incorrect diagnostic test were developed. The theoretical positive predictive value (SPPV), theoretical negative predictive value (SNPV), theoretical accuracy (SAc) and theoretical incorrect diagnosis rate (SIDR), which are not affected by a different constituent ratio of disease and nondisease groups and are obtained under the theoretical standard condition that the sample size in the disease group equals that in the nondisease group, were defined. Based on these concepts and the principles and methods of statistics and evaluation of diagnostic tests, corresponding formulas were deduced.

The formulas are: SPPV = a(b + d)/[a(b + d) + b(a + c)] = Se/(1 + Se - Sp), SNPV = d(a + c)/[c(b + d) + d(a + c)] = Sp/(1 - Se + Sp), SAc = [a(b + d) + d(a + c)]/[2(a + c)(b + d)] = (Se + Sp)/2, and SIDR = [b(a + c) + c(b + d)]/[2(a + c)(b + d)] = (2 - Se - Sp)/2. Here, a, b, c and d refer to the case numbers of true positives, false positives, false negatives and true negatives; Se and Sp refer, respectively, to sensitivity and specificity.

SPPV, SNPV, SAc and SIDR are very useful for expressing and evaluating the value of a diagnostic test under standardized and comparable conditions.