Problematic Prostatic Prediction

To the Editor.—  The article by Chodak and Schoenberg¹ about a prostatic cancer screening program demonstrates an incorrect use of the term "specificity." These authors report "specificity of the rectal examination was 29%" because 11 cancers were found among 38 cases with positive results from rectal examinations. The correct term for this ratio is the "positive predictive value," which is defined as the percentage of patients with a positive finding (ie, prostatic abnormalities found on rectal examination) who are eventually shown to truly have the disease (ie, prostatic carcinoma).² Specificity, on the other hand, is the percentage of patients without the disease whose test results (in this case negative results from examination) also indicate they do not have the condition.From the data given in the article, the true specificity cannot be calculated. Using a previously published sensitivity of rectal examinations (0.69),³ the false-negatives can be estimated to