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November 16, 1994

Comparability of Capillary and Venous Blood Samples for Lead Screening-Reply

Author Affiliations

Salt Lake Health Department Salt Lake City, Utah

JAMA. 1994;272(19):1482. doi:10.1001/jama.1994.03520190024017

In Reply.  —Drs Rainey and Schonfeld have suggested that some confusion surrounds the use of the term "false-positive rate" in our article comparing capillary blood lead levels with simultaneously drawn venous samples. The quantity we use (false positives/total screenings) is, strictly speaking, a proportion and not a rate.1,2 This proportion, the proportion preferred by Rainey and Schonfeld (false positives/[false positives + true positives]) called a false-positive rate, and the positive predictive value are all useful in evaluating screening tests but need to be translated into practical terms.Our data show excellent correlation between the two sampling methods. Even when capillary results differed from venous results, they were usually not very far off the mark. How best to quantitatively represent this was the subject of much discussion among the authors. The false-positive rate, in our opinion, is limited by its necessarily strict construction. For example, using 0.72 μmol/L (15 μg/dL) of

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