Using state lead poisoning surveillance system data to assess false positive results of capillary testing

The purpose of this study was to determine the false positive percentage of capillary blood lead screening in a statewide surveillance system and to explore potential predictors of false positive results.

Data were all blood lead tests of 0-5 year old children in Maine during 2002-2003. We determined the proportion of children with elevated (>/=10 microg/dL) capillary test results who received a venous confirmatory test, and calculated the percentage of false positive tests, defined as a capillary test of >/=10 microg/dL with a confirmatory venous test of <10 microg/dL. Multivariable binomial regression was used to determine whether capillary blood lead level and length of time between capillary and venous tests predicted false positive results, after controlling for potential confounders. We also examined the positive bias of the capillary test among both false positive and true positive results.

Seventy-three percent of elevated capillary screening tests (2.2 percent of all capillary screening tests) were false positives. False positive results were less likely for capillary levels of 15-19 microg/dL (RR=0.78; 95% CI 0.5-0.92) and 20 microg/dL or above (RR=0.83; 95% CI 0.71-0.96) compared to 10-14 microg/dL. The percentage of false positives did not vary by interval between screening and confirmatory tests. The capillary test exhibited a positive bias compared to the venous test, even among true positive results.

False positive results may have been caused by sample contamination, rather than laboratory error or true variation in blood lead level between screening and confirmatory tests. Capillary screening could be improved by training in proper sample collection methods.