BACKGROUND: A typical strategy for identifying children with elevated blood lead levels (EBLL) is to use a capillary test as a screening tool, then perform a confirmatory venous test on children with an elevated result. However, capillary tests are prone to false positive results. EBLL surveillance has been inconsistent between state and federal agencies in the way that elevated capillary tests are classified. This could be improved by a better understanding of capillary test false positives. We examined the false positive prevalence among initial elevated capillary tests through blood lead surveillance data in Minnesota.
METHODS: We analyzed blood lead test results for children aged 0–6 years tested during 2011–2015 in Minnesota. A false positive was a capillary test of at least 5 µg/dL followed by a venous test less than 5 μg/dL within 90 days. Binomial regression was used to estimate the probability of false positive results dependent on the initial capillary result and the time between the capillary and venous tests.
RESULTS: Among 8,560 children with elevated capillary results, 3,701 (43%) received follow-up venous tests within 90 days. Of these, 65% had a venous result below 5 μg/dL and were classified as false positives. The proportion of false positives changed with the time between tests, dependent on the initial result. Extrapolating model estimates to zero days between tests, without time for any change in the child’s true blood lead level, those with an initial result 5.0–6.9 μg/dL were 78% false positives, 7.0–9.9 μg/dL were 55% false positives, and 10.0+ μg/dL were 48% false positives. The average percent false positive with no time between tests was estimated to be 60% (95% Confidence Interval: 58%, 62%).
CONCLUSIONS: Given that 65% of initial high capillary tests were classified as false positives and 60% were estimated to actually be false positives, it is likely that 5% of initial high capillaries were incorrectly classified as false positives. However, if the potential for false positives is disregarded by counting all initial high capillaries as true cases, 60% of initial high capillaries would be incorrectly classified as true EBLL cases. EBLL surveillance should consistently classify children with elevated capillary tests followed by non-elevated venous tests as non-cases. Caution should be taken when interpreting elevated capillary tests without follow-up tests as EBLL cases, given that 60% are expected to be false positives. Providers should be encouraged to follow-up all initial elevated capillary tests with venous tests.