A typical strategy for identifying children with elevated blood lead levels (BLL) is to use a test on a capillary sample as a screening tool and then perform a confirmatory test on a venous sample following an elevated capillary screen. However, tests on capillary samples are prone to false-positive results, which affect surveillance estimates of the prevalence of elevated BLL.
We sought to measure the prevalence and risk of false-positive results in elevated tests on capillary samples based on various potential predictors.
Design, Setting, Participants, and Main Outcome Measures:
We analyzed blood lead results for children aged 0 to 6 years tested during 2011 to 2017 in Minnesota. A false positive was defined as a test on a capillary sample of at least 5 μg/dL, followed by a test result on a venous sample less than 5 μg/dL within 90 days. Binomial regression was used to estimate the probability of false-positive results dependent on the initial test result and the time between initial and confirmatory tests.
Results from 3898 children were included in analyses. Of these, 2330 (60%) had confirmatory results below 5 μg/dL and were classified as false positives. The proportion of false positives varied with time between tests, dependent on the initial result. Extrapolating the model to zero days between tests, without time for any change in the child's true BLL, we predicted 55% false positives in this study group (95% confidence interval: 53%-57%).
Caution is warranted when interpreting elevated tests on capillary samples without confirmatory tests on venous samples. Providers should be encouraged to follow up all elevated capillary screens with confirmatory tests on venous samples.