Lead is one of the most common environmental hazards affecting children. Children are at highest risk for permanent neurological damage from lead exposure because their brains are still developing, they absorb lead more readily from their gastrointestinal tracts, and they are more likely to put lead-contaminated objects into their mouths.1,2 There is no known level of lead in blood that is safe for children; even low blood lead levels (BLLs) are associated with decreased intelligence in children and worse performance in school.3–5
Blood lead surveillance is essential in identifying patterns and changes in exposure to lead among children. In 2012, the Advisory Committee on Childhood Lead Poisoning Prevention recommended a reference level for BLLs of the 97.5th percentile, or 5 μg/dL, in order to differentiate between “elevated” and “not elevated” results.6 Both the Centers for Disease Control and Prevention and the Minnesota Department of Health (MDH) recognize BLLs equal to or greater than the reference value of 5 μg/dL as elevated BLLs.7,8 For prevalence estimates, the MDH defines an elevated BLL case as a child who has had either a test result on a venous sample of at least 5 μg/dL or who has had 2 tests on capillary samples of at least 5 μg/dL within 90 days of each other.9
Often, children are first screened with a blood lead test on a capillary sample; if the lead level of the capillary sample is elevated, a test on a venous sample is performed. Tests on capillary samples can be less expensive and easier to perform on children.10,11 Tests on capillary samples analyzed on point-of-care machines also allow BLLs to be determined within minutes, eliminating the need to send samples to outside laboratories.12
LeadCare is the only point-of-care machine brand for testing BLLs on the market. LeadCare machines were approved for analysis of both capillary and venous samples.13,14 However, a safety communication and class 1 recall were issued in 2017 by the US Food and Drug Administration for all LeadCare machines because they were sometimes giving falsely low test results when processing tests on venous samples. This recall did not affect tests on capillary samples analyzed on LeadCare machines, and LeadCare machines are still frequently used to perform blood lead tests on capillary samples.13,14
Despite the benefits of time, cost, and ease of doing tests on capillary samples, tests on venous samples remain the criterion standard because of the potential for false-positive results with tests on capillary samples. Capillary samples can be easily contaminated if there is lead dust on the child's fingertip, leading to an increased risk of false-positive results in tests on capillary samples.11,15 Blood lead levels slightly below the reference value may have false-positive capillary results because of the margin of error in the testing; the acceptable margin of error for blood lead tests was determined to be the greater of ±4 μg/dL or ±10% for proficiency testing programs approved under the Clinical Laboratory Improvement Amendments of 1988.16 There may also be test variability across laboratories or other unknown factors.17
In addition, natural decreases in BLLs over time may lead to initial tests being classified as false positives if their corresponding confirmatory tests occur at a later date, especially if parents limit their child's lead exposure following an initial elevated result. While some studies have found that longer delays between initial and confirmatory tests result in a higher rate of false positives, others have found that the false-positive rate does not vary by interval between screening and confirmatory test.15,17 Current MDH guidelines recommend that test results on capillary samples between 5 and 14.9 μg/dL be followed up with a confirmatory test on a venous sample within 1 month, results between 15 and 44.9 μg/dL be confirmed within 1 week, results between 45 and 59.9 μg/dL be confirmed within 2 business days, and results of 60 μg/dL or greater be confirmed immediately.18 Determining whether the length of time between tests influences the false-positive rate could inform guidelines on appropriate time intervals between the initial tests on capillary samples and confirmatory tests on venous samples.
Previous studies have examined false positives in blood lead tests on capillary samples.11,15,17,19 However, these studies had differing conditions, exclusions, and time intervals between the test on the capillary sample and the confirmatory test on a venous sample. In addition, these studies have used older reference values of 10 or 15 μg/dL to define an elevated BLL.11,15,17,19 As the first study to examine the proportion of false positives in tests on capillary samples after the reference value was lowered to 5 μg/dL, this study provides critical data on the proportion of false positives for tests on capillary samples for BLLs between 5 and 10 μg/dL and for all elevated BLLs of 5 μg/dL or greater. Having a better understanding of the frequency of false positives among tests on capillary samples has the potential to impact surveillance estimates of the prevalence of elevated BLLs among children. The national mean BLL decreased from 1.65 μg/dL in 1999 to 0.84 μg/dL in 2014; as elevated BLL becomes less common in the population, the positive predictive value of screening tests is expected to decrease.20,21
In this study, we used statewide lead surveillance data to determine the consistency of initial test results on capillary samples with corresponding confirmatory results on venous samples, in order to assess the proportion of false positives of tests on capillary samples. Our main objective was to quantify the proportion of false positives for tests on capillary samples when test results for BLLs of 5 μg/dL or greater were included. We also sought to determine whether the risk of a test being classified as a false positive increased as the time interval between the tests on capillary and venous samples increased. Since BLLs are expected to naturally decrease over time in the absence of ongoing exposure, we hypothesized that a longer time between initial and confirmatory tests would be associated with an increased risk of false-positive classification. We also assessed whether the magnitude of the initial elevated BLL was an independent predictor of the risk of the test on a capillary sample being a false positive, and whether there was an interaction between the initial result and the time interval between the initial and confirmatory tests. We hypothesized that the association between time to confirmatory test and the proportion of false positives may vary by initial BLL.
We analyzed records of blood lead tests drawn between January 1, 2011, and March 31, 2018, from MDH's blood lead information system, which receives results from laboratories of all blood lead tests given to people living in Minnesota. Test results of at least 5 μg/dL were considered elevated. Children included in the study were younger than 72 months (6 years of age) at the time of the lead tests. To be included in the study, a child's first elevated test had to be a test on a capillary sample on or before December 31, 2017. The child also had to have a confirmatory test on a venous sample within 90 days of his or her initial elevated test on a capillary sample.
Children whose venous sample was tested on a LeadCare machine were excluded from analysis. Those with a confirmatory test result on a venous sample below 5 μg/dL were classified as false positives, and those with a confirmatory test result on a venous sample of 5 μg/dL or greater were classified as true positives. Only the first elevated result from a capillary sample and the first subsequent result from a venous sample for a child were included in the analysis. An alternate definition of false positives was used for sensitivity analysis. The alternate false-positive definition was a test result on a venous sample that was at least 20% lower than the test result from a capillary sample.
We used multivariable binomial regression to measure the relative risk of false-positive status in elevated tests on capillary samples based on various factors. Our main outcome of interest for the model was whether the time interval between tests was related to an increased risk of the initial test on the capillary sample being classified as a false positive. We examined the percentage of false positives for time intervals between initial and confirmatory tests and for different magnitudes of the initial test result on a capillary sample and possible associations between time interval and magnitude of the initial test.
Based on previous studies, we examined whether age, season, urban/rural residence, or gender was associated with false-positive status, and whether these factors confounded the relationship between the initial BLL and the time interval between initial and confirmatory tests on the risk of false positive in an elevated capillary screen.17,19 Comparisons between categories were made using Pearson χ2 tests. To determine urban residence, a child's county of residence at the time of the first elevated capillary was classified on the basis of the 2013 Rural Urban Continuum Code.22 Child age, county of residence, gender, and season of initial test on a capillary sample were all obtained from the blood lead test results reported to the blood lead information system. Finally, we extrapolated the regression model to estimate the expected proportion of false positives given zero days between initial and confirmatory tests. This step was taken to account for the natural decline in BLLs over time in the absence of ongoing exposure.
Of the 392 601 total children who had at least 1 blood lead test between January 1, 2011, and December 31, 2017, 8689 (2.2%) had an elevated result on a capillary sample with no previous elevated test (Figure 1). Of these children, 4729 (54.4%) received a confirmatory test on a venous sample within 90 days; however, 831 (17.6%) children received confirmatory testing completed on LeadCare devices and were excluded. A total of 3898 children were included in the analysis. Of these, 2330 (59.8%) children had their initial test on a capillary sample classified as a false positive.
As initial test results increased, the percentage of false positives decreased (P < .001). Of initial tests on capillary samples with results between 5 and 6.9 μg/dL, 77.8% were classified as false positives, whereas 35.9% of the tests on capillary samples with results at or above 15 μg/dL were classified as false positives (Table 1). For initial results of 10 μg/dL or greater, the relative risk of classification as a false positive compared with initial results between 5 and 6.9 μg/dL was 0.48 (95% confidence interval: 0.44-0.52). These trends are visualized in Figure 2, which plots initial and confirmatory test results and displays the variation in the distribution of true and false positives. Children with an initial test result on a capillary sample in the 5 to 6.9 μg/dL range represented 46.2% (1799/3898) of the sample population, and 60.0% (1399/2330) of all children determined to have a false-positive initial blood lead test in our study.
As the time between initial and confirmatory tests increased, the percentage of false positives also increased: 55.0% of tests on capillary samples that received a confirmatory test on a venous sample within 10 days were classified as false positives, while 71.3% of tests on capillary samples receiving a confirmatory test on a venous sample within 31 to 90 days were classified as false positives (P < .001). As initial test results increased, the time to receive a confirmatory test on a venous sample decreased (P < .001). Of initial tests on capillary samples with results between 5 and 6.9 μg/dL, 46.1% received a confirmatory test within 10 days, while 76.3% of those with an initial result of 15 μg/dL or greater received a confirmatory test within 10 days (Table 2).
We found that urban residence (P < .001), older age (19-72 months) (P = .02), and an initial test completed in the winter (P = .002) were all significant predictors of a false-positive result. However, we did not find that these factors or gender had a significant confounding effect on the association between the exposures (initial test result on a capillary sample and time between tests) and the outcome (probability of a test being classified as a false positive). Thus, they were removed from the final model. Similarly, we did not find any significant differences in our results when the same model was run using the alternate definition of a false positive. Therefore, in our final analyses, we used our primary definition of a false positive: children who had an initial test result on a capillary sample of 5 μg/dL or greater and a confirmatory test result on a venous sample less than 5 μg/dL within 90 days.
As previously stated, both the time interval between initial and confirmatory tests and the level of the initial test result were independent predictors of a test being classified as a false positive. The relationship between these factors was significant (P < .001); together, they comprised our model to quantify the risk of false positive. Among children with lower initial BLLs, the probability of a test being classified as a false positive increased as the time to the confirmatory test increased (P = .001). Children with a higher initial BLL were more likely to receive confirmatory testing sooner (P < .001) and less likely to have their initial result classified as a false positive than children with lower initial test results (P < .001).
Extrapolating the model to simulate zero days between initial and confirmatory tests, we predicted 74.7% false positives among those with an initial result of 5.0 to 6.9 μg/dL, 47.3% false positives in the 7.0 to 9.9 μg/dL group, 37.6% false positives in the 10 to 14.9 μg/dL group, and 41.8% false positives in those with an initial result of 15 μg/dL or greater. We predicted that if there had been zero days between tests, 54.9% of our study population would be false positives (95% confidence interval: 53.0%-56.8%). This estimate was slightly lower than the observed 59.8% false positives in the sample group. The predicted 74.7% false positives given zero days between the initial test on a capillary sample of 5.0 to 6.9 μg/dL and the confirmatory test on a venous sample were an estimate slightly lower than the observed 77.8% false positives in this group.
This study is novel in its assessment of false-positive results in tests on capillary samples for lower elevated BLLs of 5 to 9.9 μg/dL. This study found that 59.8% of all tests on capillary samples were false positives. Previous research examined the risk and rates of false positives based on time between initial and confirmatory tests as well as initial BLLs and found that 12% to 73% of tests on capillary samples were false positives; however, they utilized older reference values of 10 or 15 μg/dL.11,15,17,19 The group with the lowest initial BLLs (5-6.9 μg/dL) had the highest proportion of false positives; this group may capture the margin of error for true BLLs slightly less than 5 μg/dL. It may also include those who experienced natural decreases in BLLs (to levels <5 μg/dL) over time. This is supported by the association between the length of time between tests and the risk of false positive. Those in this group who received confirmatory testing at 31 to 90 days had the highest proportion of false positives of all groups examined in our study. These findings supported our hypotheses that a longer time between initial and confirmatory tests would be associated with an increased risk of false-positive classification, and the association between time to confirmatory test and false-positive probability would vary by initial BLL.
Overall, we found that the risk of a test being classified as a false positive increased as the time interval between initial and confirmatory tests increased. However, this change in risk was not consistent over all initial BLLs. For those with lower initial test results, the percentage of false positives increased over time. The same trend was not observed for those with higher initial test results. This might reflect the fact that a majority of those with higher initial BLLs (≥10 μg/dL) would not be expected to experience a natural decrease in BLL to less than 5 μg/dL between tests. It is also possible that false positives resulting in higher initial BLLs are due to contamination, which would not be affected by the time between tests. In addition, our estimate lacks precision for those with higher initial test results on capillary samples and more time between tests due to the small sample size of those groups.
A major strength of this study is that it used statewide surveillance data, which represent field conditions for blood lead testing. Since reporting test results is mandated in Minnesota, using statewide surveillance data gave us a complete data set of all tests done in the state as well as a much larger sample size than previous studies. While previous studies used sample populations ranging from 464 to 1278, our analysis included test results from 3898 children.11,15,17,19 The large sample size, use of surveillance data, and inclusion of blood lead test results between 5 and 10 μg/dL increase the utility of this study.
This study has several limitations. As the percentage of elevated BLLs varies greatly across geographic areas and time and the underlying prevalence of a condition in the population affects the positive predictive value of a diagnostic test, the observed percentage of false positives in our study may not be applicable to populations with a different prevalence of elevated BLL. In addition, a number of tests on capillary samples had to be excluded from these results because they did not receive a confirmatory test on a venous sample (n = 2276), their confirmatory test was completed after 90 days (n = 1184), or their confirmatory test was completed on a LeadCare device affected by the recall (n = 831). We are unable to determine what effect excluding these samples might have had on the percentage of false positives. Finally, we were unable to quantify the variation in clinical and laboratory practices and their effect on the false-positive rate.
The results of this study have implications for blood lead testing guidelines and surveillance. Blood lead testing guidelines should recommend that all elevated initial tests on capillary samples be followed by a confirmatory test on a venous sample. Practitioners should make efforts to reduce the possibility of contamination when collecting tests on capillary samples by following current recommended procedures for drawing capillary samples, such as thoroughly washing the child's hands before drawing blood.23 Because of the high likelihood of false positives in tests on capillary samples, blood lead surveillance programs should strongly consider including only confirmed elevated BLLs of at least 5 μg/dL in their surveillance case definitions to avoid greatly overestimating the prevalence of lead poisoning in children.
Future research should examine how the effectiveness of using a second test on a capillary sample compares with doing a test on a venous sample for confirmatory testing, and how this might vary across different BLLs and time intervals between tests. With increased awareness of the limitations of testing methods and their potential for error, we are better equipped to standardize case definitions in blood lead surveillance efforts.
Implications for Policy & Practice
- Blood lead tests on capillary samples are a useful screening tool to identify those with potentially elevated blood lead levels. However, they are prone to false-positive results.
- Confirmatory tests on venous samples should be completed for those with an elevated test result on a capillary sample. Confirmatory tests should be completed as soon as possible to achieve the most accurate estimate of the prevalence of true cases of elevated blood levels in a given population.
- To achieve a more robust blood lead surveillance program nationally, state and federal agencies should utilize consistent criteria to define a case of elevated blood lead level.
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Keywords:Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.
lead poisoning; predictive value of tests; public health surveillance