Diagnostic Testing Unwarranted for Children With Blood Lead 10 to 14 μg/dL

Background. Recent statements from the American Academy of Pediatrics and Centers for Disease Control and Prevention recommend diagnostic venous blood lead testing within 90 days of a marginally elevated screening test (10–14 mg/dL). Objective. To evaluate the ability of a marginally elevated capillary (CScr) or venous (VScr) blood lead screening test to predict venous diagnostic (VPb) blood lead (taken within 90 days of the screening test) that would prompt environmental evaluation (>20 mg/dL). Design. Population-based follow-up study comparing CScr and VScr with VPb drawn within 90 days of the screening sample. This study population was drawn from all children aged 0 to 4 years who were screened in Worcester County, Massachusetts, and Providence County, Rhode Island, with CScr and VScr during calendar year 1994. Outcome Measures. To evaluate predictive validity, CScr and VScr were correlated with VPb. CScr, VScr, and VPb results were then separated into the following categories: <10, 10 to 14, 15 to 19, and >20 mg/dL. CScr and VScr categories were cross-tabulated against VPb categories, and logistic regression analysis was used to evaluate categorical elevations of CScr and VScr as predictors of VPb >20 mg/dL. Results. Of 31 904 children screened with CScr, 5450 (17.1%) were elevated and 1278 were followed up with VPb within 90 days. Of 14 623 children screened with VScr, 2979 (20.4%) were elevated and 614 were followed up with VPb within 90 days. CScr was only weakly correlated with VPb (r 5 0.39), whereas VScr was more strongly correlated with VPb (r 5 0.73). Compared with CScr <10 mg/dL, CScr in the 10 to 14 mg/dL range did not identify a higher percentage of children with VPb elevation in any category, and falsely misclassified as lead poisoned some 77% of children. Compared with VScr <10 mg/dL, VScr in the 10 to 14 mg/dL range identified higher percentages of children with VPb in the 10 to 19 mg/dL range but not with VPb >20 mg/dL, and falsely misclassified as lead poisoned 42% of children. Compared with screening tests <10 mg/dL, the odds of identifying a child with VPb >20 were no different from 1 for CScr of 10 to 14 mg/dL (adjusted odds ratio 1.4 [95% confidence interval 0.3, 6.6]), CScr of 15 to 19 mg/dL (3.2 [0.7, 15.7]), or VScr of 10 to 14 mg/dL (0.9 [0.3, 3.0]). CScr and VScr in the 15 to 19 mg/dL range were associated with significantly higher odds of having VPb >20 mg/dL when compared with screening tests <10 mg/dL. Conclusions. These data indicate that special diagnostic testing within 90 days for children with CScr and VScr in the 10 to 14 mg/dL range does not result in greater identification of VPb >20. Raising the set point for diagnostic testing to 15 mg/dL in this sample would eliminate the unnecessary follow-up of 5162 children, of whom 3360 were falsely misclassified as having undue lead exposure. Pediatrics 1999;103(4). URL: http://www. pediatrics.org/cgi/content/full/103/4/e51; lead poisoning, screening, diagnostic testing, follow-up study. ABBREVIATIONS. CDC, Centers for Disease Control and Prevention; BLL, blood lead level; VPb, venous lead confirmation results; CScr, capillary screening test; VScr, venous screening test. The Centers for Disease Control and Prevention (CDC) statement of 19911 identified a group of children with blood lead levels (BLL) between 10 and 24 mg/dL, for whom little data were available to guide clinical practice. Since that time, there have been four randomized clinical trials evaluating prevention strategies for children with blood lead elevations in this range (one calcium supplementation,2 two household dust control,3,4 one soil removal5); none of these prevention strategies has resulted in clinically significant declines of BLL in the treatment group. These data on treatment outcomes suggest that screening for BLL elevations in this range violates a critical concept underlying screening: that early diagnosis and treatment should result in a more favorable outcome for the patient than would otherwise have occurred had screening not taken place.6 A second issue complicates screening of children with mild BLL elevation; laboratory error in the BLL analysis is high in relation to the threshold of concern.7,8 Most clinical laboratories operate at a level of precision for which the 95% confidence intervals for assay results are 64 mg/dL for samples in the 10 to 19 mg/dL range.9,10 These laboratory errors have been highlighted in a recent article in which 2 of 18 commercial laboratories misclassified clinical specimens with BLL of 18 mg/dL as below 10 mg/dL.11 Combined with normal fluctuations in BLL throughout time,12 laboratory errors decrease our confidence in the ability to determine blood elevation in the 10 to 19 mg/dL range with accuracy. The lead screening debate has focused on the appropriateness of universal screening, with little discussion of the screening BLL cut-off point for an abnormal test result. However, BLL is a continuous variable, and the threshold of concern, 10 mg/dL, is From the Department of Pediatrics, Dartmouth Medical School, Hanover, New Hampshire. Received for publication May 18, 1998; accepted Oct 19, 1998. Reprint requests to (J.D.S.) Pediatrics and Adolescent Medicine, DartmouthHitchcock Medical Center, One Medical Center Dr, Lebanon, NH 03756. PEDIATRICS (ISSN 0031 4005). Copyright © 1999 by the American Academy of Pediatrics. http://www.pediatrics.org/cgi/content/full/103/4/e51 PEDIATRICS Vol. 103 No. 4 April 1999 1 of 5 by guest on November 7, 2017 http://pediatrics.aappublications.org/ Downloaded from based strictly on concerns about effects of lead on development. The determination of the screening cut-off point for an abnormal level of BLL should be amenable to an evidence-based approach,13 which considers other factors, such as the precision and predictive validity of the screening test, the net cost of a false-positive result, and the benefits of treating a true positive. Raising the screening set point would be expected to decrease false positive misclassification, but is also often associated with an increased likelihood of false negative tests.14 We considered this trade-off in a recent study of paired capillary and venous BLL data. Using receiving-operator characteristics curves, we evaluated the capillary result as a predictor of the venous result15 and suggested that a rational clinical threshold for BLL elevation was 15 mg/dL because this set point minimized the chances of falsely misclassifying a child with BLL ,10 mg/dL as elevated and minimized the chances of falsely misclassifying a child with BLL $20 mg/dL as only mildly elevated, two outcomes that we wished to avoid.16 In this study, we evaluate the generalizability of our suggestion in a large population-based sample. We focus on the ability of a marginally elevated screening test (10–14 mg/dL) to predict a diagnostic venous lead confirmation result (VPb) that would prompt environmental investigation (VPb $20 mg/ dL) according to current guidelines.17,18