RETHINKING THE THRESHOLD FOR AN ABNORMAL CAPILLARY BLOOD LEAD SCREENING-TEST

Objectives: To examine the test characteristics of the capillary blood lead screening test as a predictor of elevated venous blood lead leve ls, using receiver operating characteristic (ROC) curves. To consider a rational capillary blood lead cutoff value in the context of what ha s been learned about the screening test and what is understood about t he clinical course of children with elevated blood lead levels in the mild range (0.48-0.92 mu mol/L [10-19 mu g/dL]).Design: In a clinical trial, 513 urban children aged 6 years and younger were screened for l ead exposure. Paired samples of venous blood were drawn from all child ren. For these children we examine the ROC curves for capillary blood lead levels as a predictor of elevated venous blood lead levels above 2 thresholds, 0.48 and 0.97 mu mol/L (10 and 20 mu g/dL). Contaminated capillary specimens were de fined as those in which the capillary res ult exceeded the venous result by 0.12 mu mol/L (2.5 mu g/dL) or more (n=49). Main Outcome Measures: Test sensitivity and false-positive rat e (equal to 1- specificity) as a function of the capillary screening c utoff value. Area under the ROC curve as a measure of screening test p erformance. Results: Venous blood lead levels were 0.48 mu mol/L (10 m u g/dL) or more in 20.5% and 0.97 mu mol/L (20 mu g/dL) or more in 2.3 % of children. Measurement of capillary blood lead levels performed ve ry well as a screening test with an area under the ROC curve of 0.97 a t the 0.48-mu mol/L (10-mu g/dL) threshold and 0.99 at the 0.97-mu mol /L (20-mu g/dL) threshold. For a capillary cutoff value of 0.39 mu mol /L (8 mu g/dL) and an elevated blood lead level threshold of 0.48 mu m ol/L (10 mu g/dL), test sensitivity is 100% and the false-positive rat e is 23%. Test sensitivity drops to 91%, 63%, and 49% at capillary cut off values of 0.48, 0.58, and 0.68 mu mol/L (10, 12, and 14 mu g/dL), respectively. The false-positive rate drops to 8%, 2%, and 1% at capil lary cutoff values of 0.48, 0.58, and 0.68 mu mol/L (10, 12, and 14 mu g/dL), respectively. Changing the contamination rate by appending or deleting contaminated capillary specimens from the data set had little effect on the area under the ROC curve at either threshold. Conclusio ns: In this sample of children, capillary blood lead measurement perfo rmed well as a screening test for elevated venous blood lead levels. A ltering the capillary specimen contamination rate has little effect on the test characteristics because much of the misclassification error resulted from random analytic error in the analysis of blood lead leve ls, which is high compared with the threshold of concern (0.48 mu mol/ L [10 mu g/dL]). Because of lack of data on clinical outcomes for chil dren with elevated blood lead levels in the 0.48- to 0.92-mu mol/L (10 - to 19-mu g/dL) range, we suggest that the greatest utility be placed on avoiding false-positive misclassification. A clinical capillary sc reening cutoff value of 0.72 mu mol/L (15 mu g/dL) would avoid most fa lse-positive results and would permit 100% sensitivity in detecting ch ildren with blood lead levels of 0.97 mu mol/L (20 mu g/dL) or higher.