Rethinking the threshold for an abnormal capillary blood lead screening test
J. D. Sargent and M. A. Dalton
Department of Pediatrics, Dartmouth Medical School, Hanover, NH, USA.
OBJECTIVES: To examine the test characteristics of the capillary blood lead
screening test as a predictor of elevated venous blood lead levels, using
receiver operating characteristic (ROC) curves. To consider a rational
capillary blood lead cutoff value in the context of what has been learned
about the screening test and what is understood about the clinical course
of children with elevated blood lead levels in the mild range (0.48-0.92
mumol/L [10-19 micrograms/dL]). DESIGN: In a clinical trial, 513 urban
children aged 6 years and younger were screened for lead exposure. Paired
samples of venous blood were drawn from all children. 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 mumol/L
(10 and 20 micrograms/dL). Contaminated capillary specimens were defined as
those in which the capillary result exceeded the venous result by 0.12
mumol/L (2.5 micrograms/dL) or more (n = 49). MAIN OUTCOME MEASURES: Test
sensitivity and false-positive rate (equal to 1-specificity) as a function
of the capillary screening cutoff value. Area under the ROC curve as a
measure of screening test performance. RESULTS: Venous blood lead levels
were 0.48 mumol/L (10 micrograms/dL) or more in 20.5% and 0.97 mumol/l (20
micrograms/dL) or more in 2.3% of children. Measurement of capillary blood
lead levels performed very well as a screening test with an area under the
ROC curve of 0.97 at the 0.48 mumol/L (10-micrograms/dL) threshold and 0.99
at the 0.97-mumol/L (20-micrograms/dL) threshold. For a capillary cutoff
value of 0.39 mumol/L (8 micrograms/dL) and an elevated blood lead level
threshold of 0.48 mumol/L (10 micrograms/dL), test sensitivity is 100% and
the false-positive rate is 23%. Test sensitivity drops to 91%, 63%, and 45%
at capillary cutoff values of 0.48, 0.58, and 0.68 mumol/L (10, 12, and 14
micrograms/dL), respectively. The false-positive rate drops to 8%, 2%, and
1% at capillary cutoff values of 0.48, 0.58, and 0.68 mumol/L (10, 12, and
14 micrograms/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.
CONCLUSIONS: In this sample of children, capillary blood lead measurement
performed well as a screening test for elevated venous blood lead levels.
Altering the capillary specimen contamination rate has little effect on the
rest characteristics because much of the misclassification error resulted
from random analytic error in the analysis of blood lead levels, which is
high compared with the threshold of concern (0.48 mumol/L [10
micrograms/dL]). Because of lack of data on clinical outcomes for children
with elevated blood lead levels in the 0.48- to 0.92-mumol/L (10- to
19-micrograms/dL) range, we suggest that the greatest utility be placed on
avoiding false-positive misclassification. A clinical capillary screening
cutoff value of 0.72 mumol/L (15 micrograms/dL) would avoid most
false-positive results and would permit 100% sensitivity in detecting
children with blood lead levels of 0.97 mumol/L (20 micrograms/dL) or
higher.
