Estimate of uncertainty of measurement from a single-laboratory validation study: Application to the determination of lead in blood

Abstract

Background: Lead is an environmental pollutant, and human exposure is assessed by monitoring lead concentrations in blood. Because the main source of environmental exposure has been the use of leaded gasoline, its phase-out has led to decreased lead concentrations in the general population. Therefore, validated analytical methods for the determination of lower lead concentrations in blood (<150 μg/L) are needed. In addition, new ISO standards require that laboratories determine and specify the uncertainty of their results. Methods: We validated a method to determine lead in blood at concentrations up to 150 μg/L by electrothermal atomic absorption spectrometry with Zeeman background correction according to EURACHEM guidelines. Blood samples were diluted (1:1 by volume) with 2 mL/L Triton X-100. NH4H 2PO4 (5 g/L) and Mg(NO3)2 (0.5 g/L) were used as modifiers. Matrix-matched standards were used for calibration. Results: We determined the limits of detection (3.1 μ/L) and quantification (9.4 μ/L). Repeatability and intermediate imprecision within the range 35-150 μ/L were <5.5% and <6.0%, respectively. We assessed trueness by use of certified reference materials, by recovery tests, and by comparison with target values of other reference materials (candidate external quality assessment samples). The expanded uncertainty ranged from 20% to 16% (with a confidence level of 95%) depending on concentration. Conclusions: This study provides a working example of the estimate of uncertainty from method performance data according to the EURACHEM/CITAC guidelines. The estimated uncertainty is compatible with quality specifications for the analysis of lead in blood adopted in the US and the European Union. © 2004 American Association for Clinical Chemistry.