Received on December 4, 2003
Accepted on May 17, 2004

Laboratory Management

Estimate of Uncertainty of Measurement from a Single-Laboratory Validation Study: Application to the Determination of Lead in Blood

¹ Clinical Biochemistry Laboratory, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy

^* To whom correspondence should be addressed. E-mail: marina.patriarca{at}iss.it.

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. NH₄H₂PO₄ (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 µg/L) and quantification (9.4 µg/L). Repeatability and intermediate imprecision within the range 35-150 µg/L were <5.5% and <6.2%, respectively. We assessed trueness by use of certified reference materials, by recovery tests, and by comparison with target values from other reference methods (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 for lead in blood adopted in the US and the European Union.