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

Development and application of a high accuracy method for measuring Pb in blood

BACKGROUND

Lead (Pb) is an environmental pollutant, and human exposure is assessed by measuring blood Pb concentrations. Today, the use of Pb-based products is restricted, but the health effects of low Pb concentrations require investigation. For this, a high-accuracy method is needed. Here, we report a new inductively coupled plasma mass spectrometry (ICP-MS) method for quantifying Pb in blood.

METHODS

Blood samples and calibrators were gravimetrically spiked with bismuth as an internal standard, digested with ultrapure nitric acid, and diluted 100 times. The calibrators were made from high-purity reference materials supplied by the National Institute of Standards and Technology, and the sample concentration was measured using the calibration bracketing method. The analytical performance and application of the new method were investigated.

RESULTS

The method had good specificity and a negligible matrix effect. The intra- and inter-assay coefficients of variation (CVs) were <1.34 % and <1.88 %, respectively, and the total CV was <1.74 %. The range of recovery was 98.9 %-99.4 %. SRM955d, BCR-635, and BCR-636 were within the certified intervals, with mean relative bias values of 0.93 %, -0.13 %, and -0.56 %, respectively. The limit of quantification was 1.1 µg/l. The method was used to assign target values to external quality assessment samples and to assess routine methods, showing good results at high concentrations. However, at low concentrations, two laboratories did not pass.

CONCLUSIONS

This ICP-MS method is a simple, accurate method and can be a candidate reference method for blood Pb measurement. But more research will be needed to verify this.

Verification of quantitative analytical methods in medical laboratories

Background

Globally, all medical laboratories seeking accreditation should meet international quality standards to perform certain specific tests. Quality management program provides disciplines targeted to ensure that quality standards have been implemented by a laboratory in order to generate correct results. The hallmark of the accreditation process is method verification and quality assurance. Before introducing a new method in your laboratory, it is important to assess certain performance characteristics that reflect the concept of method verification.

Methods

In this review, we illustrated how to verify the performance characteristics of a new method according to the recent guidelines. It includes an assessment of precision, trueness, analytical sensitivity, detection limits, analytical specificity, interference, measuring range, linearity, and measurement uncertainty.

Conclusions

Although the presence of several updated guidelines used to determine the performance characteristics of new methods in clinical chemistry laboratories, the real practice raised several concerns with the application of these guidelines which in need for further consideration in the upcoming updates of these guidelines.

Lead and trace element levels in milk and blood of buffaloes (Bubalus bubalis) from Hyderabad, India

The presence of lead (Pb) in milk and its interaction with trace elements is a serious health concern. Present study is aimed at determining Pb and trace element (Fe, Zn and Mg) levels in milk and blood/serum samples of lactating buffaloes (Bubalus bubalis) living in a market-area (Group-A) and a dairy-experimental station (Group-B), Hyderabad, India. In addition, kidney and liver function tests were assessed. Fodder, milk and blood Pb levels were significantly (p < 0.01) higher in Group-B. Elevated Pb levels correlated positively with reduced Fe and Zn levels in both serum and milk. A significant (p < 0.01) positive correlation between blood Pb and milk Pb levels was observed. Kidney and liver function markers were significantly higher in Group-B buffaloes. The results suggest that contaminated fodder might be one of the responsible factors for elevated Pb levels. In addition, lower levels of Fe and Zn might have led to bioaccumulation of Pb in blood and milk.

Category-specific uncertainty modeling in clinical laboratory measurement processes

BACKGROUND

A statement of measurement uncertainty describes the quality of a clinical assay analysis result, and uncertainty models of clinical assays can be used to evaluate and optimize laboratory protocols designed to minimize the measurement uncertainty associated with an assay. In this study, we propose a methodology to lend systematic structure to the uncertainty modeling process.

METHODS

Clinical laboratory assays are typically classified based on the chemical reaction involved, and therefore, based on the assay analysis methodology. We use this fact to demonstrate that uncertainty models for assays within the same category are structurally identical in all respects except for the values of certain model parameters. This is accomplished by building uncertainty models for assays belonging to two categories--substrate assays based on optical absorbance analysis of endpoint reactions, and ion selective electrode (ISE) assays based on potentiometric measurements of electromotive force.

RESULTS

Uncertainty models for the substrate assays and the ISE assays are built, and for each category, a general mathematical framework for the uncertainty model is developed. The parameters of the general framework that vary from assay to assay for each category are identified and listed.

CONCLUSIONS

Estimates of measurement uncertainty from the models were compared with estimates of uncertainty from quality control data from the clinical laboratory. We demonstrate that building a general modeling framework for each assay category and plugging in parameter values for each assay is sufficient to generate uncertainty models for an assay within a given category.

Validation and verification of measurement methods in clinical chemistry

The present overview of validation and verification procedures in clinical chemistry focuses on the use of harmonized concepts and nomenclature, fitness-for-purpose evaluations and procedures for minimizing overall measurement and diagnostic uncertainty. The need for mutually accepted validation procedures in all fields of bioanalysis becomes obvious when they implement international accreditation and certification standards or their equivalents. The guide on bioanalytical method validation published by the US FDA in 2001 represents a sensible compromise between thoroughness and cost-effectiveness. Lacking comprehensive international agreements in the field, this document has also been successfully adapted in other fields of bioanalysis. European and international efforts aiming for consensus in the entire field of bioanalysis are currently being made. Manufacturers of highly automated in vitro diagnostic methods provide the majority of measurement methods used in unmodified in clinical chemistry. Validated by the manufacturers for their intended use and fitness-for-purpose, they need to be verified in the circumstances of the end-users. As yet, there is unfortunately no general agreement on the extent of the verification procedures needed.

Estimation of the measurement uncertainty of methamphetamine and amphetamine in hair analysis

The measurement uncertainties (MUs) were estimated for the determination of methamphetamine (MA) and its main metabolite, amphetamine (AP) at the low concentrations (around the cut-off value of MA) in human hair according to the recommendations of the EURACHEM/CITAC Guide and "Guide to the expression of uncertainty in measurement (GUM)". MA and AP were extracted by agitating hair with 1% HCl in methanol, followed by derivatization and quantification using GC-MS. The major components contributing to their uncertainties were the amount of MA or AP in the test sample, the weight of the test sample and the method precision, based on the equation to calculate the mesurand from intermediate values. Consequently, the concentrations of MA and AP in the hair sample with their expanded uncertainties were 0.66+/-0.05 and 1.01+/-0.06 ng/mg, respectively, which were acceptable to support the successful application of the analytical method. The method precision and the weight of the hair sample gave the largest contribution to the overall combined uncertainties of MA and AP, for each.