A study to examine the accuracy of potassium measurements in clinical laboratories across Canada

OBJECTIVE

To assess the accuracy of potassium measurements in clinical laboratories across Canada.

DESIGN AND METHOD

The flame atomic emission spectrophotometry reference method for the determination of potassium was established at the Canadian Reference Laboratory by using National Institute of Science and Technology standard reference materials. The method was subsequently used to assign target values for potassium to Canadian Reference Laboratory's External Quality Assessment human-serum-based testing material. A total of 503 laboratories participated and 9,279 individual External Quality Assessment test results were included in the study. Bias was determined by using difference plots.

RESULTS

Clinically significant bias (>1.6%) was observed in 45.9% of the laboratories. Bias ranged from 0.34 mmol/L to -0.54 mmol/L. At low concentrations (<3.5 mmol/L) a positive bias was most frequently observed (14.7% of analytical systems). At high potassium concentrations (>5.1 mmol/L) a negative bias was most frequently observed (31.4% of analytical systems).

CONCLUSION

Inaccuracy in potassium results can contribute to test redundancy and mismanagement of patients, while prohibiting the merger of laboratory data from disparate testing sites for the purpose of trending and consolidation within a "universal health record." Inaccurate test results and the lack of standardization among laboratories adversely impact our ability to establish common reference intervals and critical limits. This inability has an adverse effect on medical decisions and patient care.

A reference system for cortisol

OBJECTIVES

The International Federation of Clinical Chemistry (IFCC) initiated a pilot study, with cortisol as an example, that aims to implement the concept of standardization of hapten immunoprocedures on the basis of metrological traceability. In fact, this standardization concept comes down to correct calibration of measurement procedures, so that measurement results for patient samples can be traced back to the metrologically highest reference of a result, i.e., an SI unit as embodied in the primary reference material. In consequence, demonstration of standardization of a test system on the basis of traceability requires evaluation of measurement results pertaining to patient samples for accuracy. Such an evaluation shall be done by a correlation study of the routine test system with an accuracy-based reference measurement procedure.

DESIGN AND METHODS

The IFCC project will select a panel of single donation human serum samples, and assign them with values for cortisol by measurement with at least two isotope dilution-gas chromatography/mass spectrometry reference methods. Limited amounts of the panel will be distributed to manufacturers, with the object to measure the samples with their calibrated immunoprocedures. The patient correlation studies between the routine and the reference methods will then be interpreted in terms of specificity and accuracy. It will also be investigated whether the performed comparison can be used as a basis for re-calibration. From the experience gained through this pilot study for cortisol, IFCC will formulate recommendations and guidance to manufacturers on how to perform and reliably interpret patient correlation studies. In this way, it is to be expected that the project might form a basis for general implementation of the concept of standardization of hapten immunoassays on the basis of metrological traceability.

Creatinine measurement proficiency testing: assignment of matrix-adjusted ID GC-MS target values

The results of an external quality-assessment experiment for serum creatinine measurement are described. Fifty-one laboratories performed quintuplicate analyses during three different analytical runs on six lyophilized sera and two frozen human serum pools. Isotope dilution gas chromatography–mass spectrometry (ID GC-MS) target values were assigned to all the materials. Intralaboratory within- and between-run imprecision results were very similar for all the materials tested (CV ≤2.20% and ≤4.70%, respectively). The overall imprecision obtained was high (CV 6.5–20.0%) because of increased interlaboratory–intermethod variability. A significant positive bias (+9.2–+43.7%) was found for all the materials at lower creatinine concentration. By using two human sera at different concentrations, we could calculate the constant and the proportional calibration bias displayed by each peer group. The majority of the lyophilized materials showed a behavior divergent from the frozen pools, indicating matrix-related problems. We propose a new algorithm for calculating matrix bias correction factor instrument–reagent specific for each material.

Serum iron and proficiency testing: a dilemma

We present and discuss observations regarding the results of the serum iron determination as seen in various surveys. The reference method proved to score different in our hands judging on the performance in the Dutch and German quality assessment schemes. Furthermore, it is clear and frustrating that these scores, representing the most accurate results, are becoming more and more labeled "out of acceptable range".