Intrinsic and routine quality of serum total potassium measurement as investigated by split-sample measurement with an ion chromatography candidate reference method.

We evaluated the intrinsic quality of eight routine test systems for the measurement of serum total potassium (K+), as well as the routine quality of four of these systems, using a group of 60 single-donation serum samples that had been certified with an ion chromatography reference method. The intrinsic quality of the tests was evaluated by analysis of the sera in the manufacturers' application laboratories under strict internal quality control. The routine quality was evaluated by analysis of the same sera in five (per system) routine laboratories under daily working conditions. The results of the study were interpreted in light of the most stringent specifications derived from the biological variation of K+, which require limits of 6.3% for total error and 1.6% for systematic error. The study revealed that the intrinsic quality of all systems was excellent. None of the test systems yielded a substantial number of results outside the 6.3% total error limit, and only one test system exceeded the 1.6% systematic error limit. The majority of the routine laboratories reproduced the manufacturers' intrinsic quality. In particular, most laboratories satisfied the 6.3% total error limit. However, several laboratories exceeded the 1.6% systematic error limit. Generally, there was a considerable difference in quality between the participating laboratories. This showed that the major problems for serum K+ analysis (for samples with no unusual matrices and with concentrations within the reference interval) are at the routine laboratory.