Commutability of external quality assessment materials for serum magnesium and calcium measurements

The commutability of pooled patient sera (PPS) and control materials (CM) should be evaluated to investigate their suitability for use in an external quality assessment (EQA) program. Individual human samples, PPS and CM were analyzed by four routine methods and inductively coupled plasma mass spectrometry reference methods for magnesium and calcium measurements. The commutability was analyzed according to EP14-A3 protocol and the difference in bias approach, respectively. For magnesium measurements, all PPS were commutable and 3/5 CM were commutable for all measurement systems according to the EP14-A3 protocol. For calcium measurements, most PPS were commutable for all measurement systems, but the CM were only commutable with the Cobas c702 system. The IFCC approach produced similar commutability profiles, except that a large number of inconclusive results appeared. The routine methods exhibited excellent linearity and precision. The majority of relative biases between the routine and reference methods were beyond the bias limits. The commutability of the CM and PPS vary depending on which evaluation approach and criterion is applied. Superiority in the commutability of PPS over CM was observed whichever evaluation approach is applied.