Performance of electrolyte assays in the United Kingdom - reference target values

Wales External Quality Assessment Scheme (WEQAS) is one of the largest external quality assessment (EQA) providers in the United Kingdom, with over 600 participants. Over the last two and a half years, reference target values have been used for the electrolyte panel in the WEQAS General Chemistry scheme. Reference methods have a vital role in that they ensure the transfer of accuracy from definitive methods to routine methods. Validated reference methods, using flame emission/atomic absorption spectrometry were established for sodium, potassium, calcium, magnesium and lithium. These methods were validated by direct comparison with the definitive method via the use of primary reference material. Comparison against the all laboratory trimmed mean (ALTM) showed a small mixed systematic error for sodium and potassium, with a maximum bias from the reference value of 1% and 2%, respectively. Lithium and magnesium data showed predominant systematic constant errors of +0.03 and +0.02 mmol/l, respectively. For calcium, ALTM results showed a mixed systematic error with a cross over in the reference range (5% positive bias and constant error of -0.1 mmol/l). Over 40% of participants used the cresolpthalein complexone method, greatly influencing the ALTM value, (11% bias, constant error -0.26 mmol/l). Other methods such as the ion selective electrode (ISE) method were in closer agreement to the reference method (-4% bias, +0.05 mmol/l constant error). The study highlights the pitfalls of using the overall mean data as an accuracy target in EQA schemes.