The commutability of external quality assurance serum and reference serum materials between the VITROS® Albumin Slide and four other routine albumin methods

Background Between-method differences in external quality assurance schemes (EQAS) are assumed to be indicative of between-method differences in results for fresh diagnostic serum samples. However, this assumption is only valid if the EQAS fluid is shown to be commutable between methods with the fresh human samples. Using fresh human serum samples, we studied the commutability of six UK National EQAS (UKNEQAS) samples and two reference serum preparations using five methods for the measurement of albumin.Methods The VITROS ® Albumin Slide was compared with two liquid reagent bromocresol green-based methods and two immunoturbidimetric methods. Thirty fresh human serum samples, six UKNEQAS samples and two reference materials were analysed over a period of five days by all methods. The reference materials were: a certified protein reference material, CRM470 (protein reference preparation) and the Scandinavian NFKK Reference Serum X.Results A concentration-dependent tendency towards non-commutability was seen for the six UKNEQAS samples. Two of the methods recovered the CRM470 above the upper limit of the expected range and the remaining three were within the expected range.Conclusions Assessment of commutability is important in the investigation of between-method differences in EQAS. External quality assurance and reference materials should have been demonstrated to be commutable and be available at multiple concentrations, before they may be considered appropriate for the assessment of accuracy.