Calcium, Magnesium, Albumin, and Total Protein Measurement in Serum as Assessed with 20 Fresh-Frozen Single-Donation Sera

To the Editor:The scope of external quality assessment (EQA)1 in laboratory medicine has evolved considerably (1). With the increasing worldwide interest in the use of common reference intervals and/or medical-decision limits, modern EQA schemes need to be better at assessing the standardization status of commercial in vitro diagnostic tests. This need has led to new challenges in the design of EQA surveys.We report the outcomes of a Norwegian pilot study that investigated the use of commutable, fresh-frozen single donations to assess the current standardization status as part of an initiative toward producing common reference intervals (2). The study covered measurements of calcium, magnesium, albumin, and total protein in serum from 20 single-donation blood samples obtained from Solomon Park Research Laboratories. Serum was generated according to CLSI protocol C37-A, with 2 U human thrombin (Sigma-Aldrich) added per milliliter of plasma to facilitate clotting (3); filtration was not used. Aliquots of these samples were sent to laboratories that used the same test systems (instrument, reagent, and calibrator from the same source). Five peer groups (n ≥ 6 laboratories each; N = 47) were considered: Abbott Diagnostics ARCHITECT, Ortho Clinical Diagnostics VITROS, …