Evaluation of automated immunoassays for 25(OH)-vitamin D determination in different critical populations before and after standardization of the assays

• 25(OH)D results obtained with different immunoassays are not comparable.
• A standardization program is ongoing that will allow a better comparability of these assays.
• In special populations, this comparability will not be achieved.
• For these patients, alternative methods, like LC–MS/MS methods or some immunoassays only can be used.

IntroductionStandardization of immunoassays for 25(OH)-vitamin D determination is a major problem in clinical practice. A worldwide standardization program has started to address this and will reduce the bias observed between immunoassays. We aimed to calibrate 5 immunoassays on a LC–MS/MS traceable to the SRM 2972 and the ID-LC–MS/MS 25(OH)D Reference Method Procedure to see if the re-standardization would be efficient in a population of 3rd trimester pregnant women (PW), hemodialysis (HD) and osteoporosis (OP) patient.Material and methods184 serum samples (25(OH)D: 8.4–87 ng/ml) were selected to calibrate the immunoassays (Abbott-Architect, Roche-Elecsys, DiaSorin-Liaison, Siemens-Centaur and IDS-iSYS). Chromsystems MassChrom method was used as the referenced. Serum obtained in 34 PW, 25 HD and 34 OP patients were used as comparatives.ResultsAfter adjusting to LC–MS/MS, immunoassays had regression slopes nearly identical to 1.0 with intercepts < 0.5 ng/ml. However, in special populations, a systematic bias was still observed, except for iSYS.ConclusionsRe-standardization of 25(OH)D immunoassay will globally improve the differences. However, patients with a different serum matrix will still present significantly different results when they will be run with different methods. For those patients, the LC–MS/MS method seems to be the method of choice, even if some immunoassays are less influenced than others.