Analytical quality goals for 25-vitamin D based on biological variation

BACKGROUND: Measurement of 25-hydroxyvitamin D, (25D) is central in the investigation of pathologies of bone and mineral ion metabolism and in determining a patient's vitamin D status. More recently much research interest has lead to investigating the role it can play in decreasing the risk of many chronic illnesses, including common cancers, autoimmune diseases, infectious diseases, and cardiovascular disease. Knowledge of the biological variation of an analyte forms an essential part of evaluating a new analyte enabling the objective assessment of the changes in serial results, the utility of reference intervals as well as establishing laboratory quality specifications.

METHODS: This study determined the biological variation of 25D in 20 healthy individuals that was calculated according to the familiar methods outlined by Fraser and Harris.

RESULTS: The within-subject variation was 12.1% and the between subject variation was 40.3%. The critical difference for sequential values significant at P<0.05 was calculated as 38.4%. The within-subject variation forms a relatively small part of the reference interval shown by the low index of individuality of 0.3. Objective analytical quality goals have also been established which have shown achievable minimum performance for imprecision of ∼6%. The desirable analytical bias goal was ∼10%.

CONCLUSION: This study has objectively shown that the analytical precision of current instruments is being achieved contrary to the known problems surrounding the analytical bias for 25D assays. The limitations of using reference intervals for 25D, both in diagnoses and monitoring are shown.