In the latest Institute of Medicine report, experts concluded that <12 ng/ml is indication of deficiency and 20 ng/ml is the optimum level for most North Americans. This report didn’t find any negative correlation with the PTH which was earlier assumed to be the basis of determining optimum Vitamin D for sufficiency.
In contrast, labs have been raising the cut-off for deficiency levels every year and some labs may have flagged results between 20-30 ng/ml to be low. How quickly will/should labs change their reference ranges? Should labs revise/recall their reports since lab results may have triggered treatment in thousands of patients?
When should the test be utilized?
Should clinical labs even offer or charge for 25-OH-vitamin D for screening vitamin D deficiency when there is no consensus on the optimum value or reference ranges?
There is probably no need to have this test done or ordered in healthy individuals. However, vitamin D should always be ordered and interpreted in the context of disease (osteomalacia, secondary hyperparathyroidism) and interpreted in relation to markers like PTH and renal function. Vitamin D is part of the endocrine system and levels of active vitamin D (1,25 dihydroxy vitamin D) are influenced by the circulating calcium levels. Calcium intake influences PTH which further regulates the conversion of vitamin D to 1,25 dihydroxy vitamin D.
What is the value to patient care?
Is it cost effective to screen populations for determining vitamin D deficiency when the treatment could be a change in life style (reasonable sun exposure) or inexpensive Vitamin D supplementation? Despite the performance of millions of tests in the last five years, I am not sure if we have added any value to the outcomes for patient care in any disease.
What about test performance?
What should the coefficient of variation (CV) of the 25-OH-vitamin D test be at LOQ? Calcium, like most automated assays performed in various labs, has CVs down in the 1-2% range. Other testing related to calcium metabolism, like PTH, have precision performance that is better than 25-OH-Vitamin D tests (10-20%) as observed in proficiency testing data.
Are we measuring, and should we be reporting both the D3 and D2 forms?
Like Vitamin D3 (Cholecaliferol), Vitamin D2 (Ergocalciferol) is metabolized to 25-OH-vitamin D2. Assays should quantify these two forms equally, but do we know if this is true for various assays used clinically? Immunoassays may not measure D2 at all, or may measure it at 100%. The HPLC and LC-MS/MS assays, by design, separate and quantify the two vitamin forms, yet 25-OH-Vitanmin D is always reported as total by adding the two together, rather than reporting them individually.