**5. Challenges for estimation of serum vitamin D level**

The estimation of calcitriol is very challenging as calcitriol (1,25(OH)2 vitamin D) has very short t1/2 and thus does not reflect long-term vitamin D status. Also, it is observed that the total 25(OH) vitamin D is the most reliable marker for vitamin D

status which measures both vitamin D-binding protein (DBP)-bound and free 25(OH) vitamin D. In the light of widespread variation in measured results and divergent results in response to vitamin D supplementation, there is a need for a distinct method for estimation of vitamin D levels. Further, free 25(OH) vitamin D levels may vary according to genotype and single nucleotide polymorphisms (SNPs) in the DBP gene for which the assays are still not well established [5].
