On March 19th, 2018, Hocher et al. published a paper entitled "Reference intervals for measured and calculated free 25-hydroxyvitamin D in normal pregnancy." showing that Free 25(OH) Vitamin D may better reflect the 25OH Vitamin D status in normal pregnancy.

This is the result of the largest study comparing free- and total 25(OH) Vitamin D during pregnancy. The following differences were found between both measurements:

• Free 25(OH)D significantly lowers in the 3rd trimester comparing to the 1st trimester, whereas total 25(OH)D was not decreased in late pregnancy.

• Free 25(OH)D was significantly correlated with serum calcium, whereas total 25(OH)D was not.

• Free 25(OH)D better correlated with serum bone-specific alkaline phosphatase than total 25(OH)D.

The group established pregnancy trimester-specific reference intervals for free 25(OH)D, and claimed that the measurement of free 25(OH)D in pregnant women is suitable as a novel laboratory parameter for vitamin D status monitoring during human pregnancy and might replace routine total 25(OH)D assessment in the future.

More and more studies support the importance of free 25(OH) Vitamin D, suggesting that this fraction of 25(OH) Vitamin D may be a better measurement of Vitamin D status rather than the total 25(OH) Vitamin D in pregnancy, renal disease, liver disease and intensive care units.

More information: Free 25(OH) Vitamin D ELISA