Name of the Test :    Bioavailable  Vitamin D (25 hydroxy)

Alias Names  :  Weakly bound Vitamin D, Albumin bound Vitamin D, Bioactive Vitamin D for mineral metabolism

Bioavailable Vitamin D determination is useful when :

–  There is a discrepancy with total vitamin D ( 25 OH ) with calcium & PTH levels(i.e) Renal dialysis patients

–  There is discrepancy in Bone mineral density with total Vitamin D ( 25 OH ) concentration

–   There is a increased level of  Vitamin D binding protein as in pregnancy 

Clinical applications:

Vitamin D deficiency is determined by measuring circulating 25 hydroxy Vitamin D (25(OH) D).Over 85 % of circulating 25(OH) D is tightly bound to a specific vitamin D binding protein (DBP). A lesser amount is bound loosely with albumin. Less than 1% is free Vitamin D (Free D). DBP levels are high during pregnancy and individuals with estrogen treatment. The free fraction along with the albumin bound fraction, called Bioavailable Vitamin D, is readily available for metabolic function.

Recent studies indicate that bioavailable, and not total 25(OH) D, have increased correlation with serum calcium. There has been poor correlation of 25(OH) D levels with bone mineral density. However, the correlation between Bioavailable Vitamin D and bone mineral density has been reported to be a positive correlation.  Similarly, measurement of Bioavailable D in hemodialysis patients has shown a better correlation in terms of mineral metabolism and PTH levels compared to total Vitamin D. It is therefore important to measure Bioavailable D in clinical conditions associated with potential mineral metabolic changes.

Method :

Bioavailable 25(OH) D is vitamin D (25 OH) not bound to DBP. To obtain the bioavailable fraction, total vitamin D, DBP,  and albumin were quantitated. Using an affinity constant of 25(OH)D for DBP and albumin, Bioavailable Vitamin D is calculated. Reported results include the concentration of Bioavailable Vitamin D , Calculated Free Vitamin D , Vitamin D binding protein and Total Vitamin D (25 OH)

Specimen requirements: 1.0 ml (minimum 0.3 ml) of serum or Plasma.

Shipping requirements: Ship the samples frozen or in ice pack. Shipping samples at room temperature is not recommended.

Turnaround time:  1 day (Set up every Tuesday)

Reference Range (Adults):

Bioavailable Vitamin D              : 1.92 – 8.82     ng / ml

Vitamin D Binding protein        : 104 –  477       ug / ml

Calculated free Vitamin D          :     5 – 18         pg / ml

Total Vitamin D (25 Hydroxy)  :       15 – 60      ng / ml

 

References:

  1. Bhan I et al ( 2012 ) Bioavailable vitamin D is more tightly linked to mineral metabolism than total vitamin  D in incident hemodialysis patients .Kidney Internl  82 : 84 – 89 .
  2. Powe CE et al (2011) Vitamin D-Binding protein modifies the VitaminD – Bone mineral density relationship.J of Bone Mineral  Research 26: 1609 – 1616 .
  3. Powe CE et al (2010) First trimester Vitamin D , Vitamin D binding protein and subsequent Preeclampsia . Hypertension 56: 758 – 763.

Leave a Reply

Your email address will not be published. Required fields are marked *