Name of the Test:   “Bioavailable” Vitamin D (1, 25 dihydroxy)

Alias Names :  Weakly bound Vitamin D 1,25 ; Albumin bound Vitamin D 1, 25 dihydroxy(1,25 di OH)

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 an increased level of  Vitamin D binding protein as in pregnancy  

Clinical applications:

Vitamin D is produced in the skin. Exogenous Vitamin D is absorbed in the intestine and converted to 25 hydroxy vitamin D. 25 hydroxy(OH)vitamin D is converted into bioactive vitamin D (1, 25 dihydroxyl vitamin D) in the kidney .Hence, patients with chronic renal failures results in vitamin D deficiency. Over 85 % of circulating 1, 25 dihydroxy D is tightly bound to a specific vitamin D binding protein (DBP). A lesser amount is bound loosely with albumin. Less than 1% is the free form of Vitamin D 1, 25 dihydroxy. DBP levels are high during pregnancy and individuals with estrogen treatment. The free fraction along with the albumin bound fraction, called bioavailable Vitamin D (1, 25 dihydroxy), which is readily available for metabolic function.

Recent studies indicate that bioavailable, and not total 1, 25 dihydroxy D, have increased correlation with serum calcium.

Similarly, measurement of bioavailable D (1,25 dihydroxy) 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, especially in renal dialysis patients.

Method :

Bioavailable 1, 25 dihydroxy is vitamin D (1, 25 dihydroxy), not bound to DBP. To obtain the bioavailable fraction, total vitamin D  1,25 dihydroxy, DBP, and albumin were quantitated. Using an affinity constant of 1, 25 dihydroxy D for DBP and albumin, Bioavailable Vitamin D( 1,25 dihydroxy is calculated. Reported results include the concentration of bioavailable Vitamin D (1, 25 dihydroxy), Calculated Free Vitamin D (1, 25 dihydroxy), Vitamin D binding protein and Total Vitamin D (1, 25 dihydroxy) .

Test Information:

Specimen requirements: 3.0 ml of serum or Plasma (minimum 0.6 ml).

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

Turnaround time:  2 days (Set up every Tuesday)

Reference Range (Adults):     Bioavailable Vitamin D (1, 25 dihydroxy)             :      3.4 – 14.2       pg / ml

Vitamin D Binding protein                                      :     104  –  477       ug / ml

Calculated free Vitamin D (1, 25 dihydroxy)        :      0.1 – 0.44      pg /ml

Total Vitamin D (1, 25 dihydroxy)                          :         15 – 80         pg / ml



  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. Bikle DD, Siliteri PK, Ryzen E et al (1985) Serum protein binding of 1,25-dihydroxy vitamin D: a reevaluation by direct measurement of free metabolite levels. J Clin Endocrinol. Metab 61:969-975.
  3. Brown AJ, Coyne DW (2012) Bioavailable vitamin D in chronic kidney disease. Kidney Internl 82:5-7.

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