Name of the Test: Free Vitamin D (25 Hydroxy)

Alias Names : Unbound Vitamin D , Calculated Free Vitamin D

Free Vitamin D determination may be useful when,

– There is a discrepancy with total vitamin D ( 25 OH ) with calcium & PTH levels(e.g) Renal dialysis patients
– There is discrepancy in Bone mineral density with total Vitamin D ( 25 OH ) concentration
– There is an increased level of Vitamin D binding protein as in pregnancy
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 or Free, and not total 25(OH) D, correlate with serum calcium. The correlation is poor between 25(OH) D levels with bone mineral density. On the other hand, the correlation between Bioavailable Vitamin D and bone mineral density is better. Similarly, measurement of Bioavailable D in hemodialysis patients show a better correlation in terms of mineral metabolism and PTH levels than total Vitamin D measurement. It is therefore important to measure free and Bioavailable D in clinical conditions associated with potential mineral metabolic changes.

Methodology:
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 affinity constant of 25(OH)D for DBP and albumin, Bioavailable Vitamin D and Free Vitamin D are calculated. Reported results include the concentration of calculated Free Vitamin D ,Bioavailable Vitamin D, Vitamin D binding protein and Total Vitamin D (25 OH).

Test request information:
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 week
Reference Range (Adults): Free Vitamin D ( Calculated) : 5.0 – 18.0 pg/ml
Bioavailable Vitamin D : 1.92 – 8.82 ng / ml
Vitamin D Binding protein : 104 – 477 ug / ml
Total Vitamin D (25 Hydroxy) : 30 – 80 ng / ml

References:
1. Powe CE et al (2011) Vitamin D-Binding protein modifies the Vitamin D – Bone mineral density relationship.J of Bone Mineral Research
26 : 1609 – 1616 .
2. Powe CE et al (2010) First trimester Vitamin D , Vitamin D binding protein and subsequent Preeclampsia . Hypertension 56: 758 – 763.
3. Pandian R , Pandian J , Elias AN ( 2013)Clinical importance of “Bioavailable” vitamin D:Development and
analytical validation of bioavailable 25 hydroxy vitamin D assay . Amer Assoc ClinChem meeting Abs # A342 .
4. Powe CE et al ( 2013) Vitamin D- Binding protein and Vitamin D status of Black Americans and White Americans.
New England J of Medicine 369 : 1991 – 2000.
5. Johnsen MS et al ( 2014 ) Serum free and Bioavailable 25-hydroxy Vitamin D correlate better with bone density than serum total 25-
hydroxy vitamin D .Scand J Clin and Laboratory Medicine Early on line 1- 7 .

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