What is being tested?
There are two forms of vitamin D that can be measured in the blood - 25 hydroxy-vitamin D and 1,25 dihydroxy-vitamin D.  25 hydroxy-vitamin D is the major form of the hormone found in the blood and is the inactive component from which the active hormone, 1,25 dihyroxy vitamin D is made.  Because 25 hydroxy-vitamin D stays for a long time in the blood and is at a higher concentration than 1,25 dihydroxy-vitamin D, 25 hydroxy-vitamin D is commonly measured to find out about vitamin D status in individuals. The test for 1,25 dihydroxy-vitamin D is used to find out whether the kidney is converting an appropriate amount of inactive 25-hydroxy-vitamin D to the active 1,25 dihydroxy-vitamin D.

Vitamin D comes from 2 sources:  a) produced in the skin on exposure to sunlight (endogenous) and b) that present in foods and supplements (exogenous).  The chemical structures of these types of vitamin D are slightly different and are distinguished by the subscripts in their chemical name.  Vitamin D₂ (ergocalciferol) is the form found in foods of animal origin and in most vitamin preparations and supplements while vitamin D₃ (cholecalciferol) is the form produced in the body and is also used in some high dose formulations used to treat severe vitamin D deficiency.

Both D₂ and D₃ vitamin D are converted to the 25-hydroxy vitamin D and then to 1,25 di-hydroxy vitamin D.The main role of vitamin D is to help regulate the absorption of calcium, phosphate, and (to a lesser extent) magnesium. Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in the disease called rickets in children and osteomalacia in adults. Vitamin D has also been implicated in the regulation of autoimmunity.

How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. No test preparation is needed.