What is the best test to diagnose vitamin D deficiency?

Serum 25(OH)D.

The circulating half-life of 25(OH)D is 2 weeks. This is the best test to determine vitamin D status.

A 25(OH)D level of less than 32 ng/mL is considered vitamin D insufficient because intestinal calcium absorption is optimized at levels above 32 ng/mL.

A 25(OH)D level of less than 15 or 20 ng/mL have been used to define vitamin D deficiency.

Parathyroid hormone levels start to rise at 25(OH)D levels below 31 ng/mL, which is another marker of vitamin D insufficiency. Although not always required for the diagnosis of vitamin D insufficiency, a serum PTH may be used to help establish the diagnosis of vitamin D insufficiency. Vitamin D is a steroid hormone and a component of a complex endocrine pathway sometimes called 'vitamin D endocrine system' (Medscape, 2012).

The word vitamin was originally derived from Funk's term "vital amine."

Calcifediol, also known as calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D (abbreviated 25(OH)D), is a prehormone which is produced by hydroxylation of vitamin D3 (cholecalciferol) in the liver.


Calcidiol.

Calcidiol is then converted in the kidneys into calcitriol (1,25-(OH)2D3), that is the active form of vitamin D.


Calcitriol, 1,25-dihydroxycholecalciferol.

Calcidiol can also be converted into 24-hydroxycalcidiol in the kidneys via 24-hydroxylation.

References:

Vitamin D Deficiency and Related Disorders: Differential Diagnoses & Workup. eMedicine.
Vitamin D -- Elixir of Life?
A vitamin D3 dosage of 800 IU/d increased serum 25-(OH)D levels to greater than 50 nmol/L in 97.5% of women http://bit.ly/GzBCcA
Vit. D may play a role in your allergies and want to get tested? Cost is no longer covered in ON, Canada - see why: http://goo.gl/1eyW4
Calcifediol, from Wikipedia, the free encyclopedia.
Image sources: Wikipedia, public domain.