Best form of Diagnostic test to detect Vitamin D deficiency

Here is how the Vitamin D is formed in the body

In the skin, UV rays help metabolise 7-dehydrocholesterol to cholecalciferol, which then by 1-alpha hydroxylase, forms 25-hydroxyvitamin D in the liver and 1,25-dihydroxyvitamin D in the kidney.

The most accurate form of Vitamin D which is tested to guage vitamin D deficiency in patients, is the 25-Hydroxyvitamin D

A level of less than 20 ng/ml is considered deficiency