25-hydroxyvitamin D (calcidiol)

What it measures

25-hydroxyvitamin D is the standard test for assessing vitamin D status (the major circulating storage form, with a longer half-life than active calcitriol, making it the more reliable status marker) - testing should be targeted to specific at-risk groups rather than performed as routine screening in the general population
Vitamin D undergoes hepatic 25-hydroxylation (producing the stable, measured storage form) then renal 1-alpha-hydroxylation to the active hormone calcitriol (1,25-dihydroxyvitamin D) - 25-hydroxyvitamin D, not calcitriol, is the correct test for assessing overall vitamin D status, since calcitriol levels are tightly regulated and can remain normal or even elevated despite significant vitamin D deficiency (via compensatory secondary hyperparathyroidism driving renal hydroxylase activity)

Current guidance (Endocrine Society, Choosing Wisely) recommends against routine population-level vitamin D testing, instead targeting testing to specific higher-risk groups
Targeted testing in higher-risk groups: older adults, limited sun exposure, darker skin, malabsorption, chronic kidney disease, obesity, osteoporosis, and patients on long-term corticosteroid or anticonvulsant therapy - not as routine testing in asymptomatic, otherwise healthy individuals
Calcitriol testing has a narrow, specific role (e.g. suspected disorders of renal 1-alpha-hydroxylase activity, some granulomatous disease-related hypercalcaemia) rather than routine vitamin D status assessment

Confirmed deficiency in an at-risk individual -> vitamin D repletion (dose/duration matched to severity of deficiency) with calcium co-supplementation where dietary calcium intake is inadequate
Routine testing in low-risk asymptomatic individuals is not recommended and does not change management, given the low pre-test probability of clinically significant deficiency and the low cost/low risk of empirical supplementation in genuinely at-risk groups without testing

Calcitriol's misleading normal/elevated level in true vitamin D deficiency (a frequently tested pitfall), targeted vs universal testing recommendation, granulomatous disease-associated calcitriol-mediated hypercalcaemia (a specific alternative indication for calcitriol testing)
Calcitriol (active 1,25-dihydroxyvitamin D) is not the correct test for assessing vitamin D status - because it is tightly regulated by the body and can appear normal or even elevated despite significant deficiency of the storage form, testing calcitriol instead of 25-hydroxyvitamin D risks a falsely reassuring result and missed diagnosis

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