Tests
1st tests to order
serum 25-hydroxyvitamin D
Test
25-hydroxyvitamin D is the major circulating form of vitamin D used to determine vitamin D status.
Testing is recommended for those with symptoms of deficiency, as well as for those with an established clinical indication for vitamin D testing; for example, hypocalcemia, osteomalacia, or following a fall.[4][7][81] Furthermore, some clinicians also measure serum 25-hydroxyvitamin D in select people at increased risk for vitamin D deficiency.[81]
Testing may also help to rule out other potential causes for vitamin D deficiency, such as syndromes due to acquired or inherited disorders of vitamin D metabolism or phosphorus metabolism.
Result
vitamin D deficiency: ≤20 nanograms/mL; vitamin D insufficiency: between 21 and 29 nanograms/mL (note that definitions vary)
serum calcium
Test
Usually normal (unless longstanding vitamin D and calcium deficiency) due to secondary hyperparathyroidism, which mobilizes calcium from the skeleton and conserves calcium in the kidney to maintain serum calcium within normal range.
Result
normal; may be decreased with inherited disorders of vitamin D metabolism such as pseudovitamin D-deficiency rickets and vitamin D-resistant rickets, severe calcium and vitamin D deficiency, severe calcium deficiency (even when vitamin D sufficiency is present)
fasting serum phosphate
Test
Due to secondary hyperparathyroidism causing phosphaturia. However, if the serum phosphate level is not obtained in the fasting state, it provides little information about the actual phosphate status of the patient.
Result
low-normal or rarely low
serum alkaline phosphatase
Test
Elevated levels may indicate a high bone turnover state.
Result
normal or elevated; usually high in rickets and osteomalacia
Tests to consider
plain film radiographs of knees and wrists
Test
Not frequently needed but should be performed when rickets or osteomalacia is suspected.[88] Widening of the epiphyseal plate and loss of definition of the zone of provisional calcification at the epiphyseal/metaphyseal interface are early signs of rickets. [Figure caption and citation for the preceding image starts]: X-rays of a wrist from a child with vitamin D-deficiency rickets before (left panel) and after (right panel) treatment with vitamin DFrom the collection of M.F. Holick, PhD, MD; used with permission [Citation ends].
Looser pseudofractures, which lie perpendicular to the cortical margins with the characteristic sclerotic borders, are a pathognomonic finding in osteomalacia. Classic metaphyseal lesions also known as bucket-handle fractures can be observed at the growth plates of long bones.[95][100][101][102][Figure caption and citation for the preceding image starts]: Radiograph of the femoral shaft in a patient with osteomalacia demonstrating a "pseudofracture" (also known as Looser's zone) on the medial aspect of the mid-femoral shaftFrom the collection of Bridget Sinnott, MD [Citation ends].
Result
cupping, splaying, and fraying of the metaphysis; Looser zone (pseudofracture)
intact PTH
Test
Usually elevated in chronic severe vitamin D deficiency (osteomalacia and rickets). However, in mild vitamin D deficiency and insufficiency, intact PTH levels are usually normal. Therefore, PTH is not a reliable diagnostic marker for vitamin D deficiency in these situations. Furthermore, if the patient is also magnesium-deficient they may be unable to mount a PTH response to vitamin D deficiency, which can result in a normal intact PTH level despite low vitamin D status.
Consequently this test should only be obtained when calcium is elevated or low.[7]
Result
normal or elevated
serum 1,25-dihydroxyvitamin D
Test
Should not be used to determine vitamin D status. However, this measurement is of great value in distinguishing the inherited or acquired disorders of vitamin D metabolism.[82][104]
Patients with pseudovitamin D-deficiency rickets and vitamin D-resistant rickets often have a normal serum level of 25-hydroxyvitamin D. However, 1,25-dihydroxyvitamin D level differs, and is either low or undetectable in pseudovitamin D-deficiency rickets and very elevated in patients with vitamin D-resistant rickets.
Patients with X-linked hypophosphatemic rickets, autosomal-dominant hypophosphatemic rickets, and oncogenic osteomalacia have low levels of serum 1,25-dihydroxyvitamin D and phosphorus. This combination is pathognomonic for these conditions.[7]
Result
normal or elevated
bone density (DEXA) scan
Test
Adults with chronic vitamin D deficiency may have a low bone density due to increased serum PTH, which removes matrix and mineral, causing osteopenia and ultimately osteoporosis. In addition, the mineralization defect of the skeleton caused by vitamin D deficiency can result in a lower bone density.[7][105]
Result
low bone density
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