Repeated Measurements of Vitamin D: Common and Very Unlikely to Indicate Deficiency

Abstract

Objective

Vitamin D testing uses considerable resources and has increased recently. We investigated repeating testing in Auckland, New Zealand.

Design, Measurement and Patients

We analysed repeated 25-hydroxyvitamin D (25OHD) tests in three databases from the Auckland region from 2005 to 2024 (362,618 tests). We assessed annual numbers, time between tests, characteristics associated with repeat tests, and relationships between requestors and repeated tests.

Results

Twenty-four percent of individuals had ≥ 2 25OHD tests. 34% of all 25OHD tests were repeats. A disproportionately high number of repeat tests were done in a small number of individuals: 57% had one repeated test (i.e., two tests total) but 5.3% had ≥ 6 and 1.4% had ≥ 11 repeated tests. Fifty eight percentage of repeat tests were done within 1 y and 77% within 2 y. 85% of people with repeated tests had a first 25OHD > 25 nmol/L, but in only 9.8% a repeated 25OHD was ≤ 25 nmol/L. Most repeat tests (77%) when the first 25OHD was > 75 nmol/L were > 75 nmol/L A small number of high-volume requestors ordered most tests: 3/7637 ordered > 5000 tests, collectively 7% (21,092) of the total, and 98 ordered > 500 tests (collectively 44%). The most frequent requestors were primary care doctors and bariatric surgeons. Simple restrictions based on the result and time since previous 25OHD could reduce repeated tests by > 60%–90% with only 4%–5% of 'avoided' tests ≤ 25 nmol/L.

Conclusion

Repeated vitamin D testing is common, and most results are not low. Restricting test availability and targeting those at risk of clinical complications of vitamin D deficiency are likely to reduce unnecessary tests.