The longitudinal variability of serum 25(OH)D levels.

Background: The extent to which a single serum 25(OH)D measurement represents long-term vitamin D status remains unclear. This study aims to assess the variability of serum 25(OH)D between tests taken at different time intervals.

Methods: Using the computerized database of the largest healthcare provider in Israel, we identified subjects in whom a serum 25(OH)D test was performed on at least two different occasions between January 2008 and September 2011 (n = 188,771). For these subjects we selected the first and the last dated tests, then we identified those who were not treated with supplements during the last 6 months before the first and before the last test (n = 94,418). Of these we analyzed subjects in whom the first and the last tests were performed in the same month of the year (n = 8881).

Results: The mean serum 25(OH)D level at the first test was 51.7 ± 24.0 nmol/L and was 56.7 ± 24.7 at the last test (P<0.001); the overall correlation was 0.63 (P < 0.001). For vitamin D status in two categories (<50 versus ≥ 50 nmol/L), the percentage of agreement between the first and last tests was 74.4%, and was 50.8% for vitamin D status in four categories (<30, 30-49.9, 50-74.9, and ≥ 75 nmol/L). The correlation decreased with increasing time between the tests ranging from 0.83 for tests done at the same year to 0.55 after 3 years. The more the first levels were higher or lower, the more likely subjects remain in their first category (≥ 50 versus <50 nmol/L).

Conclusions: Long-term month specific serum 25(OH)D levels are relatively stable.