The longitudinal variability of serum 25(OH)D levels

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.

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).

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).

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