Although serum 25(OH)D concentrations are often used to characterize vitamin D levels in population-based studies, the extent to which a single measurement is representative of long-term vitamin D status is unclear. The findings of this study suggest that measurements of 25(OH)D from serum collected at the same time of year are reasonably stable in healthy subjects over a five-year period. We observed relatively low within-subject variability and fairly high correlations in 25(OH)D measured from samples collected at study baseline, after 1 year, and after 5 years.
To evaluate the impact of within-person variability on relative risk estimates in studies of vitamin D status and cancer risk, we used the following formula derived from Rosner et al. (16
) to estimate the degree of relative risk attenuation:
Given the ICC of 0.71 that was observed in this study, we estimated that for true relative risks of 1.5, 2.0, and 2.5, we would expect observed relative risks of 1.3, 1.6, and 1.9, respectively. Investigators should consider this attenuation when designing studies of various diseases in relation to long-term vitamin D status to ensure that they have adequate statistical power.
If possible, investigators might also consider taking multiple 25(OH)D measurements from each subject at different times to reduce misclassification of vitamin D status due to within-person variability over time (17
), although this may not be feasible in many studies. In this study we observed a higher correlation when the average of baseline and +1yr measurements was compared to 25(OH)D concentration at +5yr. The average of two measurements taken one year apart may thus provide a better assessment of long-term vitamin D status than a single measurement, although the increase in agreement observed in this study was relatively small.
Seasonal variation in 25(OH)D concentration has been well-described (18
). However, to our knowledge, relatively few studies have evaluated temporal variability beyond seasonal effects. Platz et al. (11
) measured 25(OH)D concentrations in two blood samples obtained an average of 3.0 years apart from 144 cancer-free men in the Health Professionals Follow-up Study. They reported a Pearson correlation coefficient of 0.70, which is consistent with the level of agreement observed in this study. Rejnmark et al. (20
) evaluated variability between serial measurements using the same assay at three time points over a four-year period among 187 postmenopausal women in the Danish Osteoporosis Prevention Study; median within-person CVs ranged from 13% to 19%.
An important strength of the present study is the use of serum from PLCO blood specimens collected up to five years apart, at the same time each year, which provided a unique opportunity to control for seasonal variation in 25(OH)D while assessing variability over a long period. However, the five-year range in serial specimens limits the generalizability of our findings, in that the level of agreement of a single measure with 25(OH)D levels over longer time periods remains unclear. It is worth noting that a five-year time window may be biologically relevant to the relationship between vitamin D and cancer, in that vitamin D has been postulated to exert effects which influence late-stage carcinogenesis, metastasis and survival for some cancers (1
). This study is also limited by its small size, although the 95% confidence intervals surrounding the CV and ICC estimates still support the inference of low within-person variability and high agreement in 25(OH)D across the time points.
In conclusion, the findings from this study suggest that serum 25(OH)D concentration at a single point in time may be a useful biomarker of season-specific vitamin D status over a five-year period. Further studies are needed to evaluate the extent of within-person variability of 25(OH)D over a longer time.