BMC Public Health 2009, 9:22 http://www.biomedcentral.com/1471-2458/9/22 2F5ig( OurHe)D2 plotted against serum PTH concentration 25(OH)D plotted against serum PTH concentration: a) winter, age under 50 years; b) summer, age under 50 years, c) winter, age 50 and above; d) summer, age 50 and above. L in summer, with 2/3 of the population reaching suffi- ciency, the majority of the subjects (around 90%) still stayed below the optimal level of 75 nmol/L. The finding is similar to the results of other studies in the region dem- onstrating that the summer build-up of 25(OH)D is inad- equate in northern countries [26]. For overview purposes the currently available vitamin D status data in similar populations performed to date is given in Table 3. Due to the inter-assay and also inter-lab- oratory variability of different vitamin D measurement techniques some caution is advocated in interpretation [34]. At 44 nmol/L the winter mean 25(OH)D concentra- tion in Estonia is comparable with levels in several south- Page 5 of 8 (page number not for citation purposes)