Study population and study locations
This nationwide study included 33 reference hospital laboratories, where vitamin D measurement is available in Turkey. In total, 21 biggest cities from south to north of country between 36° N -42° N lattitudes with third-level reference hospitals were selected; [Adana (37° N), Ankara (39° N), Antalya (36° N), Aydın (37° N), Diyarbakir (37° N), Eskisehir (39° N), Erzurum (39° N), Giresun (40° N), Istanbul (41° N), Izmir (38° N), Kahramanmaras (37° N), Kayseri (38° N), Kocaeli (40° N), Konya (37° N), Malatya (38° N), Mersin (36° N), Samsun (41° N), Tekirdag (40° N), Trabzon (41° N), Yozgat (39° N), Zonguldak (41° N)].
Data were collected retrospectively for the date of sample, age of the participant on the date the sample was taken, sex, and vitamin D levels. Consecutive measurement of vitamin D levels in the tertiary care hospitals’ clinical biochemistry laboratories around Turkey between 2 January 2016 and 31 December 2016 was recorded.
Data of age over 18 yrs with no additional exclusion criteria were included in the final analysis.
Thirty labs were used chemiluminescence immunoassays (CLIA), and 3 labs were used high-performance liquid chromatography (HPLC) for the measurement of serum 25(OH)D levels. The measurement range was 2-167 ng/ml for the CLIA method, and 1.2-160 ng/ml for HPLC.
The study protocol was approved by the Marmara University Medical School Ethics Committee (09.2017.519) and conducted following the International Conference on Harmonization Guidelines for Good Clinical Practice and the Declaration of Helsinki.