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.