Preparation of blood samples
Blood was collected by venipuncture after an overnight fast. Samples clotted 30 min at room temperature, and serum was separated by centrifugation at 1700 g for 10 min. Aliquots were immediately stored at 20 °C ( 80 °C for assays of vita- min B-2, vitamin C, and 25-hydroxyvitamin D). Samples pre- pared at Vestfold Hospital were kept on dry ice ( 57 °C) for up to 16 h during transportation to Aker University Hospital. Lab- oratory assays were performed within 10 d of blood sampling, except for vitamin B-2 (within 90 d) and 25-hydroxyvitamin D (within 1 y). We previously performed extensive assessment of protocols for preparing specimens for vitamin C analysis with respect to deterioration of ascorbic acid concentrations at differ- ent storage conditions. The results are highly reliable when se- rum aliquots are frozen at 80 °C within2h of blood sampling and analyzed within 2 wk.