In view of increasing interest in vitamin D, the New York Times recently suggested that it might become "the nutrient of the decade" (1 ). Vitamin D plays a major role in calcium and bone metabolism and is an essential (pre)hormone involved in cell maturation and proliferation (2 ). Vitamin D deficiency is linked to an increased risk of breast, colon, and prostate cancer as well as autoimmune diseases like juvenile diabetes (2 ). Analysis and method standardization of the key indicator of vitamin D homeostasis, 25(OH)-vitamin D 3 , has been studied (3, 4 ). Information about preanalytical stability of 25(OH)-vitamin D 3 in human blood is hard to find, however, especially for "on the bench" conditions. One older study is incomplete and another focuses on freeze-thaw cycles (5,6 ). Medical laboratory sample-handling guidelines currently require freezing the sample and protecting it from artificial light and repeated freezethaw cycles. On the other hand, food industries report very good stability of vitamin D in natural matrices like milk or fat. We tested the stability of 25(OH)-vitamin D 3 in 8 different human blood samples under several sets of routine laboratory conditions.Because dietary supplementation of vitamin D in the Netherlands is confined to vitamin D 3 , we studied only 25(OH)-vitamin D 3 stability. This restriction is a potential limitation of our study, but our aim was to investigate the natural form in human samples. Based on the close resemblance in structures of vitamin D 2 and D 3 and previous but limited experience with 25(OH)-vitamin D 2 stability (5 ), we expected the stabilities for 25(OH)-vitamin D 2 and D 3 to be similar under the conditions of our study.We performed the study with leftover samples from 8 anonymous outpatients. Our study was approved by the hospital's ethics committee. Samples were collected in BD plastic blood collection containers free of anticoagulant. The original 25(OH)-vitamin D 3 concentrations ranged from 35-110 nmol/L. The baseline concentration values for our experiment (time ϭ 0) were the means of quintuplicate analyses, and for subsequent measurements they were the means of triplicate analyses. After exposure to the specified conditions in the original BD container, serum sample aliquots in stoppered plastic tubes were frozen immediately at Ϫ20°C until analysis. Storage conditions examined were extended common routine laboratory conditions: storage of serum at 6°C and at room temperature (about 20°C) in the dark and on the RT, room temperature. 55:8 1584-1595 (2009) This small increase can be attributed to evaporation or freeze-drying processes, but we believe there is also a small positive effect due to increased turbidity of the samples after freeze-thawing cycles. A 4.0% decrease in the mean concentration was seen following storage at Ϫ20°C for up to 2 months. Clinical ChemistryMean 25(OH)-vitamin D 3 concentrations (n ϭ 8) during storage under these common laboratory conditions are presented in Fig. 1. A mean decrease of 2.3% was noted after 72 h stora...
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