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Endemic hypovitaminosis D contributes to osteoporosis development. However, variation in 25-hydroxyvitamin D (25OHD) measurement is reported and confounds the diagnosis of vitamin D insufficiency/deficiency. This report emphasizes the marked variability observed in serum 25OHD measurements between laboratories.Initially, postmenopausal women had serum 25OHD determinations: 42 in laboratory A, 20 in laboratory B. Their mean (sem) serum 25OHD concentrations were 46 (2.1) and 21 (2.3) ng/ml in laboratories A and B, respectively. Furthermore, there was little overlap in serum 25OHD among these clinically similar individuals. Specifically, 17% of those measured in laboratory A but 90% in laboratory B were below an arbitrary threshold value of 32 ng/ml.Subsequently, serum was obtained from 10 healthy adults. Two aliquots from each individual, one of which was spiked with 20 ng/ml 25OHD, were sent to six laboratories. Substantial variability was noted between these six laboratories. The mean serum 25OHD concentration ranged from 17.1-35.6 ng/ml. Similarly, the mean increase produced by spiking with 20 ng/ml ranged from 7.7-18.0 ng/ml.In conclusion, 25OHD assays yield markedly differing results; whether an individual is found to have low or normal vitamin D status is a function of the laboratory used. If the medical community is to make progress in correcting widespread hypovitaminosis D, 25OHD measurement must be standardized.