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This study compared the reference intervals (RI) of B12 vitamin concentration in blood found in the literature with RIs extracted from data accumulated from a large number of patients by E. Gulbis Laboratory in Latvia. This paper investigated and demonstrated the possibility of using large amounts of random patient data to establish the RI for clinical laboratory tests. The blood level of B12 vitamin was selected as the model system for this study. The study used blind data for B12 blood level measurements from 132 379 patients accumulated in E. Gulbis Laboratory over a period of 15 years. In order to establish the reference intervals, the frequency distribution of log transformed B12 values was fit to a Gaussian distribution. The established B12 reference interval of 196 pg/ml and 942 pg/ml was found to be in good agreement with RIs reported elsewhere.