Effects of preanalytical factors on serum vitamin B12 test results

INTRODUCTION: While conclusions regarding the effects of serum separator tubes on some laboratory tests have been reached in previous studies, the effects of these tubes on the B12 test have not been adequately investigated. This study aimed to determine the effects of the serum separator tube position after centrifugation on serum B12 test results. MATERIALS AND METHODS: Two blood samples were collected by venipuncture into 5-mL Pirmax serum separator tubes from each of the 30 volunteers. The effects of the tube position after centrifugation, gentle mixing 5-10 times, recentrifugation, and short-term (15 min) standing of the tubes in an upright position on serum B12 test results were examined in a step-by-step manner. The B12 analyses were performed in an Advia Centaur XPT autoanalyzer using a chemiluminescence method. Comparisons between the groups were evaluated for statistical and clinical significance. RESULTS: B12 levels were higher in the samples positioned horizontally than in the samples positioned vertically (p < 0.001). The total effect of horizontal position and mixing was abolished by recentrifugation (p = 0.091). The horizontal tube position and sample mixing led to an exceedance of the total allowable error limit. CONCLUSIONS: The use of serum separator tubes may be associated with spuriously elevated serum B12 concentrations in the Advia Centaur XPT assay. The serum separator tubes should be kept upright in the post-centrifugation period prior to B12 analysis. We recommend recentrifugation if the tubes have been held in a horizontal position and/or the samples have been mixed.