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Background and aims: The aim of this study was to investigate the possible preanalytical effect of incomplete filling of blood tubes on molecular biology assays. Materials and methods: The study population consisted of 13 healthy volunteers from whom 11 mL of whole blood was collected and then distributed in different volumes (1.5, 3.0, and 6.0 mL, respectively) into three 6.0 mL spray-dried and evacuated K2EDTA blood tubes. Automated RNA extraction was performed using the Maxwell® CSC RNA Blood Kit. DNA was extracted with a MagCorePlusII, with concomitant measurement of glyceralde-hyde-3-phosphate dehydrogenase (GAPDH) gene expression. The nucleic acid concentration was calculated using the NanoDrop 1000 spectrophotometer, and purity was assessed using A260/280 and A260/230 absorbance ratios. Results: The RNA concentration was higher in the tubes filled with 1.5 and 3.0 mL of blood than in the reference 6 mL filled tube. The RNA 260/280 and RNA 260/230 ratios did not differ significantly between the differently filled blood tubes. The DNA concentration remained constant in the differently filled tubes. Compared to the 6.0 mL reference filled tube, the 1.5 mL and 3.0 mL filled blood tubes displayed a lower DNA 260/280 nm ratio. The DNA 260/230 ratio did not differ significantly in any of the variously filled tubes. Compared to the 6.0 mL reference filled blood tube, the 1.5 mL and 3.0 mL filled blood tubes showed a significant increase in the GAPDHcycle threshold. Conclusions: Our results suggest that underfilling of K2EDTA blood tubes may be a modest but analytically significant source of bias in molecular biology testing. [ABSTRACT FROM AUTHOR]