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The influence of using a roller mixer just after blood collection on the ratio of rejected samples in coagulation tests was investigated retrospectively.Numbers of blood samples collected in centralized units of outpatient clinics for prothrombin time (PT) were 24 050 in group I and 24 500 in group II (H. Aral, unpublished data). In group I, the tubes were inverted manually. In group II, the tubes were left standing on roller mixer with 30 rpm. The ratios of rejected samples of the two groups were compared statistically with ki-square analysis.Using a roller mixer, rate of rejection decreased from 0.79% (group I) to 0.20% (group II) (P < 0.001).We suggested using roller mixer to improve the reliability of coagulation testing. Such standardization in preanalytical phase may be helpful in preventing laboratory errors and obtaining correct test results in coagulation tests.