OBJECTIVES:  There are numerous international standards and recommendations by pharmaceutical manufacturers for the order of draw to avoid contamination of the sample caused by additives in blood collection tubes. Our goal was to examine whether changing the order of draw results in biases in the analyzed diagnostic parameters. METHODS:  The quantitative, cross-sectional study was carried out in Karolina Hospital, Mosonmagyaróvár, Hungary between September 1. and November 31, 2016.We used non-randomized, purposive sampling in both experimental and control groups (N1=30, N2=30). In the experimental group, four incorrect orders of draw for phlebotomy tubes were used (Clot-activator, Na-citrate; Li-heparine, Na-citrate; K3 EDTA, native; Na-citrate, native). In the control group, the correct order of draw was followed (Na-cytrate, native, K3 EDTA). The following parameters were examined: prothrombin, INR, Potassium, Calcium, Magnesium, ALP. Data were analyzed using descriptive (absolute and relative frequencies, median with interquartile range [IQR]) and mathematical (χ2-test, Mann- Whitney U tests, linear and logistic regressions) statistics (p <0.05). RESULTS:  Incorrect order of draw (K3EDTA- serum tube) resulted in significantly decreased calcium levels in the experimental group compared to the control group (2.39 mmol/l [IQR: 2.33-2.46] vs. 2.60 mmol/l [IQR: 2.57-2.63], p <0,001) Any other deviation in the order of draw failed to show significant biases. CONCLUSIONS:  Incorrect order of draw for the sampling tubes during phlebotomy resulted in decreased level of calcium ion, which is a statistically significant difference, however, the value still remained in the normal clinical range.