Heparin is commonly used to prevent obstruction of indwelling arterial catheters with blood clots. It is known to affect the outcomes of analysis of coagulation parameters with thromboelastography (TEG); therefore, it has been recommended to neutralize its effect with heparinase. However, heparinase may also neutralize the effect of low molecular weight heparin and endogenous heparinoids present in critically ill patients and thus yields unreliable results. The aim of this study was to evaluate the minimal discard blood volume needed to eliminate the effect of heparin flush on TEG parameters without the use of heparinase. Ten patients with indwelling arterial catheter were included in the study. Coagulation parameters were evaluated with kaolin-activated TEG. Blood samples were obtained after discarding 1, 2, 3, 4, 5 or 10 ml of blood to eliminate the effect of heparin. We investigated the influence of the discard volume on time until the first detectable clot (R), speed of clot development (alpha angle), maximal amplitude of the measured clot and time to maximal amplitude of the measured clot. We found an increase in coagulation (reflecting the heparin elimination) with the increasing discard volume between 1 and 4 ml. This was obvious from an increase in alpha angle and maximal amplitude of the measured clot and a decrease in R and time to maximal amplitude of the measured clot (P < 0.001). However, values obtained after discarding 4, 5 and 10 ml of blood did not differ markedly. To obtain valid information about TEG parameters, it is necessary to discard volume of at least 4 ml of blood (i.e., five times the volume of catheter dead space).