Modern photo-optical coagulometers collect optical data in the form of reaction curves and offer a possibility to determine clotting times at different points of clot formation by using different evaluation modes. The objectives of this study were to determine the possible impact of an evaluation mode on activated partial thromboplastin time (APTT) results and to investigate potential benefits from visual inspection of obtained reaction curves. APTT was determined by using actin FS as reagent on two coagulometers (Siemens Medical Solutions) in 174 plasma samples with three different evaluation modes: fixed absorbance (FA), drifting baseline (DB), and point of inflexion (POI) on Behring coagulation timer (BCT), and with DB mode on Behring coagulation system (BCS). Statistically significant difference of APTT results applying the Friedman's test (P<0.0001) followed by Dunn's multiple comparison test (P<0.05) was obtained in all tested samples between POI mode and all other evaluation modes, independently of analyzer used. The differences obtained indicated that laboratory professionals must be aware of possible different evaluation modes on different analyzers, and establish evaluation mode-specific reference intervals. Moreover, correctness of a reported result could be confirmed only by visual analysis of the reaction curve.