A method of determining prothrombin time (PT) in a whole blood, anti-coagulated blood, blood plasma or anti-coagulated blood plasma sample at an ambient temperature in the range of 15° C. to 45° C. is described. The method is performed with liquid reagents and the PT, preferably expressed as International Normalized Ratio (INR), is calculated based on said temperature and the clotting time (CT). A test kit is also described for analysis of PT which comprises temperature recoding means, and one or several separate sealed vessels containing reagents, optionally in lyophilized form for reconstitution prior to use, for clotting one or more defined volumes of whole blood, anti-coagulated blood, blood plasma or anti-coagulated blood plasma sample, and optionally time registration means and volume determining means.