The thrombin time (TT) assay is a core test in clinical laboratories that perform coagulation testing. This assay screens for abnormalities in the conversion of fibrinogen to fibrin. The time necessary for fibrinogen to clot is affected by hypofibrinogenemia, dysfibrinogenemia, and the presence of inhibitors of the fibrinogen-to-fibrin reaction (heparin, hirudin, fibrin degradation products, and paraproteins) (1). The TT is used primarily to evaluate plasma specimens with prolonged activated partial thromboplastin time (APTT) values and, to a lesser extent, prolonged prothrombin time (PT) values for heparin or other thrombin inhibitors. The test is also useful to detect quantitative and qualitative fibrinogen abnormalities. Important criteria for selecting a TT reagent are high sensitivity to heparin, sensitivity to hypofibrinogenemia, and acceptable precision. We compared five commercial TT reagents on our automated coagulation analyzer and evaluated their sensitivity to fibrinogen and heparin concentration, their precision, and their accuracy. Information on each of the five TT reagents evaluated is provided in Table 1⇓ of the data supplement (available with the online version of this Technical Brief at http://www.clinchem.org/content/vol49/issue1/). Each commercial reagent was reconstituted according to the manufacturer’s instructions and used to measure TT in singleton measurements on the STA-R coagulation analyzer (Diagnostica Stago) according to the manufacturer’s specifications and laboratory standards. Clotting times for the TT assay were initiated when the instrument pipetted 100 μL of patient plasma, which then incubated for 120 s. The addition of 100 μL of thrombin reagent activated the final measurement of analyte. The Diagnostica Stago and BioPool reagents required a longer incubation time of 240s. Fibrinogen was measured on the STA Compact (Diagnostica Stago) with use of the STA-Fibrinogen reagent (Diagnostica Stago) containing human thrombin of 100 000 IU/L (100 IU/mL). The assay was initiated when the plasma sample was diluted 1:20 (5 μL + 95 …