The thrombin time assay is a versatile clinical assay used for screening fibrinogen related disorders and quantification of fibrin degradation products, as well as for detecting the presence of anticoagulant drugs. In situations where other screening tests may not be used to reliably diagnose fibrinogen related disorders, use of the thrombin time to discriminate these disorders may be the most effective choice. With new classes of direct oral anticoagulants in clinical use, interest in the thrombin time has increased. The purpose of this program is to describe the methodology and optimal use of the thrombin time assay for regular use in the clinical laboratory. Case studies highlighting the various applications of the thrombin time will also be presented.