Prothrombin times and activated partial thromboplastin times have long been used as tests of overall ("global") clotting function. Laboratory coagulation testing issues should be at the forefront of the reader's consciousness whenever critically evaluating and extrapolating published study conclusions reliant on the results of these tests. Thus, this article reviews laboratory issues and known variables influencing prothrombin time and partial thromboplastin time results and international normalized ratio determinations.