An instrument for measuring the coagulation characteristics of whole blood samples is provided with disposable parts for contacting the blood. The pin has a blood-compatible plastic sleeve surrounding a metal stem which weights the pin assembly so it sinks into the blood sample. The metal stem also engages into the pin suspension apparatus of the instrument. A blood-compatible plastic cup receives the blood sample to be tested, and is surrounded by a metal collar to transfer heat to the cup for the time required for a test. The plastic surfaces which contact the blood sample are slightly roughened, to a matte finish, to improve adhesion so as to approximate the results obtained with stainless steel pins and cups.