OSMT (Ontario Society of Medical Technologists) Practice Exam

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All of the following tests should be collected in a purple top tube except:

RBC
Differential
Plts
PT

The correct answer is: PT

Purple top tubes are used for collecting blood specimens for tests that require an EDTA anticoagulant. EDTA is used to preserve cell morphology and prevent clotting by chelating calcium ions. Therefore, tests that involve counting and examining blood cells, such as RBC (red blood cell) count, WBC (white blood cell) count, and platelet count (Plts), are typically collected in a purple top tube. On the other hand, the PT (prothrombin time) test is used to evaluate the extrinsic pathway and common pathway of the coagulation cascade. PT requires a different anticoagulant, usually sodium citrate, which is typically collected in a light blue top tube. Therefore, PT should not be collected in a purple top tube.