Diagnostic measures of hemostasis include platelet count, prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen, and fibrinogen degradation products. During venipuncture in humans, the shear force exerted by a needle smaller in diameter has been shown to affect platelet count but not PT or aPTT. As compared with human platelets, feline platelets have greater aggregability in response to shear stress. This prospective, observational, randomized clinical study evaluated whether needle size (22-g vs 25-g) affected routine coagulation variables in cats. Blood was sampled from the left and right jugular veins of 20 healthy, client-owned cats. Results showed no difference between the needle sizes in aPTT, platelet count, fibrinogen degradation products, or fibrinogen. PT was significantly higher when blood was drawn with the smaller diameter needle, but the degree of elevation was considered to have little clinical impact. The authors concluded that jugular venipuncture with either a 22-g or 25-g needle did not introduce any clinically meaningful difference in routine coagulation variables or platelet counts in cats.