The comparative evaluation of the hydrodynamic properties of prosthetic heart valves is a valuable indicator for assessment of expected clinical performance. Pressure drop in steady flow is a primary indication of performance. Concerning the procedure of these experiments, the cardiac valve prostheses ISO 5840 document recommends the use of a unique 36-mm internal diameter straight tube for all valve sizes. Recently the European Committee for Standardization proposed the use of three straight tubes of different sizes matched with valve dimensions. In order to argue this proposal, we measured the pressure drop in three tubes of different diameters across four different valves of small caliber and across a reference diaphragm for several flow rates. Results are discussed from a fluid-dynamic point of view. The results show that the use of a unique 36-mm tube is sufficient for a basic evaluation, but matching tubes with valve dimensions is more pertinent for an optimization goal.