The clinical applications of mean platelet volume (MPV) have recently broadened far beyond the differential diagnosis of platelet (PLT) disorders to embrace diagnosis and prognostication of a variety of thrombotic conditions. As the potential usefulness of this simple and inexpensive parameter may be challenged by instrument heterogeneity, we investigated the degree of analytical quality and interinstrument comparability. One hundred consecutive inpatient samples were simultaneously assessed on Abbott Sapphire, Mindray BC6800, Siemens Advia 2120, and Sysmex XE5000. The within-run imprecision of the four hematological analyzers was also assessed according to the Clinical and Laboratory Standards Institute document EP5-A2. The imprecision of PLT count ranged between 1.4 and 4.3%, and hence was always within the desirable quality specifications. The within-run imprecision of MPV ranged between 1.1 and 3.8%, and hence was also within the desirable quality specifications. The optical and impedance measurements displayed excellent correlations. Overall, the PLT count exhibited a modest instrumental variation, with bias always within the desirable quality specifications. A large bias was instead recorded for MPV, with between-instrument variations exceeding the desirable quality specifications in five out of six interinstrumental comparisons. No significant correlation was also observed between PLT count and MPV with any of the instruments tested. These results attest that although there is an optimal degree of analytical quality and comparability for PLT counting among different hemocytometers, the harmonization of MPV is poor, thus making the adoption of universal cutoffs virtually impossible.