Mean platelet volume analysis needs more standardization

To the editor

With great interest we follow the development of mean platelet volume (MPV) as a surrogate marker for different diseases. Unfortunately many papers do not even mention the impact of anticoagulants on MPV nor do they refer to standardized reference ranges. In this light the investigations of Lee and Kim and also Ozhan et al. are interesting [1], [2].

Both groups report on liver disease and its relationship with MPV. The authors take into account that platelets will swell in different media when they describe the methods. Actually, Ozhan et al. chose the method, which Bath showed to be valuable. Bath demonstrated that a high concentration of citrate (4:1) diminished swelling and make the analysis more time independent [3]. Nevertheless, there is concern about diluting effects, which alter cell counting. The use of non-standard concentrations makes daily work uncomfortable and may hamper MPV use as a standard parameter. This aspect was mentioned in the letter of Lee and Kim. For this reason the authors decided to use standard EDTA tubes. They argue that in a paper by Dastjerdi et al. the analysis should be done within 1 hour when preserved in EDTA. Here the authors doubt that there is a clinical relevance if the difference is 0.66 fl only for MPV [4]. That might be correct for clinical purpose but if one wants to distinguish between two diseases small differences may be important. Moreover if one will compare two investigations it is reasonable to analyse blood under the same conditions. Otherwise it is not possible to draw good conclusions.

Diaz et al. report in a recent study that platelets will swell if preserved in EDTA. The authors report a progressive increase in MPV during a 6-hour period when preserved in EDTA. They suggest additives such as wortmannin and tyrphostin which both are metabolic inhibitors to keep the MPV stable over that period [5].

Our study on the effects of anticoagulants and time dependency revealed a maximum growth of platelets – for EDTA up to 120 minutes and for citrate at standard concentration up to 60 minutes. After 120 minutes our results revealed a plateau and platelets did not swell significantly [6]. These results are comparable to the results of Bath et al. Only the optimal time to measure MPV in EDTA differed. In our investigation it took 60 minutes longer to reach a steady state than in Bath's study. Moreover, we created a reference range for the two standard anticoagulants (citrate and EDTA) based on 100 healthy blood donors.

In order to set MPV in the right context, a standard time window is recommended for standard anticoagulants and preservation media, and also a standard time window for analysis.