Abstract
I read with great interest 2 articles about the relationship between mean platelet volume (MPV) and venous thrombosis published online in your journal. 1, 2 Venous and arterial thromboses have been considered as distinct physiopathological concepts through thrombus formation. Arterial thrombi are found in areas of high shear stress and consist mostly of platelets and fibrin. Venous thrombi usually develop in low flow vessels and consist predominantly of erythrocytes and fibrin. Although the data suggest the MPV may potentially be useful as a predictive marker, there are a number of concerns related to the technological limitations and variations in the measurement methods of the MPV that need to be considered. Mean platelet volume does not report a routine part of the complete blood count because of the EDTA induced changes over time. The EDTA-induced platelet shape changes result in a progressive increase in MPV with impedance technology. In general, the MPV increases up to 30% within 5 minutes of exposure and increases further by 10% to 15% over the next 2 hours. 3 ,4 However, some investigators have reported inconsistent increases in MPV with EDTA storage up to 50%. 3 –5 The effect of EDTA on MPV with optical analysis appears to be unpredictable, decreasing in many patient samples while increasing in the others. Therefore, with the use of EDTA in complete blood count, the MPV is not a very reliable index. The MPV was measured within 30 minutes after sampling in the study by Gulcan et al and within 1 hour after sampling in the retrospective study by Cil et al. Also, measurements of MPV were conducted within 12 hours in the Tromsø study which is the first article about MPV and venous thromboembolism. 6 If the MPV have to be reliably measured, the potential effect of anticoagulant on the MPV must be carefully controlled, either using an alternative anticoagulant (such as citrate) or standardizing the duration between sampling and analysis. Lance et al performed a study to standardize the measurements of MPV. 7 They found citrate samples revealed significantly smaller MPV than EDTA. Platelets swell until 120 minutes in EDTA and until 60 minutes in citrate. They suggested that timing is important when measuring MPV and optimal measuring time should be 120 minutes after venipuncture.
On the other hand, it is not proven that MPV is a marker for platelet function. The MPV is the geometric mean of the transformed lognormal platelet volume data in impedance technology systems. In some optical systems, MPV is the mode of the measured platelet volume. 3 ,5,8 When platelets decrease in number, bone marrow megakaryocytes are stimulated by thrombopoietin, and their nucleus becomes hyperlobulated, with much higher DNA content. These stimulated megakaryocytes produce larger platelets. Thus, platelets with a higher MPV are expected to be seen in destructive thrombocytopenia when megakaryocytic stimulation is present. Conversely, platelets with a lower MPV are expected in thrombocytopenic states associated with marrow hypoplasia or aplasia. Turbidometric platelet aggregometry, developed in the 1960s, revolutionized the ability to identify and diagnose alterations in platelet function, and soon became the “gold standard” for platelet function testing. We did not observe correlation between any of platelet indices measured including MPV and platelet aggregation responses obtained with turbidometric method in healthy participants. 9 Although our study contained important data, this study was performed in a very controlled population and searching how the platelet indices compare to platelet function in pathologic conditions such as immune thrombocytopenia and so on could provide important clues in understanding of these data.
Furthermore, a cutoff value should not be defined for MPV because different technologies for measuring the MPV yield different results. The platelet parameters derived by the automated cell counter are highly specific to the individual technologies developed for each type of analyzer. Studies comparing results from these instruments have shown MPV differences of up to 40%.
As a result, MPV could potentially provide useful clinical information for venous thromboembolism. However, before it can be used for this purpose, the measurement of MPV must be carefully standardized.