Modern hematological instruments calculate several indices related to platelets, which provide useful information regarding the morphology and turnover of these cellular elements.
Mean Platelet Volume
The mean platelet volume (MPV) population, unlike erythrocytes, does not have a Gaussian distribution but is rather shifted to the left so that some graphs represent the curve after log transformation. The reference interval is between 7 f l and 12 fL. Newly produced platelets from the marrow are larger in size than older platelets, and greater MPV is observed when platelet production increases. MPV may be useful as an index to assess the extent of platelet production when accelerated destruction is suspected. In the latter case, increased MPV can be found in several inflammatory gastro intestinal diseases, thrombotic thrombocytopenic purpura (TTP), Bernard–Soulier syndrome, myeloproliferative dis eases, preeclampsia, and some cancers. In many of these clinical conditions, an increase in MPV is accompanied by a significant decrease in the number of circulating platelets due, as mentioned, to early destruction in the circulation. On the contrary, a decrease in MPV can be observed in some genetic thrombocytopenias (Wiskott–Aldrich syndrome) and in bone marrow aplasia.
Platelet Distribution Width
The platelet distribution width (PDW) represents a measure of the range of variation of platelet volumes in terms of the percentage coefficient of variation. Similarly to RDW, PDW can be expressed both as a coefficient of variation(PDW-CV) or standard deviation (PDW-SD):
• PDW-CV = SD (MPV)/MPV× 100
• PDW-SD, which reports in femtoliters the true value of the standard deviation obtained from the platelet volume distribution measured at the height of 20% on the curve itself.
The mode of expression depends on the type of instrumentation. The reference range for PDW-CV reported in the literature is 10.0–17.9%, with a mean value of 13.3%, depending on the instrumentation used. PDW generally increases in relation to an increase in the number of young circulating platelets, as seen in idiopathic thrombocytopenic purpura (or immune thrombocytopenia or immune thrombocytopenic purpura [ITP]) or macrothrombocytopenia. Although PDW is significantly increased in ITP compared to controls, its diagnostic power is limited when analyzed on its own. PDW should be considered a useful supporting parameter in association with MPV and immature platelet fraction, a parameter known as IPF. For example, PDW should be evaluated together with other platelet parameters for differentiating idiopathic thrombocytopenic purpura from Wiskott–Aldrich syndrome in patients with macrothrombocytopenia. In ITP, young platelets are produced with a larger size, and, thus, MPV is often elevated along with a high PDW. On the other hand, patients with Wiskott–Aldrich syn drome have uniformly large platelets, thus a high MPV but a normal PDW.
Immature Platelet Fraction
Immature platelet fraction (IPF) is a parameter recently introduced in automated analyzers. It provides the fraction of cross-linked platelets on the total number of platelets, that is, the youngest platelets, which, similar to reticulocytes, have a cytoplasmic reticular structure due to polyribosomal RNA. The reference range is between 1% and 8% of the total platelets, and the value increases when the pro duction of platelets by the marrow increases. It is therefore indicative of the efficiency of the bone marrow and, in particular, the proportion of megakaryocytes produced. The parameter is useful in the diagnosis and treatment of thrombocytopenia, such as idiopathic thrombocytopenic purpura and thrombotic thrombocytopenic purpura, as well as in the differentiation between forms with destruction in the circulation and those due to suppression of bone marrow production.
Plateletcrit
The plateletcrit (PCT), similar to the hematocrit for erythrocytes, represents the portion of the volume occupied by platelets in the blood as a whole. The value of the PCT is expressed as a percentage and is calculated according to the following formula:
PCT= platelet count × MPV / 10,000
The reference range for PCT is 0.22–0.24%. In normal subjects, the total mass of platelets is a constant ratio between the number of platelets and MPV, which therefore shows an inverse correlation between them. This ratio is altered in some conditions, including physical efforts, consumption of substances of abuse, such as alcohol and smoking.
