Platelets with abnormal morphology
Large or giant platelets with mainly normal appearance
- Definition: large platelets are arbitrarily defined as having a diameter of 4μm or greater (less than half the diameter of a normal red cell)
- Large platelets can occur in normal blood, but when frequent they are associated with particular pathological states:
The context of large platelets is really important. When platelet counts are raised (A) large platelet forms are more likely associated with a myeloproliferative disorder (in reactive states they tend to be smaller and less variable). When platelet count is low the cause is most often premature platelet destruction (circulating platelets that are less mature are larger) - shown is a case of immune thrombocytopenia (B) with a Howell Jolly body indicating a previous splenectomy. Less common, but important, are the range of congenital conditions that can have large platelets; shown here is a MYH9 disorder (May Hegglin anomaly) with the characteristic pale blue inclusion within the neutrophils (C).
Causes of enlarged platelets with normal morphology
Small platelets with reduced number but otherwise normal features
These are seen in Wiscott-Aldridge syndrome (A): the mean platelet volume measured by automated counters is <5fl in the Wiscott-Aldridge syndrome, this contrasts with the 7.5-12fl volume of normal platelets. Therefore when flattened on a slide, the mean area will be around half that of a normal platelet. Platelet number is also reduced.
Platelets with abnormal or absent granules
Where platelet count is substantially elevated (A) then the diagnosis is a myeloproliferative disorder - often the platelets will vary in both size and granulation. Marked morphological abnormalities may indicate transforming disease. When counts are low, consider myelodysplasia or even an AML with dysplastic features - shown here is a very abnormal platelet together with small megakaryoblasts (B). Or more rarely the inherited grey platelet syndrome (C) - note the absent platelet granules while neutrophil granulation is normal.
|If platelet count is elevated|
Myeloproliferative disorders: the platelets may also show substantial variability of form and size
|If platelet count is reduced|