Normal platelets, variability and artefact
- Circulating platelets are bi-convex discs of around 2–3 µm diameter (less than a quarter the area of normal red cell) but are more variable in size than red cells or white cells
- Platelets with small size may occur in normal blood, but when very frequent may indicate the rare condition Wiscott Aldridge syndrome
- Large platelets - arbitrarily defined as being more than one quarter the size of a red cell. These are present in the blood of normal individuals but should be infrequent (generally <5% of platelets). They are more numerous where platelet production is increased (for example in immune thrombocytopenia)
- Giant platelets have size equal to or greater than a red cell, and are rare in normal blood. When more than the occasional giant platelet is found, the blood should be checked carefully for underlying disease.
Normal platelets have a relatively wide range of sizes, but large forms are uncommon (A); however when early release of platelets is needed they emerge with larger form, so large platelets may be seen in conditions such as immune thrombocytopenia (B), large size is also a feature of myeloproliferative disorders (C)
- Electron microscopy describes four zones within platelets: (i) the peripheral membrane zone rich in adhesive molecules; (ii) the submembrane area of tubules and filaments determines platelet shape; (iii) the granular zone with alpha and dense platelet granules; (iv) a tubular system that promotes shape change.
- In light microscopy detailed structure cannot be seen - the most common appearance is to be densely-stained and have a round or irregular shape.
- However, platelets do have a morphology that can be seen on well-stained smears. Furthermore, immediately following spreading the interaction between platelet and glass slide may cause some activation with features of partial activation apparent, and structural detail may be seen - particularly in larger forms.
Platelets may appear uniformly granular (A), but partial activation may cause short membrane projections (B), or spread forms with a central granular and peripheral membrane zone (fried egg appearance) (C)
Occasionally platelets may overlie red cells, or may lie against them - in such cases the platelet cytoplasm can cause a characteristic ‘halo’ that displaces haemoglobin (A) or may indent the cell membrane of adjacent red cells (B)
DISTINGUISHING PLATELETS FROM OTHER ELEMENTS
A number of features may be mistaken for platelets; and it is important to distinguish these morphologically. The key is to know how a platelet should appear – the stained shade, density and shape all contribute - if something appears unusual then examine it closely. Some examples are shown in the images below:
Stain debris is the most frequently encountered problem, and may superficially resemble platelets and may occasionally be a source of confusion when platelet counts are very low, but is generally distinguished based on colour, density, or shape (images A and B)
Other elements that can cause issues (including to automated counters) include infecting organisms (A: streptococci), cryoglobulin (B: in an unusual needle-shaped form), or red cell fragments (C: burns). Often recognizing the presence of these elements is easy, but you need to think about them!
ARTEFACTS AFFECTING PLATELETS
A number of platelet artefacts may be encountered in blood.
The most frequent artefact is platelet aggregation or clumping (A). This phenomenon is caused by non-specific antibody particularly associated with EDTA anticoagulant and has no clinical significance, but may cause difficulty in interpreting platelet counts. Platelet clumps will usually be seen throughout the film, but are most obvious at the end of the blood smear. A related, but distinctive, phenomenon is ‘satellitism’ (B) where the platelets form a rosette around neutrophils and again this has no clinical significance. Finally, in samples damaged by heat there may be extensive platelet clumping and degranulation (C) with amorphous clumps that are difficult to recognise.