What does a platelet look like on a Wright‑Giemsa stained peripheral blood smear?

Platelet Appearance on Wright-Giemsa Stained Blood Smear

Platelets appear as small, discoid, anucleate cellular fragments measuring approximately 2-4 μm in diameter with a pale blue cytoplasm containing fine purple-red granules when viewed on a Wright-Giemsa stained peripheral blood smear. 1

Morphologic Characteristics

Size and Shape

• Normal resting platelets are discoid structures approximately 2-4 μm in diameter, making them the smallest type of cells found in any human body fluid 1
• The typical platelet volume ranges from 7.1 to 10.5 fL in healthy individuals 1
• Platelets maintain their characteristic discoid shape in the resting state, though they are agile cells with large intracellular membrane reservoirs that change size, volume and shape upon activation 1

Cytoplasmic Features

• The cytoplasm stains pale blue to light purple with Wright-Giemsa stain 2
• Fine purple-red granules are visible throughout the cytoplasm, representing α-granules, dense granules, and other organelles 2
• No nucleus is present, as platelets are anucleate cellular fragments derived from megakaryocyte cytoplasm 2, 3

Size Variations and Clinical Significance

Normal Variation

• Some degree of anisocytosis (size variation) may be present even in normal blood smears 4
• Careful analysis should distinguish normal size variation from pathologic changes 4

Pathologic Size Changes

• Small platelets (3.8-5.0 fL) with homogeneous size distribution suggest Wiskott-Aldrich Syndrome, particularly in male patients with thrombocytopenia 1
• Large or giant platelets indicate myeloproliferative neoplasms such as essential thrombocythemia or polycythemia vera, especially when accompanied by granulocytic left shift or dysplasia 5, 6
• Microplatelets or excessive anisocytosis may indicate inherited thrombocytopenia or acquired platelet disorders 4

Key Morphologic Identifiers

Distinguishing Features

• Platelets can be identified by their unique molecular marker CD41 (Glycoprotein IIb) on flow cytometry, though this is not visible on light microscopy 1
• On blood smear, platelets are distinguished from other cellular elements by their small size, discoid shape, and absence of nucleus 2, 7
• The density of platelets ranges from 1.061-1.091 g/mL 1

Common Pitfalls in Platelet Morphology Assessment

• Do not rely solely on automated platelet sizing—always examine the blood smear directly when platelet abnormalities are suspected, as automated counters may report normal size in conditions like Wiskott-Aldrich Syndrome 1
• Platelet clumping can occur during blood collection and handling, particularly with cold activation, leading to falsely low platelet counts and altered morphology 1
• Activated platelets change from discoid to spherical shape with pseudopod formation, which may be seen as an artifact of blood collection rather than true pathology 1