This week’s Morphology Monday case featured a fascinating artefact, platelet satellitism, where platelets appear to “cluster” around white blood cells, particularly neutrophils though it has also been reported around other white cells as well.

Platelet satellitism is an in vitro phenomenon in which platelets surround and stick to neutrophils in a rosette-like fashion on a peripheral blood smear. It’s not a sign of disease but rather an artefact linked to the anticoagulant EDTA used in blood collection tubes. In automated analysers, platelet satellitism can cause a falsely low platelet count (pseudothrombocytopaenia), potentially leading to unnecessary investigations or clinical concern.

The exact mechanism isn’t fully understood, but it’s thought to be due to:

• EDTA-dependent IgG autoantibodies
• These antibodies bind platelets to Fcγ receptors on neutrophils, forming rosettes on the film
• This occurs only in EDTA-anticoagulated blood and not in samples collected in citrate or heparin

If platelet satellitism is suspected:

• Repeat the sample in a citrate tube
• Confirm true platelet count with a manual film review
• Communicate with clinicians to avoid unnecessary intervention

This case highlights why morphology remains essential — even when automation does most of the counting.

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Special thanks to Charlotte Pattison for sharing the images for this case.