Answer to Parasite Case of the Week 754: Platelets overlying RBCs, an intraerythrocytic blood parasite mimic. Many thanks to Dr. Jacob Rattin who donated this case and provided the following helpful explanation.
Normal platelets are 1.5-2.5 μm in diameter and are morphologically round-to-oval and granular (Figure 1, arrowheads).
On Wright stain they are usually blue/purple and on Giemsa stain they are normally pink/purple. As seen in Figures 1 through 4 (arrows), platelets can overly red blood cells (RBCs), mimicking a blood parasite (e.g., Babesia or Plasmodium). In Figure 5, the green arrow is pointing to stain precipitate overlying an RBC.
Helpful morphologic clues to differentiate this pitfall from a true parasite are the following:
1. Platelets overlying RBCs often have a “halo” around them which is likely from it pressing down upon the RBC, which signifies it is not actually intraerythrocytic.
2. The platelets lack any definitive morphology of a parasite.
3. Other normal platelets in the field should resemble the platelet/s overlying the RBC (which is apparent in Figures 1 through 5).
4. As always, clinical information integration is crucial! Checking the patient’s medical and travel history can aid in the differential diagnosis.
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