Haematologica vo l. 84(7): J uly 1999 Haematologica 1999; 84: 655 t he i rrepl aceabl e imag e Anticoagulant pseudothrombocytopenia with platelet satellitism MONICA MORSELLI , G IUSEPPE L ONGO, G ORETTA BONACORSI , L EONARDO P OTENZA, G IOVANNI E MILIA, GIUSEPPE T ORELLI De partment of Medical Sciences , Section of Internal Medicine, Oncology and Hae matology, Univ ersity of Modena and Reggio Emilia, Italy Correspondence: Giuseppe Longo, Dipartimento di Scienze Mediche, Oncologiche e Radiologiche, Sezione di Medicina Interna, via Del Poz- zo 71, 41100 Modena, Italy. Phone: international +39-059-422782 – Fax: int ernational +3 9-05 9-4 245 49 – E- mail: longo@ unimo.i t aware that some thrombocytopenias of uncertain etiology may be artifactual in particular conditions; spurious thrombocytopenia due to EDTA-induced pl atelet s atell it ism is an e x ample of s uch a phe- nomenon. References 1. S hahab N, E va ns ML. P latelet s atellit ism. N E ngl J Med 1998; 338:591. 2. P eters MJ , Heyderman RS , Klein NJ . Plat elet satel- litism. N Engl JMed 1998; 339:131-2. 3. S hahab N, E vans ML. P lat ele t satelli t ism. Authors R eply. N E ngl JM e d 1998; 339:131-2. 4. Bizz aro N, Gol dschmeding R, von dem Borne AE . Platelet satellitism is Fc RIII (CD16) receptor-medi- ated. Am JClin Pathol 1995;103:740-4. 5. Christopoul os C, Mat t ock C. P lat e let s atelliti sm and -granule proteins. JClin Pathol 1991;44:788-9. T here has been a discussion recently in the liter- ature about platelet-leukocyte interactions in vivo and their well recognized importance in modulating inflammation and hemostasis. Occa- sionally, and only under particular conditions, plat e lets may int e ract wi th polymorphonuclea r neu- tr ophil s forming complex e s that may le ad t o a spuri- ous thr ombocy tope nia. 1-3 We obse rve d a 35-ye ar old healthy man in whom repeated routine laboratory tests showed a mild thrombocytopenia (70- 90,000 / mm 3 ). The blood film revealed scattered platelet rosetting around polymorphonuclear neu- t rophil s ; t he phenome non was e vide nt onl y whe n the blood was treate d wi t h EDT A as a nt icoagulant and incubated at room temperature (Figure 1). A blood ca pil lary film pe rfor me d wi thout E DTA did not rev e al platelet satellitism and the direct count using capil- lary blood with ammonium oxalate was normal. Of int ere s t , pl atele t s atellit ism with p olymorphonuclea r eosinophils or basophils, was never observed (Figure 2) . Ros e t t ing was not e vide nt whe n cit rat e or heparin was used. Platelet phagocytosis by monocytes was sometimes seen. In this case, the in vitro interaction between platelets, neutrophils and EDTA was the likely cause of the spurious thrombocytopenia. The me chanis m of plat e let sate llitism is not com- pletely understood. Ig autoantibodies dir e ct agains t the gly coprot e in I Ib/ IIIA complex of t he plate lets a nd the Fc ( ) receptor III (Fc RIII) of neutrophils have been implicated. EDTA at low temperature might alter the conformation of platelet glycoproteins and ne ut rophi ls F c rec e pt or by che lati on o f calc ium ions. Bridge for mati on betwee n plat e lets a nd neutrophil s may t ake place t hrough t he binding of t he F ab fr ag - me nt t o t he IIb/ IIIa pl atele t glycoprot ein and the F c fragment to the Fc RIII of the PMNs. Alternatively, but less likely, autoantibodies may recognize the s ame epi t ope in t he t wo struct ures. 4 A non-immuno- logic mechanism has also been proposed involving thrombospondin (or other -granule proteins such as P-selectin) that, when an activation stimulus is present, is rapidly expressed on platelet surface favor- ing adhesion to neutrophils. 5 Physicians should be Figure 1. Platelet satellitism around 2 neutrophils. Figure 2. Platelet satellitism around the neutrophil but not around the eosinophil.