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Hirudin‐induced pseudothrombocytopenia in a patient with EDTA‐dependent platelet aggregation: report of a new laboratory artefact

Sir, anticoagulant-induced pseudothrombocytopenia (PTP) is an important cause of spuriously low platelet counts described for all haematology analysers used in clinical routine diagnostics. If misinterpreted, it may lead to serious diagnostical and therapeutical consequences like bone marrow biopsy, initiation of corticosteroid therapy, platelet transfusion or even splenectomy (Zandecki et al., 2007). PTP has been reported for different anticoagulants such as ethylenediamine tetra-acetic acid (EDTA), citrate, heparin and oxalate, occurring with an incidence of 0.1–0.2% for EDTA, which is the most commonly used agent for automated blood counts (Schrezenmeier et al., 1995). We describe the case of an 82-year-old male patient admitted to hospital for trauma surgery. Initial laboratory exploration performed with EDTA (Sarstedt, Nuembrecht, Germany) as anticoagulant on a Cell-Dyn-Sapphire (Abbott, Abbott Park, IL, USA) showed a platelet count of 98 000/ll. A platelet flag and a note reporting platelet aggregates was given, the white blood cell (WBC) scattergrams yielded small dots above and within the lymphocyte area. Blood smear investigation confirmed the suspected occurrence of platelet aggregates. In suspicion of EDTA-induced PTP, further work-up is including the collection of a capillary blood specimen and a venous sample anticoagulated by sodium citrate for performing an automated platelet count and examination of blood smears for revelation of platelet aggregates (Payne & Pierre, 1984). In addition to these routine procedures, we collected a hirudin-anticoagulated venous blood specimen (Sarstedt, 25 lg/ml hirudin) searching for alternative anticoagulants in diagnosis of PTP. In the hirudinsample, the automated platelet count was 41 000/ll, a platelet aggregation flag was given and the WBC histograms corresponded to the results of the EDTA-examination. Exploration of a blood smear revealed large platelet aggregates. The citrate-anticoagulated specimen (Sarstedt) showed the platelet count within the reference range (206 000/ll) without reporting of any flags, the blood smear did not show platelet aggregates. In accordance to these findings, the diagnosis of EDTAand hirudininduced PTP was made. Anticoagulants and associated platelet counts are summarized in Table 1. To our knowledge this is the first description of hirudin-induced PTP. Anticoagulant-induced pseudothrombocytopenia because of chelating anticoagulants like EDTA, citrate and oxalate has been extensively studied. Chelation-dependent depletion of membrane calcium leading to antibody reactions with newly formed membrane antigens has been postulated as potential mechanism (Onder, Weinstein & Hoyer, 1980). Such immunoglobulines (Ig) are mostly of IgG or IgM type and represent so called ‘natural autoantibodies’ with antiplatelet activity, but without any pathological property in vivo (Bizzaro, 1995). The background of PTP development because of direct thrombin inhibitors like hirudin is not clear. Potential mechanisms are antibody reactions, including either unspecific or specific antibodies after contact with recombinant hirudin or the medicinal leech (hirudo medicinalis), as well as nonimmunologically mediated platelet agglutination caused by complex building of hirudin with other non-Ig proteins like thrombin. Thrombin is known to cause platelet agglutination in vitro (Watkins & Shulman, 1970). Lepirudin, a recombinant hirudin analogue, is used as alternative anticoagulant in patients with heparin-induced thrombocytopenia (HIT) II (Lubenow et al., 2004). In a study population of 196 patients with HIT II under prolonged lepirudin therapy, 44.4% developed antihirudin antibodies causing enhanced anticoagulatory drug activity (Eichler et al., 2000). Thrombocytopenia because of hirudin is a rarely reported, severe LETTER TO THE EDITOR INTERNATIONAL JOURNAL OF LABORATORY HEMATOLOGY