Piperacillin‐dependent anti‐platelet antibodies are a relevant, easy to confirm differential diagnosis in patients with rapid‐onset thrombocytopenia

Field: Medicine (Health Sciences)

Drug-dependent anti-platelet antibodies are a rare and often overlooked cause for thrombocytopenia. One reason is their rarity, but lack of testing and poorly defined test protocols may add to this diagnostic dilemma. In the present study, we report on three patients who were hospitalised between 2017 and 2018 because of severe infections. Ethical approval for this study was obtained. All patients experienced sudden isolated thrombocytopenia and bleeding within their first week in hospital. Clinicians suspected immune thrombocytopenia (ITP) and/or heparin-induced thrombocytopenia (HIT), but heparin-induced platelet aggregation (HIPA) and a direct monoclonal antibody-dependent immobilisation of platelet-specific antigens (MAIPA) assay were negative in our laboratory. After consulting with the treating physicians we suspected drug-dependent platelet antibodies as the cause for thrombocytopenia because platelet counts dropped rapidly (within 5–6 days) to very low numbers (<20 000/µl); patients had signs of bleeding and antibiotics were newly prescribed on the day of admission. All three patients were on piperacillin/tazobactam (PIP/TAZ) a fixed combination of an acyl-aminopenicillin and a β-lactamase inhibitor. This broad-spectrum antibiotic is often used for managing severe infections in hospitalised patients. In accordance with a recommendation from the International Society on Thrombosis and Haemostasis (ISTH) Subcommittee on Platelet Immunology, Arnold et al.1 freshly isolated donor platelets from a Group 0 donor that were incubated with the patients’ serum samples and the drug (at 1 mg/ml) or saline and fluorescein isothiocyanate-labelled anti-human immunoglobulin G was used to detect drug-dependent antibody binding by flow cytometry. The drug-to-saline mean fluorescence intensity (MFI) ratio was calculated (Table I). PIP/TAZ ex vivo metabolites (MET), present in the morning urine from an individual who took the drug the evening before, were used instead of the drug in an extra set of tests. All three patients tested positive (MFI ratio >2·0) for PIP-dependent and MET-dependent anti-platelet antibodies, whereas TAZ-dependent antibodies were not detected. Note that the MFI ratios with MET were stronger than with PIP. This could potentially indicate that the main immunising agent was a drug metabolite rather than the drug itself. Unfortunately known MET of PIP and/or TAZ for further studies were not available. It should be kept in mind that in rare cases antibodies may not be detected by means of the unmodified drug and that tests in the presence of MET can be helpful in identifying the responsible agent.2 Med-MAIPA (ODpat–ODctl) Because the target antigen of PIP on platelets is not well-defined,3-5 we also performed a MAIPA assay in the presence of the drug and also the MET (Table I). Glycoprotein (GP) IIb/IIIa was identified as a drug target, whereas GPs Ib/IX and Ia/IIa and platelet endothelial cell adhesion molecule 1 (PECAM-1) were excluded. GP IIb/IIIa is also a major target identified for other substances involved in drug-induced thrombocytopenia.6 We observed only very weak if any association between the strength of reactivity in flow cytometry and MAIPA (Spearman’s rho 0·64). Discontinuation of the sensitising drug led to a spontaneous recovery of the platelet counts within 3–5 days in all three patients. Of course antibiotics cannot be discontinued in patients with bacterial infections without substitute. Unfortunately, we found only sparse data on cross-reactivity between β-lactam antibiotics.7 In general drug-dependent antibodies are considered to be highly drug specific. It can be expected that pharmacological equivalents are well tolerated.6 We tested two other penicillin antibiotics (amoxicillin and mezlocillin) and five cephalosporins (first, second and third generation). No cross-reactivity of PIP-dependent anti-platelet antibodies was observed, neither in the flow cytometric assay (Table II) nor by MAIPA (data not shown). Additional clinical data were unavailable and we do not know whether any of these alternatives were used successfully. We conclude from our observations that drug-dependent antibodies should always be considered in patients with unexplained rapid-onset thrombocytopenia and bleeding signs if new medication has been prescribed during the last 7 days. We further suggest that laboratories should actively seek to contact the consulting physicians of all patients with unexplained sudden thrombocytopenia, as drug-dependent immune thrombocytopenia is often not considered as a relevant differential diagnosis. A simple flow cytometric assay as suggested by the ISTH appears to be a fast and reliable method for detecting drug-dependent antibodies, as confirmed by additional tests in our patients. Finally PIP-dependent anti-platelet antibodies are not cross-reactive with a spectrum of pharmacological equivalents indicating that these substances are probably safe to prescribe for immunised patients. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.