We describe a patient who developed thrombocytopenia after receiving mezlocillin; the condition reversed after mezlocillin therapy was discontinued. Mezlocillin-associated antiplatelet antibodies were present in vitro.

A 70-year-old man was admitted with generalized seizures. The initial laboratory evaluation showed normal blood counts (platelet count, 303 x 10⁹/L) and normal coagulation studies. Dilantin therapy was started and a left parietal hemorrhage was diagnosed. Nine days later, a left lower lobe pneumonia developed that was treated with gentamicin and ceftriaxone. On day 19, because of persistent fever, this antibiotic treatment was discontinued and mezlocillin therapy was started. The platelet count on that day was 388 x 10⁹/L. On days 20 to 22, platelet counts were between 368 and 421 x 10⁹/L; blood counts were not checked on days 23, 24, and 25. On day 26, the platelet count was 1 x 10⁹/L, with a normal leukocyte count, hemoglobin concentration, prothrombin time, and partial thromboplastin time. The patient was clinically stable with no overt bleeding. Six units of random donor platelet concentrates were transfused; the platelet count 1 hour after transfusion was 83 x 10⁹/L. Therapy with mezlocillin and dilantin was discontinued. Serial platelet counts on subsequent days were 69, 108, 146, and 163 x 10⁹/L.

Serum was collected 24 hours after discontinuing mezlocillin therapy. Platelet antibodies were measured with a solid-phase antibody assay (Capture P kits; Immucor, Norcross, Georgia) using ABO-identical platelets and ABO-identical serum as a control [1, 2]. Platelets incubated with either mezlocillin or phosphate-buffered saline were attached to the microtiter plate wells by centrifugation. Patient or control sera were incubated with either mezlocillin (400 mg/mL) or phosphate-buffered saline at 37 °C for 30 minutes. The assay was done as previously described [1, 2].

Patient serum that had been preincubated with mezlocillin reacted with platelets. In contrast, patient serum preincubated with phosphate-buffered saline did not react with either of the platelet preparations. This suggests that the serum contained an antibody that reacted with mezlocillin and subsequently attached to platelets, leading to immunologically mediated platelet destruction.

We have thus documented that mezlocillin can cause drug-induced thrombocytopenia. The Capture P test can be useful as a rapid method of proving or disproving the role of a particular drug in patients who develop thrombocytopenia while receiving multiple drugs.