A 65-year-old man with a falling platelet count and a leg DVT on day 7 of surgery

The patient is a 65-year-old man who underwent elective left total hip replacement. On the first postoperative day, he was started on unfractionated heparin 5000 units subcutaneously twice daily for thromboembolic prophylaxis. His regular medications included lisinopril 10 mg once daily for hypertension and oxycodone 10 mg every 6 hours for pain. He was also receiving cefazolin 1 g intravenously every 8 hours as surgical prophylaxis. On day 5, his platelet count was 260 x 10^9/L, compared with a baseline of 280 x 10^9/L. On day 7, he reported pain and swelling in his left calf. He was afebrile and his vital signs were stable. Examination showed left calf oedema and tenderness. Duplex ultrasound confirmed a left popliteal deep vein thrombosis. Laboratory results: platelet count 70 x 10^9/L, haemoglobin 11.2 g/dL, white cell count 12.0 x 10^9/L. A repeat platelet count was 62 x 10^9/L. He had no other risk factors for thrombosis, and there was no laboratory or clinical evidence of infection. All heparin was stopped, and he was started on an alternative anticoagulant. Over the following three days, the platelet count rose to 120 x 10^9/L and to 220 x 10^9/L by day 12. The leg swelling resolved gradually. The patient was not rechallenged with heparin.