A 65 year old woman is admitted for elective replacement of her left knee joint. Her preoperative FBC is as follows: WBC 7,000/mm3, Hb 12 g/dL, platelets 420,000/mm3. An ECG, CXR and fasting plasma glucose are normal. Her medical and surgical histories are unremarkable, and she is not on any drugs. Following an uncomplicated operation, she is started on DVT prophylaxis with subcutaneous enoxaparin. On the 5th postoperative day, she complains of pain and swelling of her left leg.
WBC: 8,700/mm3 (4,600-11,000) Hb: 11.9 g/dl (11.5-18) Platelets: 152,000/mm3 (150,000-450,000)
There is acute thrombosis of the left popliteal vein, with absence of flow and compressibility.
D-Dimer levels are elevated.
The platelet factor 4 antigen assay is positive.