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Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
A 65-year-old woman, with end-stage renal disease (ESRD) on hemodialysis, presents with pain, swelling and discoloration of her right leg and foot. She says that she started twice-weekly hemodialysis 2 weeks ago and has had no issues until 1 week ago when she noticed a warm, painful swelling of the back of her right leg and right foot after finishing her dialysis session. Over the week, she says these symptoms have steadily worsened and, in the last few days, her right foot has become discolored. Past medical history is significant for ESRD secondary to long-standing hypertension. Current medications are verapamil 200 mg orally daily and unfractionated heparin that is given during hemodialysis. Her vital signs include: temperature 37.0°C (98.6°F), blood pressure 145/75 mm Hg, pulse 88/min, respirations 15/min, and oxygen saturation 99% on room air. On physical examination, the patient is alert and cooperative. The cardiac exam is normal. Lungs are clear to auscultation. The abdomen is soft and nontender with no hepatosplenomegaly. The right calf is swollen, warm, and erythematous. Physical findings of the patient’s right foot are shown in the exhibit. Laboratory findings are significant for the following:
Sodium 141 mEq/L
Potassium 4.9 mEq/L
Chloride 104 mEq/L
Bicarbonate 25 mEq/L
BUN 32 mg/dL
Creatinine 3.1 mg/dL
Glucose (fasting) 75 mg/dL
  Bilirubin, conjugated 0.5 mg/dL       
Bilirubin, total 1.0 mg/dL
AST (SGOT) 22 U/L       
ALT (SGPT) 23 U/L       
Alkaline phosphatase 56 U/L
  Bleeding time 19 min      
Prothrombin time (PT) 11 s   
Partial thromboplastin time (PTT) 30 s
        WBC 8,500/mm3       
RBC 4.10 x 106/mm3
Hematocrit 41.5%
Hemoglobin 13.5 g/dL       
Platelet count 100,000/mm3 (previously 200,000/mm3)
Which of the following is the next best diagnostic step in this patient?
assistant
Heparin/PF4 enzyme-linked immunosorbent assay (ELISA)