Review Question - QID 213741

A 30-year-old African-American man is hospitalized for sickle cell anemia workup in the setting of acute-onset dyspnea and fatigue. The patient reports that he often wakes up tired due to his occupation as an investment banker. He had noticed his urine appearing dark but suspected that it was because of inadequate hydration. He reports that his left leg is sometimes swollen despite wearing compression socks. The night prior, he had just closed a major merger and celebrated by drinking at a bar with his colleagues. He denies any family history of a blood disorder. His temperature is 98.6°F (37°C), blood pressure is 100/50 mmHg, pulse is 120/min, and respirations are 28/min. Physical exam is notable for subconjunctival pallor and splenomegaly. His left leg appears mildly larger than the right leg. Laboratory results are below:

Hemoglobin: 7.0 g/dL
Hematocrit: 21 %
Mean corpuscular volume: 92 µm^3
Reticulocyte count: 4%
Leukocyte count: 3,900/mm^3 with normal differential
Platelet count: 27,000/mm^3

Serum:
Na+: 137 mEq/L
Cl-: 100 mEq/L
K+: 5.6 mEq/L
HCO3-: 20 mEq/L
BUN: 10 mg/dL
Glucose: 90 mg/dL
Creatinine: 0.9 mg/dL
Thyroid-stimulating hormone: 3.4 µU/mL
Ca2+: 9.2 mg/dL
AST: 15 U/L
ALT: 25 U/L
Lactate dehydrogenase: 580 U/L (Range 80-280 U/L)
Haptoglobin: 20 mg/dL (Range 30-220 mg/dL)
Bilirubin: 7.2 mg/dl
Direct bilirubin: 0.8 mg/dl

Urine:
Color: Dark
Epithelial cells: Scant
Glucose: Negative
WBC: 2/hpf
RBC: 1/hpf
Bacterial: None

Peripheral blood smear reveals microcytic hypochromic cells. Ultrasound of his left lower extremity reveals an occlusive venous thrombus in the left femoral vein. Which of the following is the most appropriate test to confirm the most likely diagnosis?