WBTF PCA is elevated in patients with SCD. EDTA-anticoagulated whole blood samples from normal controls and patients with SCD were frozen at −70°C immediately after collection. WBTF PCA was measured as described in Patients and Methods. Each symbol represents a single data point; samples are included from both Black (•) and Caucasian (○) control subjects. Patient samples include those drawn during steady-state disease (▵) as well as during pain crisis (▴).