Fibronectin plasma levels correlate with HIT antibody formation. Plasma levels of intact fibronectin (in micrograms per milliliter) (A) and of anti-PF4/heparin IgG (OD) (B) in patients with clinically confirmed HIT who showed a positive SRA and a positive PF4/heparin IgG ELISA (▲, n = 17); patients with no clinical HIT, but a positive SRA and ELISA (●, n = 18); patients with no clinical HIT, a negative SRA, and a positive ELISA (♦, n = 24); and patients who did not develop anti-PF4/heparin IgG (▪, n = 30). Data are intact fibronectin and anti-PF4/heparin IgG levels of individual plasma and median of each group. The P values are provided for the comparison between levels of all ELISA-positive and ELISA-negative patients and P_trend over all 4 groups. (C) Intact fibronectin levels (in micrograms per milliliter) are plotted against anti-PF4/heparin IgG OD levels of individual patient plasmas, including all 4 groups (n = 89). A nonlinear association between fibronectin and anti-PF4/heparin IgG plasma levels is illustrated by the nonlinear regression curve, as determined by median regression. Regression formula: y = 0.102 + 0.825*1/SQRT(x/100).