A free lignocaine index as a guide to unbound drug concentration.

A free lignocaine index was developed on the basis of measurements of plasma lignocaine and its principle binding protein, alpha 1-acid glycoprotein (AAG) in 80 samples from 16 patients admitted to the coronary care unit and given prophylactic lignocaine therapy. The free drug fraction, fu, of lignocaine was determined by equilibrium dialysis and its relationship to AAG and total lignocaine concentration (T) defined by multiple linear regression analysis as l/fu = 1.45 + 0.023 (AAG) -0.129 (T) (multiple r = 0.872, P less than 0.001). This relationship was used to calculate the 'free lignocaine index' as fu X T and compared with the observed value obtained by equilibrium dialysis of 178 samples from 41 separate subjects who received lignocaine after suspected myocardial infarction. There was a highly significant relationship (r = 0.933, n = 178, P less than 0.001) between the observed and predicted values. We conclude that the free drug index may be useful in rapidly assessing the unbound (free) concentration of lignocaine in plasma.