Since plasma protein binding of disopyramide (DP)--a class IA antiarrhythmic widely used in the prevention and treatment of various types of cardiac arrhythmias--is not only saturable within the therapeutic range but also altered under various pathophysiological conditions, the interpretation of total DP concentrations, Ctotal, measured during routine therapeutic drug monitoring (TDM) is often complicated. To circumvent this problem, we attempted to establish a comprehensive nomogram that allows estimation of unbound DP concentrations (Cu) based upon Ctotal of the drug and plasma concentration of alpha 1-acid glycoprotein (AAG), a major DP-binding protein. The nomogram was formulated with use of the in vitro binding data retrieved from 103 subjects categorized into 10 different groups each with a different mean concentration of AAG (range: 0.14-1.54 g/L). Data analysis, using a binding model assuming one specific binding site and nonspecific binding(s), revealed that alterations in plasma DP binding are attributable mainly to those in the capacity, Bmax, rather than affinity, ka, constant of the specific binding site. In addition, plasma AAG concentration correlated significantly (r = 0.90, p < 0.001) with the Bmax value over the range 0.09-2.28 g/L. For this reason, we substituted Bmax calculated by the regression equation as a function of AAG and the overall mean ka and nonspecific binding parameter values for the respective individual variables of the binding model, so that Cu of each plasma sample was estimated from the corresponding data on Ctotal and plasma AAG levels.(ABSTRACT TRUNCATED AT 250 WORDS)