Relationship between quinidine concentrations measured in saliva and erythrocytes, and in serum

The feasibility of indirect monitoring of serum quinidine concentration by determining saliva or erythrocyte levels was investigated in 16 hospitalized patients on quinidine therapy. No significant correlation was found between quinidine levels in saliva and its total or free levels in serum. The saliva to serum free drug concentration ratios were not dependent on saliva pH (r = 0.169), and they ranged from 1.238 to 6.782 among the different individuals. Quinidine serum protein binding was concentration dependent. It correlated poorly with serum levels of albumin, cholesterol, urea, creatinine, and patient's age. There was a significant positive correlation between quinidine concentration in erythrocytes (RBC) and its total or free concentration in serum (r = 0.481, p less than 0.05 and r = 0.770, p less than 0.001); however, the RBC to free serum concentration ratio varied appreciably among patients (range of ratio value 0.117 to 0.477) and did not significantly correlate with variables such as hematocrit or age. Thus, the estimation of serum protein binding of quinidine by determining its level in RBC is, as in the case of saliva, of limited usefulness. The means of the concentration of quinidine in blood, serum (total), RBC, and saliva in five patients after an overnight fast did not differ significantly from the means of the concentration determined after the consumption of lunch. Unbound quinidine levels in serum, on the other hand, were higher in the morning (1.31 micrograms/mg vs 1.01 micrograms/mg p less than 0.01). The administration of systemic heparin to one patient did not affect quinidine concentration in the various media in a consistent manner.