Trimethoprim: prediction of serum concentrations from saliva measurements

Saliva has been used in the past as a non-invasive predictor of serum drug concentration. Prediction may be made from a regression line of saliva versus serum concentration or from an equation proposed by Matin et al. (1974); such predictions have been examined for trimethoprim, a drug that has a pKa of 7.3, the degree of ionisation influencing its partition between saliva and serum. A relationship was found between serum and saliva trimethoprim concentrations. Salivary trimethoprim concentrations were marginally related to salivary flow rate, but not salivary pH. The precision and bias of prediction from linear regression and the equation of Matin et al. (1974) was considered; the equation did not describe the relationship between serum and saliva trimethoprim concentrations and we conclude that its use is inappropriate.