Fallacious results from measuring salivary carbamazepine concentrations

During a carbamazepine (CBZ) relative bioavailability study involving tablets and a syrup preparation, salivary drug concentrations appeared disproportionately high relative to simultaneous plasma drug concentrations in the first 2-3 h after oral drug intake. This raised the suspicion of contamination of saliva by retention of drug in the mouth. In a separate study CBZ was retained in the mouth in tablet form (whole or crushed) or in syrup, for only 5 s before being spat out, and the mouth was carefully rinsed. Despite this, measurable salivary concentrations, sufficient to cause substantial error if extrapolated to simultaneous plasma drug concentrations, were present for at least 2 h after drug administration. CBZ in these studies disappeared from saliva with an apparent mean half-life of 21.0 ± 4.8 min. This experience suggests that, in therapeutic drug monitoring, salivary CBZ concentrations for at least 2 h after dosage may lead to invalid conclusions about simultaneous plasma CBZ concentrations