Measurement of tobramycin concentrations in saliva

Paired serum and saliva specimens were obtained from 16 patients and three volunteers who had received tobramycin. In addition, saliva was collected from ten controls who had not taken any drugs in the previous week. Tobramycin concentrations were measured in duplicate by the Emit assay modified to obtain precise results at concentrations greater than 0.25 microgram/ml. The drug was undetectable in 32 of 34 saliva specimens and only trace amounts were present in the other two, even though corresponding steady-state serum concentrations were greater than or equal to 4.0 micrograms/ml for 29 of 34 specimens. Tobramycin values were zero for all ten saliva specimens from controls. These results probably can be extrapolated to other aminoglycoside antibiotics since most are hydrophilic and ionized at physiologic pH, which disfavors penetration into saliva. We conclude that saliva cannot be used as a noninvasive indirect method of measuring aminoglycoside serum concentrations to guide dosage adjustments.