Laboratory evaluation of the serum dilution test in serious staphylococcal infection.

Four test media were studied to determine performance characteristics of serum dilution tests used to monitor antimicrobial therapy during serious Staphylococcus aureus infection being treated with highly protein-bound antibiotics. Serum inhibitory titers and serum bactericidal titers obtained with Mueller-Hinton broth supplemented with calcium and magnesium were 3- to 16-fold higher than titers obtained with whole human serum buffered with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES). In cation-supplemented Mueller-Hinton containing 5% albumin or in cation-supplemented Mueller-Hinton combined with an equal volume of human serum, titers were 2- to 5-fold higher than in whole human serum buffered with HEPES. Clinical or animal studies are needed to establish whether the higher titers observed with patient serum containing highly protein-bound drugs diluted in low protein-content media would foster inadequate dosage regiments. In the meantime, both infectious disease clinicians and microbiologists should be aware of this potential pitfall.