Laboratory Evaluation of the Serum Dilution Test in Serious Staphylococcal Infection

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Abstract
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 S-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.

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