To the Editor.—
The description of the inhibitory quotient (IQ) by Paul D. Ellner, PhD, and Harold C. Neu, MD (1981;246:1575), was welcome, as it formalizes the thought processes that must occur whenever quantitative susceptibility data are interpreted by the clinician. It is important, however, to realize several limitations of the use of the IQ in practice.The authors are careful to point out the need to consider specific tissue levels achievable, but fail to mention that considerable variation is seen between patients in CSF penetration.1 Indeed, marked variability even of serum levels is seen with certain antimicrobial agents, underscoring the need for assay procedures to ensure therapeutic levels.2 Antibacterial activity of aminoglycosides, tetracyclines, trimethoprim, and sulfonamides is diminished under the acid-base and redox conditions often encountered in infected tissue.A fourfold to eightfold ratio between serum level of drug and minimum inhibitory concentration (MIC) is suggested as
Tack KJ. The Inhibitory Quotient. JAMA. 1982;247(11):1563–1564. doi:10.1001/jama.1982.03320360015010
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