The Inhibitory Quotient-Reply

In Reply.—  We appreciate the opportunity to respond to Dr Tack's letter. We agree that there is considerable variability in CSF drug levels. In most cases, drugs that exhibit marked variability in CSF concentration are not used, and physicians are informed that higher doses would normally be employed in the treatment of meningitis. Moreover, the flexibility of our system is such that as more data concerning CSF levels of new drugs, eg, cefotaxime and moxalactam becomes available, it would be reflected in the CSF IQs. We recognize that numerous factors may alter the concentration and, hence, activity of antimicrobial agents both in tissues and body fluids.¹The conditions that obtain in in vitro determinations of MICs or serum bactericidal levels do not approximate those that exist at the site of infection in the body. Furthermore, the methodology employed in such determinations (medium and inoculum size) are variable. However, arbitrarily