The use of antibiotic serum levels to predict concentrations in tissues.

A review of the literature shows that antibiotic concentrations in tissues and tissue fluids are often quoted as being different in profile to concurrent serum levels. To study the relationship between serum and tissue concentrations we analysed published studies where different experimental models were tested simultaneously. In some models serum levels predicted tissue levels while in others they did not. The factors likely to be responsible for the differences were examined. The most important of these factors was tissue geometry (surface area to volume ratio; SA/V). Serum levels predicted tissue levels in models where the SA/V was high (greater than 60) but not where the SA/V's were low (less than 10); here the antibiotic concentrations were lower and more prolonged than serum levels. These observations can be extrapolated to the clinical situation. In most situations involving prophylaxis or treatment of infections in non-specialised tissues (naturally high SA/V), serum levels will closely reflect levels in extracellular tissue fluid where most bacterial infections are located.