The extent to which an antibiotic reaches infected tissue.

To be effective, antibiotics must gain access to infected tissue. Clinical outcomes are dependent on antibiotic concentrations at the site of infection. These do not necessarily correlate well with antibiotic levels in the bloodstream. The degree of tissue penetration depends on both the properties of the antibiotic and the nature of the tissue affected.

Measuring tissue penetration requires an assessment of unbound drug concentrations in the interstitial space surrounding cells at the site of infection, as tissue concentrations measured from biopsy specimens may give a misleading picture of antibiotic concentrations at relevant sites. The best method to determine unbound interstitial concentrations in tissues is microdialysis. In this technique, a probe with a semi-permeable membrane is inserted into the interstitial fluid. Molecules diffuse across this membrane at a rate dependent on their concentration in interstitial fluid.