Normal glomerular filtration rate (GFR) declines with age and in disease. Diminished GFRs are seen in many patients on the ICU. The clearance of toxins and pharmaceuticals might be (temporarily) diminished especially in patients with septic or circulatory shock. Most physicians are aware of this and adjust the dosage of antibiotics accordingly: the dosage is reduced or the administration interval is prolonged. However, some patients have an increased clearance of antimicrobials, so-called glomerular hyperfiltration. Glomerular hyperfiltration is present whenever the GFR exceeds 160 ml/min/1.73 m2 in men and 150 ml/min/1.73 m2 in women. This is especially seen in young, male patients after neurotrauma, polytrauma or burn patients. The consequences of glomerualr hyperfiltration might be that antibiotics are cleared more rapidly and concentrations fall below optimal levels. This compromises effective antimicrobial therapy when it is most important: directly from the beginning of treatment. Therapeutic Drug Monitoring (TDM) of all classes of antibiotics is needed in ICUs that treat critically ill patients at risk of glomerular hyperfiltration.