Fig. 3— Procalcitonin (PCT)-guided antibiotic (AB) stewardship. Cut-off ranges of PCT were derived by calculating multilevel likelihood ratios and optimised for the setting of an emergency room and hospital. The use of a sensitive assay (Kryptor® PCT; Brahms, Hennigsdorf, Germany) assured adequate sensitivity. Briefly, use of antibiotics was more or less discouraged (<0.1 μg·L−1 or <0.25 μg·L−1) or encouraged (>0.5 μg·L−1 or >0.25 μg·L−1) based on a range of PCT levels. The same cut-offs were used regardless of whether or not patients had been pre-treated with antibiotics prior to admission to the emergency department. In patients with very high PCT values on admission (e.g. >10 μg·L−1), discontinuation of antibiotic therapy was encouraged if levels decreased to <80–90% of the initial value. In patients with an initial PCT level >10 μg·L−1 and smaller reductions during follow-up, continuation of antibiotic treatment was encouraged. Re-evaluation of the clinical status and measurement of serum PCT levels was recommended after 6–24 h in all persistently sick and hospitalised patients in whom antibiotics were withheld. As indicated above, the PCT algorithm should be overruled in patients with immediately life-threatening disease. However, in these patients, despite the immediate administration of intravenous broad-spectrum antibiotic therapy, a much more likely noninfectious differential diagnosis responsible for the critical illness must be strongly considered and actively sought, especially if PCT levels remain very low during follow-up. Physicians were advised that persistently elevated PCT levels may indicate a complicated course, while PCT levels may remain relatively low in localised infections (e.g. empyema or abscess). PCT levels were reassessed on days 3, 5 and 7 in hospitalised patients with ongoing antibiotic therapy, and in patients showing a worsening or delayed recovery of signs and symptoms, and antibiotic was discontinued using the PCT cut-offs defined above. For antibiotic stewardship in a medical or, especially, surgical intensive care unit (ICU), modified cut-off ranges might be necessary. Because mean PCT levels are increased in a cohort of critically ill patients as compared with patients in an emergency room or hospital setting, the optimal thresholds of the cut-off range are likely to be higher, especially in a immediately post-operative or post-traumatic situation. CAP: community-acquired pneumonia; PSI: pneumonia severity index; CURB65: severity score for CAP based on confusion, urea nitrogen, respiratory rate, blood pressure and age ≥65 yrs; COPD: chronic obstructive pulmonary disease; GOLD: Global Initiative for Chronic Obstructive Lung Disease staging system for COPD; Sa,O2: arterial oxygen saturation.