Table 1 Eligibility criteria

Inclusion criteria

≥ 18 years of age

A primary clinical diagnosis in the ED of acute LRTI (< 28 days duration) ^a

Clinician willing to consider procalcitonin in antibiotic decision making

Exclusion criteria

Conditions where physicians are unlikely to withhold antibiotics

 Systemic antibiotics before ED presentation

  a. All prophylactic antibiotic regimens, or

  b. Received >1 dose within 72 h prior to ED presentation

 Current vasopressor use

 Mechanical ventilation (via endotracheal tube)

 Known severe immunosuppression ^b

 Accompanying non-respiratory infections

 Known lung abscess or empyema

Conditions where PCT can be >0.25 μg/L without infection

 Chronic dialysis

 Metastatic cancer

 Surgery in the past 7 days (excluding minor surgery such as skin biopsy)

Conditions rendering follow-up difficult

 Incarcerated or homeless

 Enrolled in ProACT in the past 30 days