Community-Acquired Pneumonia Severity Scores
Published October 11, 2026 · Last updated October 11, 2026
How the CURB-65, CRB-65, and PSI/PORT scores stratify community-acquired pneumonia severity and support site-of-care decisions — what variables each uses and where its evidence comes from.
Community-acquired pneumonia (CAP) severity scores convert bedside findings and routine test results into a risk category. Their shared purpose is to support site-of-care decisions — who can be managed at home, who needs a ward bed, and who may need intensive care — but each does so with a different set of variables and a different underlying model.
The library contains three CAP severity instruments: CURB-65, CRB-65, and the PSI/PORT score. This guide explains which step each one belongs to, what it measures, and where the evidence behind it comes from.
Why several CAP severity scores exist
Severity scores differ in the data they require and in the populations and outcomes against which they were derived. A score that can be applied without laboratory results answers a different practical question from a weighted model that uses many variables. Knowing which tool was built for which setting is part of using it well.
CURB-65: five bedside variables
The CURB-65 score was derived and validated to define community-acquired pneumonia severity at hospital presentation, using five readily available features to stratify 30-day mortality risk and help guide site-of-care decisions. It relies on data that are typically available at the point of assessment rather than an extended laboratory panel.
Use the CURB-65 calculator to compute the score from its inputs.
CRB-65: the laboratory-free version
The CRB-65 score omits the blood urea nitrogen component of CURB-65 so that it can be applied without laboratory testing, which suits primary care and other settings where a result is not immediately available. It stratifies severity and mortality risk in the same general way, at the cost of one discriminating variable.
PSI/PORT: a weighted, tiered model
The Pneumonia Severity Index (PSI), also called the PORT score, was developed by Fine and colleagues as a prediction rule to identify patients with community-acquired pneumonia who are at low risk of 30-day mortality. It assigns points across demographic, comorbidity, examination, and laboratory or radiographic variables and groups patients into risk classes, so it is more detailed than either CURB-based score.
Choosing a score, and its limits
The three scores are not interchangeable: they were derived in different cohorts, use different variables, and are calibrated to different outcomes. None is a substitute for clinical judgement — a patient who looks unwell should be treated as unwell regardless of a low score, and social and functional factors that affect safe site of care often sit outside the models. For that reason this guide links only the CAP severity instruments in the library rather than padding the list with unrelated calculators.
Calculators in this guide
Related guides
References
- Lim WS, van der Eerden MM, Laing R, et al. Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study. Thorax. 2003;58(5):377-382.
- Lim WS, Baudouin SV, George RC, et al. BTS guidelines for the management of community acquired pneumonia in adults: update 2009. Thorax. 2009;64(Suppl III):iii1-iii55.
- Fine MJ, et al. A prediction rule to identify low-risk patients with community-acquired pneumonia. N Engl J Med. 1997;336(4):243-250.
- Fine MJ, et al. Prognosis and outcomes of patients with community-acquired pneumonia: a meta-analysis. JAMA. 1996;275(2):134-141.
- National Institute for Health and Care Excellence. Pneumonia in adults: diagnosis and management. NICE Clinical Guideline CG191. 2014.