CRP and lower respiratory tract infections

CRP and lower respiratory tract infections

CRP testing guides antibiotic prescribing and management of lower respiratory tract infections

CRP aids the initial diagnosis, management, and monitoring of lower respiratory infections (LRTIs), including community-acquired pneumonia, acute bronchitis, and exacerbations of chronic obstructive pulmonary disease (COPD). When CRP is measured at the initial visit and used together with the clinical examination of the patients, it aids differentiation of bacterial and viral infections and decision making whether antibiotics are needed or not. Correct and rational use of antibiotics is crucial in decelerating the development of antimicrobial resistance (AMR). Therefore, antibiotics should be prescribed only to the patients who benefit from the treatment.

Clinical guidelines in Europe recommend conventional or CRP point of care testing together with the clinical assessment to assist diagnosis and antibiotic prescribing for LRTIs in primary care.1-3

Recommended CRP cut-offs for diagnosis of LRTIs in adults1-3

CRP value and presence of bacterial infectionRecommendation for antibiotics

<20 mg/l

Bacterial infection unlikely
Antibiotics discouraged
20-100 mg/l
Bacterial infection is possible
Consider delayed antibiotic prescription to be started if symptoms become worse
>100 mg/l
Bacterial infection very likely
Antibiotics recommended

The Swedish guideline for LRTIs recommends similar CRP cut-offs to guide antibiotic prescribing and suggests probable pneumonia diagnosis if the symptoms of infection have lasted for a week, and CRP is > 50 mg/l4.

CRP cut-offs are a useful aid for antibiotic prescribing but should always be interpreted in the context of the patients’ clinical picture. Especially in high-risk patients, antibiotics may be required, although CRP levels are not significantly increased. CRP should be measured 1-2 days after the initial onset of the infection to reflect the true disease severity.

CRP is useful in monitoring disease course and treatment response

Serial measurements of CRP are suitable for the evaluation of the course of infection and efficacy of antibiotic treatment5. CRP has a half-life of 19h, and CRP levels decrease as a response to antibiotic treatment. The clinical guideline by the European Respiratory Society and European Society of Clinical Microbiology and Infectious Diseases recommends CRP testing on hospitalized pneumonia patients on admission on day 1 and at day 3 or 41. If CRP levels do not show a decline after a few days of antibiotic treatment, it may suggest a treatment failure. CRP levels, especially at day 4, have a high negative predictive value for pneumonia complications6.


  1. Woodhead M et al. Guidelines for the management of adult lower respiratory tract infections. Clin Microbiol Infec 2011; 17:E1-E9.
  2. National Institute for Health and Care Excellence (NICE). NICE guideline - Pneumonia in adults: diagnosis and management, 2014.
  3. NICE guideline – Cough (acute): antimicrobial prescribing, 2019.
  4. Andre M, Hedin K, Mölstad S. Nedre luftvägsinfektioner i primärvården. Rekommendationer för handläggning läkemedelsverkets expertgrupp. Lakartidningen 2009; 106: 1660-1663.
  5. Bruns AH, Oosterheert JJ, Hak E et al. Usefulness of consecutive C-reactive protein measurements in follow-up of severe community-acquired pneumonia. Eur Respir J 2008; 32(3): 726-732.
  6. Chalmers JD, Singanayagam A, Hill AT. C-Reactive Protein Is an Independent Predictor of Severity in Community-acquired Pneumonia. Am J Med 2008; 121(3):219-225.