Patient-Oriented Evidence That Matters

C-Reactive Protein Is the Most Useful Biomarker for Diagnosis of Outpatient CAP


Am Fam Physician. 2020 Oct 15;102(8):online.

Clinical Question

How accurate are biomarkers C-reactive protein (CRP), procalcitonin, and leukocytosis in the diagnosis of community-acquired pneumonia (CAP) in adults with respiratory symptoms?

Bottom Line

This review found that the best evidence supports CRP as the most useful and accurate biomarker for diagnosing CAP in adults presenting with lower respiratory tract symptoms. Procalcitonin has a good positive likelihood ratio (LR+) but a minimally useful negative likelihood ratio (LR−). Leukocytosis has only modest accuracy that is minimally clinically useful. (Level of Evidence = 1b)


The investigators searched PubMed and reference lists of pertinent articles for studies that evaluated the accuracy of readily available biomarkers in adult patients who presented with symptoms of acute respiratory infection and those with clinically suspected pneumonia. The authors included studies with at least one biomarker test and an acceptable reference standard (chest radiography or computed tomography) performed on all participants. They did not exclude any studies based on country, year, or language. Two or more individuals independently reviewed potential studies for inclusion and risk of bias using a standard evaluation tool. Discrepancies were resolved through consensus discussion. The authors ended with 14 studies with a total of 6,599 patients. One-half of the studies occurred in the emergency department, and one-half occurred in the primary care setting. Of the 14 studies, eight were scored at low risk of bias, with the remaining six studies considered at moderate risk of bias.

CRP was the most accurate test (less than 1 mg per dL [10 mg per L]; LR− = 0.27, and greater than 2 mg per dL [20 mg per L], 5 mg per dL [50 mg per L], and 10 mg per dL [100 mg per L]; LR+ = 2.08, 3.68, and 5.79, respectively). Procalcitonin had a good LR+ (greater than 0.25 mcg per L and 0.50 mcg per L; LR+ = 5.43 and 8.25, respectively) but a minimally useful LR−. Leukocytosis was

POEMs (patient-oriented evidence that matters) are provided by Essential Evidence Plus, a point-of-care clinical decision support system published by Wiley-Blackwell. For more information, see http://www.essentialevidenceplus.com. Copyright Wiley-Blackwell. Used with permission.

For definitions of levels of evidence used in POEMs, see http://www.essentialevidenceplus.com/product/ebm_loe.cfm?show=oxford.

To subscribe to a free podcast of these and other POEMs that appear in AFP, search in iTunes for “POEM of the Week” or go to http://goo.gl/3niWXb.

This series is coordinated by Sumi Sexton, MD, editor-in-chief.

A collection of POEMs published in AFP is available at https://www.aafp.org/afp/poems.



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