How well does procalcitonin distinguish bacterial from viral causes of community-acquired pneumonia in hospitalized patients?

Not extremely well! Although a recent multicenter prospective study in adult hospitalized patients reported that the median procalcitonin (PCT) concentration was significantly lower for community-acquired pneumonia (CAP) caused by viral pathogens ( 0.09 u/ml vs atypical bacteria [0.2 ug/ml] and typical bacteria [2.5 ug/ml]),  PCT was <0.1 ug/ml and <0.25 ug/ml  in 12.4% and 23.1% of typical bacterial cases, respectively1

This means that we could potentially miss about a quarter of CAP cases due to typical bacterial causes if we use the <0.25 ug/ml threshold (<0.20 is ug/ml has been used to exclude sepsis2). For these reasons and based on the results from another study3, no threshold for PCT can reliably distinguish bacterial from viral etiologies of CAP4.  Clinical context is essential in interpreting PCT levels! Also go to a related pearl on this site5.

Can PCT distinguish Legionella from other atypical bacterial causes of CAP (eg, caused by Mycoplasma or Chlamydophila)? The answer is “maybe”! Legionella was associated with higher PCT levels compared to  Mycoplasma and Chlamydophila in one study1, but not in another3.


  1. Self WH, Balk RA, Grijalva CG, et al. Procalcitonin as a marker of etiology in adults hospitalized with community-acquired pneumonia. Clin Infect Dis 2017;65:183-90.
  2. Meisner M. Update on procalcitonin measurements. Ann Lab Med 2014;34:263-73.
  3. Krüger S, Ewig S, Papassotiriou J, et al. Inflammatory parameters predict etiologic patterns but do not allow for individual prediction of etiology in patients with CAP-Results from the German competence network CAPNETZ. Resp Res 2009;10:65.
  4. Bergin SP, Tsalik EL. Procalcitonin: the right answer but to which question? Clin Infect Dis 2017; 65:191-93.
How well does procalcitonin distinguish bacterial from viral causes of community-acquired pneumonia in hospitalized patients?

Should I order serum procalcitonin on my patient with suspected infection?

Two things to ask before you order procalcitonin (PCT): 1. Will it impact patient management?; and 2. If so, will the result be available in a timely manner ie, within hours not days?

Whatever the result, PCT should always be interpreted in the context of the patient’s illness and other objective data. Not surprisingly then, as a “screening” test, PCT may be more useful in patients with low pre-test likelihood of having bacterial infection, not dissimilar to the use of D-dimer in patients with low pre-test probability of pulmonary embolism1.  

Several potential clinical uses of this biomarker have emerged in recent years,  including:1,2

  • Helping decide when to initiate antibiotics in patients with upper acute respiratory tract infections and bronchitis. A normal or low PCT supports viral infection.
  • Helping decide when to discontinue antibiotics (ie, when PCT normalizes) in community-acquired or ventilator-associated pneumonia.
  • Helping monitor patient progress with an expected drop in PCT of about 50% per day (half-life ~ 24 hrs) with effective therapy.

Few caveats…

  • PCT may be unremarkable in about a third of patients with bacteremia (especially due to less virulent bacteria, including many gram-positives)3.  
  • PCT levels are lowered by high-flux membrane hemodialysis, so check a baseline level before, not after, hemodialysis4.
  • Lastly, despite its higher specificity for bacterial infections compared to other biomarkers such as C-reactive protein, PCT may be elevated in a variety of non-infectious conditions, including pancreatitis, burns, pulmonary edema or aspiration, mesenteric infarction (ischemic bowel), cardiogenic shock, and hypotension during surgery2.



  1. Schuetz P, Muller B, Chirst-Crain M, et al. Procalcitonin to initiate or discontinue antibiotics in acute respiratory tract infections (review). Evid-Based Child Health (A Cochrane Review Journal) 2013;8:4;1297-137.
  2. Gilbert GN. Use of plasma procalcitonin levels as an adjunct to clinical microbiology. J Clin Microbiol 2010;48:2325-29.
  3. Yan ST, Sun LC, Jia HB. Procalcitonin levels in bloodstream infections caused by different sources and species of bacteria. Am J Emerg Med 2017;35:779-83.
  4. Grace E, Turner RM. Use of procalcitonin in patients with various degrees of chronic kidney disease including renal replacement therapy. Clin Infect Dis 2014;59:1761-7.
Should I order serum procalcitonin on my patient with suspected infection?