Clinical Progress Note: Procalcitonin in the Diagnosis and Management of Community-Acquired Pneumonia in Hospitalized Adults

Community-acquired pneumonia (CAP) accounts for more than 1.5 million adult hospitalizations and 100,000 deaths each year in the United States.¹ Antibiotic overuse in the hospital setting is an important contributor to the rise of antibiotic resistance, prompting increased efforts to limit inappropriate antibiotic use in hospitals.² Procalcitonin, a precursor of the hormone calcitonin, is upregulated in bacterial infections and downregulated in viral infections. The US Food and Drug Administration has approved it as a serum biomarker to assist clinicians with decisions about using antibiotics.³

There is no consensus on how to best use procalcitonin in the management of CAP. We provide a practical update that includes a review of recent literature, added secondary analysis, and expert opinion surrounding the use of procalcitonin in the diagnosis and management of CAP in hospitalized adults.

Initial procalcitonin levels do not sufficiently exclude bacterial etiologies of CAP to withhold antibiotic prescription safely. The largest diagnostic accuracy study of procalcitonin in the diagnosis of CAP was a subanalysis of the Etiology of Pneumonia in the Community Study.⁴ A total of 1,735 adults hospitalized with CAP received procalcitonin testing along with systematic pathogen testing. The area under the receiver operating characteristic curve for procalcitonin in discriminating bacterial pathogens from viral pathogens was 0.73 (95% CI, 0.69-0.77). A procalcitonin cut-off of 0.1 ng/mL resulted in 80.9% (95% CI, 75.3%-85.7%) sensitivity and 51.6% (95% CI, 46.6%-56.5%) specificity for identification of any bacterial pathogen.

In a secondary analysis of this study, we calculated multilevel likelihood ratios (LRs) for ranges of procalcitonin values to determine the diagnostic accuracy of procalcitonin in distinguishing bacterial from viral etiologies of CAP (Table). Multilevel LRs offer more useful diagnostic information than dichotomizing at specified cut-points.⁵ A procalcitonin result less than 0.1 ng/mL has a negative LR of 0.4 (95% CI, 0.3-0.5), which is not low enough to rule out bacterial CAP effectively when starting with intermediate or high pretest probability. For a low result (<0.1 ng/mL) to be useful in ruling out bacterial CAP, for example having less than a 10% posttest probability of bacterial CAP, the pretest probability would have to be no greater than 22%. Even then, a 10% posttest probability of bacterial CAP may still be too high for clinicians to withhold initial antibiotics. For procalcitonin values between 0.1 ng/mL and 1.0 ng/mL, the probability of bacterial CAP does not change significantly, with an LR of 1.0 (95% CI, 0.8-1.3). Procalcitonin values up to 5 ng/mL reach a modest positive LR of 2.3 (95% CI, 0.8-4.3). Very high values, such as those >10 ng/mL, yield a positive LR of 5.5 (95% CI, 3.2-9.7), are potentially useful in decisions to initiate antibiotics in situations of very low pretest probability of bacterial CAP. For example, a 9% pretest probability of bacterial CAP is likely below many physicians’ threshold for starting antibiotics. A procalcitonin of 12 ng/mL in this patient would increase the posttest probability to 35%, a value that would prompt many physicians to initiate antibiotics.

Overall, there is insufficient evidence to support the use of procalcitonin as a stand-alone test for ruling out bacterial CAP, limiting its use in withholding antibiotics in patients with suspected bacterial CAP.