Diagnostic biomarkers have been successfully implemented in several fields of medicine (e.g., D-dimers in pulmonary embolism, natriuretic peptides in acute heart failure, and troponin in myocardial infarction). However, the timely diagnosis of bacterial infections remains a challenge [1]. Reliable clinical and/or microbiological parameters from easy to obtain specimens that may be used to diagnose bacterial infections and rule out other infections are mostly lacking. The main disadvantages of many current microbiological methods are diagnostic delays, such as those that occur with culture methods, suboptimal sensitivity for samples like blood cultures, and low specificity due to contamination in samples like sputum cultures, whereas others, such as lung biopsies, are not amenable to routine diagnostics due to their invasive nature. Furthermore, inflammatory markers, including C-reactive protein (CRP) and white blood cells (WBC), lack specificity for bacterial infections [2].

Recent advances in biotechnology and the sequencing of the human genome offer unprecedented opportunities to increase our understanding of critical illness and injury [3]. The early detection of infection has both physiologic and clinical importance. The earlier an injurious process can be identified, the more timely additional preventive (i.e., removal of a stimulus for injury) and potential therapeutic measures can be initiated [4].

Procalcitonin (PCT) has emerged as a promising marker for the diagnosis of bacterial infections because higher levels of PCT are found in severe bacterial infections relative to viral infections and nonspecific inflammatory diseases. Hence, PCT may be used to support clinical decisions regarding the initiation or discontinuation of antibiotic therapy [5,6]. The clinical utility of serum PCT levels continues to evolve [7]. PCT is regarded as a promising candidate marker for making a diagnosis and antibiotic stewardship in patients with systemic infections [1].

Importantly, as with any diagnostic tool, PCT use should be embedded in clinical algorithms adapted to the type of infection and the clinical context and setting. While optimal PCT level cutoffs have been established for some types of infections and clinical settings and their safety and efficacy shown in randomized-controlled intervention trials, for other types of infection only observational studies are available. Thus, the clinical benefit and safety of using PCT remains undefined [8].

The aim of this review is to summarize the current evidence for PCT in different infections and clinical settings, to discuss the strengths and limitations of PCT, and to summarize the reliability of this marker when used with validated diagnostic algorithms.

Although PCT is the prohormone for calcitonin, its biologic activities are distinctly different [9]. In the C cells of the thyroid gland and K cells of the lung, elevated serum calcium concentrations or neoplastic changes result in transcription of the PCT gene. Subsequently, ribosomal synthesis of the 116-amino-acid PCT molecule occurs, with subsequent cleavage of amino acids 60 to 91 yielding calcitonin. Calcitonin's only recognized biologic activity is to lower the serum calcium concentration by inhibiting bone resorption [7]. Hormones are exclusively produced in endocrine cells and act systemically. Cytokines are produced by multiple cells and have local effects.

Hormokines can exhibit either classical hormonal expression, or, upon inflammatory stimulation, show more cytokine-like behavior [10,11]. Ubiquitous inflammatory release can be induced directly via microbial toxins (e.g., endotoxin), or indirectly via a humoral or cell-mediated host response (e.g., interleukin [IL]-1β, tumor necrosis factor [TNF]-α, and IL-6). Parenchymal cells, including liver cells, kidney cells, adipocytes, and muscle cells, provide the largest tissue mass and principal source of circulating hormokines in sepsis [11].

PCT is markedly elevated (up to 5,000-fold) within 2 to 4 hours in severe forms of systemic inflammation or in bacterial infections, and the level persists until recovery [7,12,13]. The biological half-life of PCT is 22 to 26 hours, an advantageous time point compared with CRP and other acute-phase reactants [14]. Distinct from CRP and other acute-phase reactants, existing data suggest that PCT levels rarely increase in response to viral infections, indicating that PCT may be useful for discrimination between bacterial and viral infections. The lack of viral response is postulated to result from the virus-stimulated synthesis of α-interferon by macrophages, which, in turn, inhibits TNF synthesis. The predictive value of PCT has been tested in several studies and in a recent prospective, multicenter study (Fig. 1) [15]. PCT is useful not only for monitoring bacterial infections but also for the differential diagnosis of systemic inflammatory response syndrome, which is a serious medical condition [15,16]. There are likely other biologic roles for PCT, beyond signaling bacterial invasion, which require further study [7,12].

Although it would be highly desirable, there is no research or clinical assay that exclusively detects the 116-kDa PCT peptide. Depending on the type of assay, all tests detect various portions of several computed tomography precursors (Table 1). Based on a highly sensitive research assay, the normal level of PCT in an uninfected individual is 0.033 ± 0.003 ng/mL. The first commercial PCT assay (LUMI test) has a functional lower limit of sensitivity of 0.5 ng/mL. The second-generation Food and Drug Administration (FDA)-approved PCT assay is technically a time-resolved cryptate emission immunoassay. The assay quantifies both PCT and part of the N-terminal end of the PCT molecule. The functional lower limit of sensitivity is 0.05 ng/mL, and the assay has reliable linear quantitation to 1,000 ng/mL. Either serum or plasma is used, and results are available in 1 hour or less [7,17].

The clinical evaluation of PCT levels continues. At present, there are four common uses of PCT levels. First, the current immunoassay was approved by the FDA for establishing the likelihood of mortality in critically ill septic patients [15]. Second, PCT levels have been used to guide empirical antibacterial therapy in patients with acute exacerbations of chronic bronchitis, community-acquired pneumonia (CAP), and sepsis [1,18]. Third, PCT levels, along with standard clinical parameters, can assist in determining whether the patient's empirical antibacterial therapy is effective [1]. Finally, the most useful application is the use of sequential PCT levels to determine when there is no longer a need for antibacterial therapy [7,19].

PCT levels may help resolve the etiology of fever in patients with fever of unknown origin (FUO) syndrome, in that PCT levels do not increase in some disease entities that cause FUO syndrome (e.g., Still's disease, systemic lupus erythematosus, and inflammatory bowel disease) [18,37,38].

Initial data indicate that PCT levels are unaffected by patient use of nonsteroidal anti-inflammatory agents or glucocorticoids. If so, PCT levels remain a valuable marker of the host inflammatory response even when nonsteroidal anti-inflammatory drugs and pharmacologic doses of corticosteroids have altered the patient's temperature curve, WBC count, and WBC differential [24].

A number of limitations exist in using PCT as a marker of infection and sepsis. Nonspecific elevations in PCT levels in the absence of a bacterial infection can occur in situations of massive stress, such as after severe trauma and surgery, or in patients with cardiac shock [1,17]. This is the reason why the power of PCT to discriminate between sepsis and sterile inflammation is better for medical than for surgical patients Also, various other causes of nonbacterial systemic inflammation have been reported, including birth stress in newborns, heat shock, and acute graft-versus-host disease, as well as different types of immunotherapy, such as granulocyte transfusions, the administration of antilymphocyte globulin or anti-CD3 antibodies, and therapy with cytokines or related antibodies (IL-2 or TNF-α) [39]. Some autoimmune diseases such as Kawasaki disease or different types of vasculitis and paraneoplastic syndromes are also associated with elevated PCT levels [24].

Among the numerous markers of sepsis and infection that have been proposed, PCT is by far the most widely evaluated. This marker may help physicians make a diagnosis earlier, differentiate infectious from sterile causes of severe systemic inflammation, and assess the severity of systemic inflammation caused by bacterial infections.

There is a lot more to learn about PCT. The use of PCT, like any biomarker, should be considered within the context of the clinical workup and should take into account all patient related and therapy related factors that may interfere with the initial magnitude and course of this parameter.