Procalcitonin (PCT), the precursor of calcitonin, consists of 116 amino acids and it barely detectable in serum under normal conditions [1]. However, in the presence of a bacterial or fungal infection or sepsis, PCT is produced by various organs, included the thyroid, and released into blood [2]. Measurements of serum PCT concentration are useful for diagnosing bacterial infections and serum PCT levels can aid in the differential diagnosis of systemic inflammatory response syndrome (SIRS) [3]. Furthermore, the rapidity of PCT testing can facilitate diagnosis and prompt treatment [4].

PCT is elevated at 3 to 6 hours after systemic bacterial infection, but not by local infection or viral infection, non-infectious inflammatory responses, autoimmune disease, or sepsis. Thus, it would appear that PCT testing could increase the specificity of the diagnosis of a bacterial infection more than testing for other inflammatory markers, such as C-reactive protein (CRP) or leukocyte counts [5,6,7]. However, other conditions, not associated with infections, such as early postoperative state, trauma, severe burns, leukopenia, and renal dysfunction can also increase serum PCT [8,9]. Additionally, it has been shown that serum PCT levels are high in patients with reduced renal function and that these levels are reduced after dialysis [10,11]. Accordingly, it seems unreasonable to apply the same PCT cutoff value to all patients.

The aims of this study were to assess the potential use of serum PCT as a diagnostic biomarker of bacterial infection in end-stage renal disease (ESRD) patients and to determine an appropriate cutoff level.

The inflammatory response manifests as symptoms of fever, redness, swelling, and pain due to a localized reaction against body tissue damage or infection and proceeds with inflammatory mediators and monocytes and neutrophils within 30 to 60 minutes after inflammation development [5,12]. The degree of inflammation is assessed by measuring levels of an acute phase reactant in blood. CRP is often used as an indicator clinically, but it is difficult to differentiate between the signs and symptoms of inflammation and infection. Serum PCT levels increase from 3 to 6 hours after a bacterial infection or sepsis, but do not increase significantly in patients with viral infections, local infections, non-infectious inflammation, or autoimmune diseases. Thus, PCT aids in the diagnosis of bacterial infections, and has higher sensitivity and specificity than CRP [4,5,7].

Furthermore, serum PCT can aid in the differentiation of bacterial pneumonia from infections caused by viruses and atypical pathogens, and from tuberculosis and also provides guidance regarding antibiotic treatment [13,14,15]. When the serum PCT level is ≤ 0.5 ng/mL in SIRS patients, a systemic bacterial infection can be excluded, but if > 2.0 ng/mL, a systemic infectious process is strongly suggested. A level of > 10 ng/mL indicates critical sepsis or septic shock, and a level of ≤ 2.0 ng/mL, indicates the need for re-examination after 6 to 24 hours even if bacterial infection or sepsis is suspected [15]. In the present study, the iESRD and cESRD group had significantly different mean PCT levels (0.50 ± 0.49 ng/mL vs. 2.95 ± 3.67 ng/mL).

Serum PCT concentrations were measured in the cESRD group because serum PCT levels depend on the severity of infection. Serum PCT levels were increased significantly in males compared to females, but further larger studies are needed to confirm this. Serum PCT was slightly higher in those older than 60 years and with hypertensive renal disease patients, but no significant difference was found by age or renal failure etiology. The mean serum PCT levels in SIRS and non-SIRS patients were similar at 2.96 ± 4.25 and 2.93 ± 2.74 ng/mL, respectively. Thus, a higher serum PCT level does indicate the presence of SIRS (p = 0.986). Kim et al. [15] concluded that serum PCT was unable to determine SIRS severity: that is, sepsis, severe sepsis, or septic shock. However, it could differentiate sepsis and severe sepsis [15]. However, we did not separate sepsis and severe sepsis for the analysis in the present study.

Serum PCT is a recognized indicator of bacterial infection, but in patients with multiple injuries, in patients in the early stages after a critical surgery, and in severe burn cases, PCT levels can be elevated regardless of infection. Levels can also increase after a severe cardiogenic shock or during an organ perfusion disorder [8,9]. Likewise, patients with chronic renal insufficiency have elevated PCT levels that decrease after dialysis [10,11,16]. Thus, the use of a PCT cutoff for differentiating healthy individuals from patient is impractical.

One study on serum PCT in ESRD patients on dialysis obtained a mean value of 0.69 ± 0.81 ng/mL before dialysis, and a level higher than the standard value of 0.5 ng/mL in 57% of patients [17]. Dahaba et al. [16] measured the PCT values of terminal renal failure patients that had not reached a state of septicemia. Measurements were taken before dialysis, and PCT levels were also higher than those of normal healthy controls. The expected lowest value for serum PCT levels was 1.5 ng/mL. In a recent study, it was reported that mean serum PCT decreased to 83% ± 25% after high-flux dialysis, but did not decrease significantly after low-flux dialysis [18]. Furthermore, PCT levels differed according to the method of dialysis [10]. In many studies, dialysis patients without infection had PCT levels slightly higher than normal people due to the presence of procedure-related inflammation (for example, an invasive procedure for HD or PD), decreased renal clearance, or an increase in peripheral blood mononuclear cell counts [19].

Regarding the reference value of serum PCT in healthy individuals, at a cutoff of 0.5 ng/mL, PCT levels had a sensitivity and specificity of 100.0% and 65.0%, respectively, for infection and 100.0% and % for SIRS. Herget-Rosenthal et al. [19] used serum PCT to diagnose severe infection or septicemia. PCT was measured before dialysis and its sensitivity and specificity were 89% and 81%, respectively. Specificity was relatively low in the present study. We used ROC curve analysis to evaluate the ability of PCT to determine the presence of infection. When a cutoff of 0.75 ng/mL was applied, its sensitivity and specificity were 76.2% and 80.0% for infection and 100.0% and 60.6% for SIRS, respectively.

Several variables differed significantly in the iESRD group and correlations were found between WBC and CRP and serum PCT. WBC and CRP are known indices of infection and were positively correlated with PCT, whereas serum albumin, a known indicator of nutritional status, was negatively correlated with serum PCT [20]. The present study had some limitations. Dialysis patients without infection were chosen as controls but there was no healthy normal control group. Moreover, the sample size was relatively small.

In summary, serum PCT levels were significantly higher in dialysis patients with a bacterial infection, demonstrating the utility of serum PCT as an index of infection. Additionally, dialysis patients without infection had higher PCT levels than the 'standard' cutoff of 0.5 ng/mL. Thus, it appears to be inappropriate to apply this cutoff to patients with renal dysfunction. The present study shows that a serum PCT cutoff of 0.75 ng/mL is more appropriate for the diagnosis of infection in ESRD patients.