Procalcitonin as a marker of sepsis

Abstract

Prompt diagnosis and treatment with appropriate antimicrobial chemotherapy is of paramount importance to reduce morbidity and mortality associated with sepsis. Inflammatory markers currently in use, such as C-reactive protein (CRP) do not reliably differentiate between the systemic inflammatory response and sepsis. Procalcitonin (PCT), a precursor of calcitonin, is a 116 amino acid protein that has been proposed as a marker of disease severity in conditions such as septicaemia, meningitis, pneumonia, urinary tract infection (UTI) and fungal and parasitic infection. In particular, serial measurements are useful in order to monitor response to therapy. Together with good clinical judgement and judicious use of antimicrobial agents, PCT should serve as a valuable adjunct in the diagnosis and management of sepsis.

Introduction

The term sepsis is used to define the systemic inflammatory response to an infectious agent: bacterial, viral, fungal or parasitic. Sepsis is a significant cause of morbidity and mortality, particularly in the immunocompromised, elderly and critically ill. It is the leading cause of death on intensive care units. In an attempt to clarify and standardise the terminology used in sepsis, some definitions have been agreed to assist researchers and clinicians dealing with sepsis and its sequelae [1]. Septic shock is a less precise term, as it may be divided into early septic shock (responds to intravenous fluids and/or pharmacological interventions), or refractory septic shock (lasting for more than 1 h despite intravenous fluids and pharmacological intervention, and requiring inotropic support). Variable terminology is used in this review because of the terms used in the cited references. However, the terminology used is defined in Table 1 and has been adapted for use in paediatrics [2].

The rapid detection of sepsis in an ill patient is of paramount importance in order to institute the prompt administration of appropriate antimicrobial chemotherapy. Various markers of inflammation such as IL-1β, TNF-α, IL-6 and IL-8 have all been studied. These cytokines are well recognised to correlate with disease severity in sepsis [3], [4], [5]. However, they are neither sensitive nor specific enough, are time consuming and expensive to perform.

Procalcitonin (PCT) is a precursor of calcitonin, and is a 116 amino acid protein with a molecular mass of 13 kDa. It undergoes successive cleavages in the neuroendocrine cells of the thyroid, lung and pancreas to form three distinct molecules; calcitonin (32 amino acids), katacalcin (21 amino acids) and an N-terminal fragment called aminoprocalcitonin (57 amino acids) (Fig. 1). The first description of elevated serum PCT concentrations in sepsis was by Assicot in 1993 [6]. This article describes high concentrations of a substance with calcitonin-like immunoreactivity in patients with various bacterial and viral infections. Serum PCT levels decreased rapidly during antibiotic therapy. Concentrations of mature calcitonin were normal in all subjects irrespective of the PCT concentration.

Bacterial lipopolysaccharide (LPS) has been shown to be a potent inducer of PCT release into the systemic circulation. This release is not associated with an increase in calcitonin. PCT levels increase from 3 to 4 h, peak at about 6 h and then plateau for up to 24 h [7], [8]. It is degraded by specific protease and has a half-life of between 25 and 30 h [9]. In contrast, C-reactive protein (CRP) levels rise between 12 and 18 h after bacterial challenge.

In healthy individuals circulating levels of PCT are very low, usually below 0.1 ng/ml. In viral infections and inflammatory states, PCT concentrations are slightly elevated up to 1.5 ng/ml, but in bacterial infection levels may exceed 1000 ng/ml [6], [8], [10]. This 3–5 log-fold increase makes it an ideal marker of bacterial sepsis.

The exact sites of PCT production are unknown but it is thought that the liver is a major site. The evidence for this comes from a study that showed that hepatocytes produced large quantities of PCT following stimulation with TNF-α and IL-6 [11]. Another study has demonstrated expression of PCT in peripheral blood mononuclear cells (PBMCs) following stimulation with LPS. This expression appears to be modulated by LPS and the pro-inflammatory cytokines TNF-α, IL-1β, IL-2 and IL-6 [12]. The finding of high PCT levels in patients who have undergone thyroidectomy makes thyroid origin unlikely [6].

The precise physiological role of PCT is unclear. One study has shown that increased concentrations exacerbate mortality in an animal model, whereas neutralisation with polyclonal antibody increases survival [13]. It suggests that PCT acts as a mediator that sustains and augments the inflammatory response in a manner similar to IL-6 and IL-8, and that it is integral to the host response and to the ultimate outcome of sepsis. The authors raise the possibility that PCT might be a potential target for therapeutic blockade. It would be unlikely phylogenetically that a substance be released in sepsis by a host in response to an invading organism that caused it to succumb to the organism. A reasonable hypothesis is that PCT may function in a similar manner to TNF-α having a beneficial effect in small quantities but detrimental effects in excessive quantities.

The assay most widely used is commercially available (Brahms Diagnostica, Berlin, Germany) and utilises a sandwich immunoluminometric method. Two antigen specific monoclonal antibodies are used, one of which binds the C-terminal region (katacalcin) and the other, which is fluorescent labelled with acridinium ester, binds calcitonin. The inner surface of the tube is pre-coated with katacalcin antibody, which binds to PCT in the patient sample, and this in turn binds to the luminescent labelled antibody creating a ‘sandwich complex’. The intensity of the signal is measured using a luminometer. This is directly proportional to the concentration of PCT and results are calculated from a standard curve. The detection limit is 0.1 ng/ml. The assay requires 20 μl of plasma and can be performed within 2 h.

A semiquantitative immunochromatographic bedside test is also available from the same company [14]. In this assay, 200 μl of EDTA plasma is added to the test strip. PCT in the sample binds to mouse anti-katacalcin antibody complexed to colloidial gold. This complex moves by capillary action through the test strip and the sandwich complex can be seen as a reddish band. The colour intensity is proportional to the PCT concentration. There are four bands; <0.5 ng/ml, 0.5–2 ng/ml, >2–10 ng/ml, and >10 ng/ml. This stratification (depending on which study is used as a reference criterion) would be translated to negative (<0.5); consistent with no infection or viral infection, positive (0.5–2) and positive (2–10); both consistent with bacterial or viral infection, and positive (>10); consistent with bacterial infection.