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Novel biomarkers of infection in the critically ill 

Novel biomarkers of infection in the critically ill
Novel biomarkers of infection in the critically ill

David T. Huang

and Ayan Sen

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date: 01 December 2020

Key points

  • There is insufficient evidence for the routine use of novel biomarkers in infection and sepsis.

  • A large number of biomarkers have been studied to date, but most studies only demonstrate correlation with outcome, rather than improvement in outcome.

  • Several biomarkers are of prognostic value with strong associations with poor outcomes and mortality.

  • Procalcitonin guidance to reduce antibiotic usage is promising, but most supporting data come from emergency department studies in a single European country.

  • The future may involve development of high sensitivity assays, molecular strategies and a ‘panel approach’, all of which need to be investigated in well-designed studies.


Over 25% of all annual deaths in the world are due to infection. Sepsis is defined as infection (probable or documented) with systemic manifestations such as systemic inflammatory response syndrome (SIRS) [1]‌. Severe sepsis is defined as sepsis with sepsis-induced organ dysfunction or tissue hypoperfusion. Severe sepsis is common, consumes considerable health care resources, and is associated with high mortality [2]. Surveillance and early response to infection and sepsis depend on rapid clinical diagnosis. The septic response is a complex chain of events involving inflammatory, humoral, cellular, and circulatory abnormalities. Early diagnosis and risk stratification facilitate timely and specific treatment, but are complicated by the highly variable and non-specific nature of the signs and symptoms of sepsis.

Biomarkers can be of value if they can improve medical decision-making by indicating the presence, absence, or severity of infection and sepsis determining aetiology (e.g. bacterial versus viral infection), and distinguishing systemic sepsis from local infection. Other potential uses include prognostication, guiding antibiotic therapy, and evaluating response to therapy. Large numbers of biomarkers have been studied in infection and sepsis. However, in many cases, reliability, validity, and clinical utility have not been fully evaluated, in part due to limitations of experimental models for human sepsis, and lack of a ‘gold standard’ for the diagnosis of infection or sepsis, prognosis of severe infections and sepsis [3]‌.

White blood cells

The white blood cell (WBC) count is the most commonly used infectious biomarker. An elevated WBC is suggestive of an infection, but cannot be used in isolation and can indicate inflammation without infection (‘false positive’) or be absent despite infection in the elderly and immunocompromised (‘false negative’). SIRS criteria include a WBC >14,000 or <4000 cells/mm3, or a bandemia >10% [2]‌.

C-reactive protein

C-reactive protein (CRP) is an acute-phase protein released by the liver after the onset of inflammation or tissue damage. It has been studied and used extensively in clinical practice as a biomarker of infection, inflammation, and sepsis. Its production is controlled by interleukin-6, an inflammatory cytokine. The median concentration of CRP in young adult blood donors is 0.8 mg/L, the 90th percentile is 3.0 mg/L, and the 99th percentile is 10 mg/L. A high-sensitivity CRP (hs-CRP) test measures low levels of CRP using laser nephelometry. The test gives results in 25 minutes with a sensitivity of 0.04 mg/L. Exact sensitivities and specificities of CRP for the diagnosis of bacterial infections vary in different studies and therefore its role should be interpreted based on context [4]‌. CRP level is also elevated during inflammatory states of non-infectious aetiologies, e.g. autoimmune disorders, myocardial infarction. As a result, its role in diagnosis of infection and sepsis is modest at best. Studies of critically-ill patients showed that elevated plasma concentrations of CRP correlate with an increased risk of organ failure and/or death. However, in studies of post-operative sepsis, procalcitonin and IL-6 levels significantly decreased in survivors from days 1 to 14, whereas CRP levels did not. Therefore, its role in prognostication in patients with sepsis is questionable. Elevated hsCRP at a stable phase of health was associated with increased risk of future sepsis events over 4.6 years of follow-up [5].


In normal physiological conditions, pro-calcitonin (PCT) is synthesized by thyroid C cells and serum levels are low (0.1 ng/mL) [4]‌. In bacterial infection, PCT is ubiquitously synthesized in multiple extrathyroidal tissues. PCT levels increase within 4–12 hours upon stimulation, and circulating PCT levels halve daily when the infection is controlled by the host immune system and antibiotic therapy. A sample can be collected and sent to the laboratory with an average turnaround time of less than 3 hours. PCT can, however, increase after trauma or surgery, particularly major abdominal surgery, and in pancreatitis. Some authors report that PCT levels only transiently increase for 12–24 hours after surgery and in the absence of infection, fall back to normal levels. This is in contrast to both CRP and WCC, which can stay elevated for a number of days after surgery without underlying infection. PCT has a half-life of less than 24 hours.

Bacteraemic infections cause the highest rises in PCT with lower or negligible rises in localized, viral, and intracellular bacterial (e.g. Mycoplasma pneumonia) infections. In patients with community-acquired pneumonia (CAP) and urinary tract infections, PCT at a cut-off of 0.1 µg/L had a very high sensitivity to exclude bacteraemia [6,7]. PCT may not only differentiate between viral and bacterial infections, but also indicate the presence of bacterial super-infection in patients with viral diseases. Gram-negative bacteraemia may cause higher PCT rises than Gram-positive bacteraemia.

Six meta-analyses have been performed on the diagnostic accuracy of PCT to detect infection in different patient populations. Some of these meta-analyses identified PCT as being helpful for the diagnosis of clinically or microbiologically documented infection [8,9] whereas others have found moderate or moving towards null effect in the detection of bacteraemia [10]. However past studies did not use the high sensitivity PCT (Kryptor); this may have contributed to some to the discrepant conclusions. Recent guidelines issued by the Infectious Diseases Society of America suggest that use of PCT as an adjunctive diagnostic marker to differentiate sepsis from SIRS of a non-infectious origin may be considered [11]. However, the 2012 Surviving Sepsis Campaign Guidelines state ‘the utility of procalcitonin levels or other biomarkers (such as C-reactive protein) to discriminate the acute inflammatory pattern of sepsis from other causes of generalized inflammation (e.g., post-operative, other forms of shock) has not been demonstrated. No recommendation can be given for the use of these markers to distinguish between severe infection and other acute inflammatory states’ [2]‌.

PCT may have a role in antibiotic stewardship and de-escalation. Thirteen randomized controlled studies, in which 4,395 patients were enrolled, showed a reduction in the prescription of antibiotics of between 74% and 11% and a reduction of days on antibiotics by 13% to 55% [12]. Most of those studies were performed with patients with clinically diagnosed respiratory tract infections in Swiss emergency departments, and less data exist for ICU patients. PCT levels in response to sepsis do not appear to be significantly affected by the use of steroids. PCT can be elevated in renal impairment, severe trauma and surgery or in patients after cardiac shock, acute graft-versus-host disease, immunotherapy, autoimmune diseases, and paraneoplastic syndromes. The role of PCT in systemic fungal infections is unclear. Candida-related severe sepsis or septic shock does not necessarily elicit a substantial increase in serum levels.


Soluble triggering receptor expressed on myeloid cells 1 (TREM-1), a recently discovered member of the immunoglobulin superfamily, is greatly upregulated in infections, but not in non-infectious inflammatory conditions. It has been suggested that plasma sTREM-1 levels as an indicator of sepsis were superior to those of CRP and PCT. A meta-analysis found that the sensitivity of sTREM-1 for the diagnosis of bacterial infection, was 0.82 and that the specificity was 0.86 [13]. Other studies have reported that it is inferior to CRP and PCT.

Coagulation biomarkers

Protein C serum concentrations in neutropenic patients have been described to be significantly decreased, before clinical signs of severe sepsis and septic shock are apparent. Biphasic waveform (BPW) analysis, a new biological test derived from the activated partial thromboplastin time, has been proposed for the diagnosis of sepsis. The major limitation of coagulation parameters is that coagulation pathway abnormalities may be triggered by other disease states, such as trauma, obstetrical disorders, or cancer.

Other biomarkers

The quest for the magic biomarker, the ‘troponin of sepsis’ continues with several other acute phase reactants having failed to show clinical utility beyond a strong correlation with adverse outcomes. These include resistin, ‘long’ pentraxin 3, alpha-1 acid glycoprotein, and hepcidin, pro-inflammatory cytokines like IL-6 and IL-8, TNF-alpha, HMB1, proadrenomedullin, Macrophage migrating inhibitory factor (MgIF) and anti-inflammatory, endothelial, and apoptotic aspects of systemic inflammation, such as interleukin-1 receptor antagonist (IL-1ra) and IL-10. These biomarkers are of limited value because they can be induced by numerous non-infectious diseases like major surgery and major trauma, autoimmune disorders, viral infections, and transplant rejection. In a large US cohort study of subjects hospitalized with community acquired pneumonia, the circulating cytokine response to pneumonia was noted to be heterogeneous with considerable overlap between those who do and do not develop severe sepsis [14].

Panels of biomarkers

Combinations of biomarkers reflecting various aspects of the host response have been proposed to overcome limitations of single biomolecules. A panel consisting of sTREM-1, PCT, and CD64 index called the ‘bioscore’ was higher in patients with sepsis. In the emergency department (ED), the usefulness of a combination of suPAR, sTREM-1, and MgIF with CRP, PCT, and neutrophils was evaluated in patients with SIRS and found to have better sensitivity and specificity than any single marker [15]. Similarly, a biomarker panel of NGAL, IL-1ra, and protein C was predictive of severe sepsis, septic shock, and death in ED patients with suspected sepsis [16].

Molecular diagnostics

Blood culture reflects the current gold standard for the detection of bloodstream infection, since viable micro-organisms isolated from the blood can be analysed to identify species and susceptibility to antimicrobial therapy. However, culture data is impaired by the delay in the time to results and the fact that positive blood cultures can be found for only approximately 30% of severe sepsis patients. A number of molecular approaches to improve conventional culture-based identification, including PCR, have been suggested, such as matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry, which may decrease the time to result to 4 hours. Another alternative strategy is the extraction and amplification of microbial nucleic acids from a positive blood culture and subsequent hybridization on a microarray platform to detect specific bacterial genes [17], which has recently been evaluated in an observational multi-centre design with conventional blood culture as the comparator. The assay had an overall sensitivity of 94.7% and a specificity of 98.8%, and 100% for meticillin-resistant Staphylococcus aureus (MRSA) bacteraemia, and resulted 18 hours faster than conventional blood culture [18]. However, shortcomings include an incomplete coverage of pathogens, the inability of the test to be applied directly to a biological sample, and restricted information regarding antimicrobial susceptibility.

PCR-based approaches, which amplify microbial nucleic acids directly from the bloodstream, carry the potential to improve the detection of infection with typical sepsis-associated bacteria, such as, streptococci and staphylococci, and also polymicrobial infections, including fastidious, multi-resistant, and fastidious species such as fungi. Comparable data are currently available only from studies which used the SeptiFast multiplex PCR. Concordance of PCR and blood culture results with respect to the recovery of blood culture-positive results by PCR was moderate to good in most, but not all studies. PCR-based approaches have potential as a supplement to blood cultures to reduce the time to results to readjust and narrow the spectrum of antimicrobial therapy, specifically for infections such as those by Candida or MRSA.


At present, there is insufficient evidence for the routine use of novel biomarkers in infection and sepsis. A large number of biomarkers have been studied to date, but have been constrained by lack of a ‘gold standard’, varying sensitivities and specificities, and inability to distinguish between infectious or inflammatory causes of a SIRS response. Most importantly, few have gone beyond correlation with outcome, to what really matters—improvement in outcome. The future is promising with development of high sensitivity assays, molecular strategies, and a ‘panel approach’, all of which need to be investigated in well-designed future studies, including randomized trials that can determine the true clinical utility of novel biomarkers of infection.


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