Current and novel biomarkers for diagnosis and monitoring of sepsis

Sepsis is a life-threatening condition affecting approximately 27 million people worldwide each year. Without appropriate treatment, sepsis can progress to severe sepsis and septic shock, which cause some eight million deaths each year. The risk of sepsis-related mortality increases with age and coexisting diseases such as diabetes or cancer; thus the incidence of sepsis has increased significantly, with hospitalizations and related costs doubling over the last 10 years.¹

Major efforts to reduce sepsis-related mortality have focused on generating consensus definitions of sepsis and related conditions and establishing guidelines for early identification and implementation of goal-directed therapy. In addition, studies have been undertaken to identify new sepsis biomarkers and therapies. Clinical studies show that mortality is significantly reduced if septic patients are identified and treated at early stages of the disease process.² However, early diagnosis is complicated by heterogeneity in clinical presentation, co-morbidities, and the scarcity of biomarkers that enable clinicians to accurately identify sepsis in early stages of disease. The recent revision and expansion of guidelines, with a deeper understanding of the pathophysiology of sepsis and availability of novel diagnostic and therapeutic tools, points to a better forecast for this severe disease in the future.

An expanded definition for sepsis 

In 1991, the American College of Chest Physicians and the Society of Critical Care Medicine introduced the international consensus definitions for sepsis and the systemic inflammatory response syndrome (SIRS) as follows:

• SIRS: Inflammatory response to infectious and non-infectious conditions characterized by the presence of at least two of four signs: fever or hypothermia, tachypnea, tachycardia, and abnormally elevated or suppressed white blood cell count 
• Sepsis: SIRS + documented infection, identified by microbial culture
• Severe sepsis: Sepsis + organ dysfunction
• Septic shock: Severe sepsis + hypotension and decreased peripheral perfusion 

These definitions were fundamental for better disease recognition and characterization of severity, and they served as a framework for inclusion criteria in clinical studies. Identifying patients with SIRS became so straightforward that informatics tools could be designed to rapidly identify patients.³ However, the definitions had their limitations. Determining which SIRS patients were septic remained a challenge and relied heavily on clinical expertise and judgment. Many non-infectious conditions also present with SIRS, including pancreatitis, burns, and post-surgery. In septic patients, infection cannot always be confirmed microbiologically. 

These shortcomings were partially addressed ten years later with a revised sepsis definition that included confirmed or suspected infection in patients with SIRS, and signs of systemic inflammation due to infection.⁴ These signs include both biochemical and clinical parameters and are grouped in four categories: general, inflammatory, hemodynamic, and tissue perfusion parameters. 

The Surviving Sepsis Campaign (SSC) Guidelines, first published in 2004 and revised in 2008 and 2013, are evidence-based guidelines for the management of severe sepsis and septic shock.^5-7 The guidelines describe two sepsis treatment bundles, resuscitation and management, which must be implemented within hours after severe septic patients present to the emergency department. Implementation of these guidelines decreases mortality.⁸ 

Both the sepsis definitions and the SSC guidelines include biochemical criteria, but they are supportive rather than diagnostic tests—because to date no single biochemical/inflammatory biomarker has had sufficient diagnostic strength to predict sepsis. SIRS criteria, pathogen identification, and clinical variables remain the cornerstone of the sepsis definition, but they are often unable to identify patients in early stages of disease. Clinical signs like the SIRS criteria are nonspecific for sepsis, and microbiological cultures can take days to produce results. There is a great need for sensitive and specific biomarkers that can identify patients in early stages of sepsis, determine disease severity and predict response to therapy. 

Sepsis pathophysiology

The pathophysiology of sepsis is complex. Recent theories describe a host’s global response to infection as having both pro-inflammatory and compensatory anti-inflammatory stages.⁹ The exact sequence of events is unclear. One theory supports a sequential model in which inflammation is followed by anti-inflammation, while a second model, known as the mixed antagonist response syndrome, suggests that these two occur simultaneously. Dysregulation of the inflammatory/anti-inflammatory response may contribute to sepsis-related mortality. A prolonged hyper-inflammatory response results in a storm of toxic cytokines, while an immunosuppressive stage impairs protection against pathogens. No matter the sequence, dysregulation of the immune response to an infection leads to altered coagulation and cellular activation, endothelial cell failure, and cellular apoptosis, all of which contribute to metabolic alterations and multi-organ damage, and eventually death.¹⁰ A clear understanding of sepsis pathophysiology is crucial for development of novel therapeutic and diagnostic tools. 

Sepsis biomarkers: the current state of the art

Lactate, C-reactive protein (CRP), and procalcitonin (PCT) are commonly used for classification and management of septic patients. Lactate is used to assess tissue perfusion and is elevated with tissue hypoxia caused by hypoperfusion in severe sepsis and septic shock but not in early sepsis. The SSC guidelines recommend measuring lactate within three hours after sepsis is suspected; a lactate > 4 mmol/L warrants fluid resuscitation. If initial concentration is above that cut-off, lactate is remeasured within a few hours to evaluate response to therapy. Patients achieving a lactate clearance > 10% have better prognoses.¹¹ 

CRP and PCT are both inflammatory biomarkers, widely investigated for sepsis diagnosis.^9,12,13 CRP is an acute-phase reactant elevated in many inflammatory conditions. PCT, the precursor of the thyroid hormone calcitonin, is also increased in the systemic inflammatory response to infection. The Food and Drug Administration-approved PCT testing is indicated in conjunction with other laboratory and clinical findings for the diagnosis of bacterial infection and sepsis in critically ill patients. Overall, most studies indicate superior clinical utility (sensitivity and specificity) of PCT over CRP for the identification of sepsis among patients with systemic inflammation.^12,14,15 The concentration of PCT correlates with severity of disease, while CRP is not helpful for stratification.¹⁴ 

The utility of PCT remains controversial and it is not universally adopted in clinical practice.¹² Both CRP and PCT are listed among the inflammatory variables that serve as criteria to diagnose sepsis, but the SSC guidelines state that the ability of PCT or CRP to discriminate between non-infectious and infectious SIRS has not been demonstrated, and they issue no recommendations for utilization of either biomarker to identify infected patients among those with systemic inflammation. The SSC guidelines endorse PCT as a tool for antibiotic stewardship.⁷ In adults, low PCT concentrations can be used to direct cessation of antibiotics in critically ill patients; however, high PCT concentrations should not be used to intensify antibiotic therapy.¹⁶ The utility of PCT is still unknown in pediatric patients and neonates. PCT-guided antimicrobial therapy reduces antibiotic use without benefits in morbidity and mortality.^16,17

In sum, lactate, PCT, and CRP are helpful markers to manage patients with suspected sepsis by providing prognostic information and guiding therapy, but they have limited diagnostic utility in sepsis and no role at the early stages of sepsis (Table 1).

Sepsis biomarkers: looking ahead to the future

Many research groups are studying novel biomarkers for sepsis diagnosis and management. The studies range in quality (sample size), inclusion criteria (SIRS, all patients, shock), hospital setting (ED, ICU, hospitalized), outcomes measured (mortality, bacterial infection, distinguishing SIRS vs. sepsis), and age (adults, neonates, children), and this variation greatly confounds the interpretation of the findings. A recent literature review identified more than 3,000 published reports of 178 sepsis biomarkers, including immune cell markers, cytokines, coagulation factors, acute-phase reactants, markers of vasculo-endothelial damage, vasodilation, and organ dysfunction.¹³ The authors surmise that none of the biomarkers alone had sufficient diagnostic strength to identify septic patients. 

The clinical setting of these studies varied, but among the most important features of a sepsis biomarker were the ability to identify patients with early sepsis in a population with systemic inflammation, the ability to stratify severity, and the ability to predict outcome and response to therapy. Because of the complexity of sepsis and the comorbidities of patients at risk for sepsis, it seems unlikely that a single biomarker will fill all of these requirements. 

The emerging theory is that a panel of biomarkers may better predict sepsis among patients with systemic inflammation.¹³ Several studies suggest that biomarker panels have a diagnostic utility to predict sepsis that is superior to that of any single biomarker.^18-20 And measuring multiple markers is now feasible in the clinical lab, with the recent availability of multi-analyte platforms, known as multiplex platforms, in which the concentration of panels of biomarkers (protein, DNA, or RNA) is determined simultaneously. These new technologies are efficient and can potentially improve sepsis diagnosis. 

The challenge to implementing these routinely is the large amount of data generated. These platforms require robust bioinformatics and statistical tools to provide easy-to-interpret information for clinicians at the bedside. Combining the results of multiple markers into a single algorithm or risk score may ease the implementation of these panels into clinical practice.^18,19,21 Finally, emerging technologies in microbiology, such as molecular and MALDI-TOF techniques, have altered the way microorganisms are identified in the clinical laboratory.^22,23 These reduce diagnostic time and improve specificity and sensitivity compared to cultures, thereby enhancing diagnosis and management of septic patients.

Early diagnosis and treatment are fundamental to reducing sepsis-related morbidity and mortality. The complexity and heterogeneity of sepsis make early diagnosis challenging. While no single biomarker has sufficient diagnostic strength to identify septic patients among those with systemic inflammation at early stages, a panel of biomarkers may better distinguish those groups. Novel technology such as multiplexing platforms may provide timely results. If results are translated into risk scores, these technologies may be adopted as standard of care. In conclusion, a better understanding of the underlying pathological events in sepsis together with development of new diagnostic approaches are key to improving care of septic patients and reducing morbidity and mortality. 

References

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