Sepsis Definition and Clinical Implications

Current Definition Framework (Sepsis-3)

• The Society of Critical Care Medicine recommends that sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection, with organ dysfunction represented by an increase in the SOFA score of 2 points or more 1, 2, 3
• Organ dysfunction can be clinically identified by an increase in the SOFA score of ≥2 points, which is associated with in-hospital mortality >10% 1
• The previous concepts of SIRS and "severe sepsis" are no longer used in the current definition framework, as recommended by the Society of Critical Care Medicine 4, 5

Septic Shock Definition

• The European Society of Intensive Care Medicine defines septic shock as a subset of sepsis with particularly profound circulatory, cellular, and metabolic abnormalities associated with a greater risk of mortality 6, 7
• Clinically identified by vasopressor requirement to maintain mean arterial pressure ≥65 mmHg and serum lactate level >2 mmol/L (>18 mg/dL) in the absence of hypovolemia, as defined by the Society of Critical Care Medicine 1, 7

Quick SOFA (qSOFA) Criteria

• The World Health Organization recommends that qSOFA consists of three clinical variables: respiratory rate ≥22 breaths/min, altered mental status (Glasgow Coma Scale score ≤13), and systolic blood pressure ≤100 mmHg, for rapid identification of patients at risk of sepsis outside the ICU setting 1, 2, 3, 6
• Presence of at least 2 of these criteria suggests higher risk of poor outcomes typical of sepsis, as recommended by the American College of Emergency Physicians 1, 2, 3

Evolution of Sepsis Definitions

• Previous definitions (pre-2016) focused heavily on inflammation and used SIRS criteria, which was defined as at least two of: temperature >38°C or <36°C, heart rate >90 beats/minute, respiratory rate >20 breaths/minute or PaCO2 <32 mmHg, and white blood cell count >12,000/mm³ or <4,000/mm³, or >10% immature (band) forms, as previously recommended by the Society of Critical Care Medicine 6, 7

Clinical Implications

• Early recognition is crucial for improving outcomes, as recommended by the American College of Critical Care Medicine 4, 5
• The pathophysiology involves a dysregulated inflammatory response that can lead to multi-organ failure, with a strength of evidence level of high, as defined by the National Institutes of Health 3
• Sepsis represents a spectrum of severity with differential responses at local, regional, and systemic levels, with a moderate level of evidence, as recommended by the European Society of Intensive Care Medicine 3
• Patient factors including age and comorbidities significantly affect the course of sepsis, with a high level of evidence, as defined by the American College of Physicians 1, 3

Special Considerations

• Immunocompromised patients (e.g., HIV-positive) have increased susceptibility to sepsis due to pre-existing immune system activation and exhaustion, with a high level of evidence, as recommended by the Infectious Diseases Society of America 1, 3
• Elderly patients may present with attenuated inflammatory responses and fewer clinical signs despite severe infection, with a moderate level of evidence, as defined by the American Geriatrics Society 3
• The inflammatory response depends on both the causative pathogen and host factors (genetic characteristics and co-existing illnesses), with a high level of evidence, as recommended by the National Institutes of Health 1

Sepsis Definition and Microbiology

Causative Microorganisms

• More than 90% of sepsis cases are caused by bacteria, with Gram-negative and Gram-positive organisms occurring with approximately equal frequency 8
• Fungi, particularly Candida species, are responsible for a significant minority of cases 8
• Understanding the microbiology of sepsis is essential for appropriate antimicrobial therapy, which significantly impacts patient survival compared to inadequate or inappropriate treatment 8

Pathophysiological Mechanisms

• Sepsis involves the recognition of pathogen-associated molecular patterns (PAMPs) by the host immune system, triggering inflammatory signaling pathways 9

Clinical Identification and Diagnosis

• Microbiological diagnosis is essential for effective antimicrobial therapy, with blood cultures being the cornerstone of diagnosis 8
• Distinguishing colonization from infection is a key challenge in interpreting microbiological findings 10

Evolution of Sepsis Definitions

• Current definition (Sepsis-3) emphasizes organ dysfunction rather than inflammatory response alone, as defined by the World Journal of Emergency Surgery 11
• Previous definitions (pre-2016) focused heavily on inflammation and used Systemic Inflammatory Response Syndrome (SIRS) criteria, which included temperature >38°C or <36°C, heart rate >90 beats/minute, respiratory rate >20 breaths/minute or PaCO2 <32 mmHg, and white blood cell count >12,000/mm³ or <4,000/mm³, or >10% immature forms 9

Diagnostic Challenges

• Prior antibiotic therapy may lead to negative cultures despite ongoing infection 10
• Correctly identifying unfamiliar organisms and determining the significance of mixed culture results present additional challenges 10
• Interpreting the importance of organisms normally of low virulence requires clinical correlation 10

Risk Stratification Using NEWS2

Components of the NEWS2 Score

• The National Institute for Health and Care Excellence (NICE) specifies that NEWS2 incorporates six physiological parameters—respiratory rate, oxygen saturation, supplemental oxygen requirement, systolic blood pressure, pulse rate, level of consciousness, and temperature—to assess patients for possible sepsis 12.

Clinical Application of NEWS2 in Sepsis

• According to NICE guidance, aggregate NEWS2 scores are used to assign patients to risk categories that determine the urgency of antibiotic prescribing and the frequency of clinical monitoring for sepsis 12.

Interpretation of NEWS2 Scores

• NICE defines the following score thresholds for sepsis risk: a score of 0 indicates very low risk; 1–4 indicates low risk; 5–6 indicates moderate risk, with higher scores prompting increasingly urgent clinical action 12.