SIRS Criteria
SIRS (Systemic Inflammatory Response Syndrome) is diagnosed when at least 2 of the following 4 criteria are present: temperature >38°C or <36°C, heart rate >90 beats/min, respiratory rate >20 breaths/min or PaCO₂ <32 mmHg, and white blood cell count >12,000/mm³ or <4,000/mm³ or >10% immature (band) forms. 1, 2
The Four SIRS Criteria
Temperature abnormality:
Tachycardia:
Tachypnea:
White blood cell abnormality:
- WBC count >12,000 cells/mm³ (>12 × 10⁹ cells/L) OR <4,000 cells/mm³ (<4 × 10⁹ cells/L) OR >10% immature (band) forms 1, 2
Clinical Context and Application
SIRS represents a systemic inflammatory response that can be triggered by both infectious and non-infectious causes, including infection, trauma, burns, pancreatitis, surgery, and other tissue injuries. 2, 3, 4
- SIRS is diagnosed solely based on readily available clinical parameters and basic laboratory values that can be obtained at the bedside, without requiring pathological specimens or cultures 3, 5
- The presence of SIRS should prompt a thorough search for the underlying cause rather than being considered a final diagnosis 3, 5
Critical Distinction from Sepsis
SIRS is not synonymous with sepsis. 2
- Sepsis requires SIRS criteria PLUS proven or suspected infection 2, 5
- SIRS can occur without infection (e.g., from trauma, surgery, pancreatitis) 2, 3
- Septic shock requires sepsis plus vasopressor requirement to maintain mean arterial pressure ≥65 mmHg and lactate >2 mmol/L 3, 5
Clinical Significance and Prognostic Value
Each additional SIRS criterion increases mortality risk linearly, without a specific threshold effect at 2 criteria. 6
- Persistent SIRS beyond 48 hours significantly increases risk of organ failure and death, with 25.4% mortality in persistent SIRS versus 8% in transient SIRS and 0.7% without SIRS 3
- In trauma patients, admission SIRS score ≥2 independently predicts nosocomial infection and worse outcomes 7, 8
- The magnitude of postoperative SIRS corresponds to the degree of surgical trauma and correlates with poorer surgical outcomes 1, 3
Important Clinical Caveats
In the postoperative period, SIRS criteria must be interpreted carefully, as SIRS commonly results from surgical stress rather than infection. 2
- Among patients with infection and organ failure, 12.1% do not meet SIRS criteria (SIRS-negative severe sepsis) yet have similar mortality to SIRS-positive patients 6
- Of the four SIRS variables, hypothermia and leukocytosis are the most significant predictors of infection when adjusted for age and injury severity 7
- C-reactive protein (CRP) is the prototypical marker of systemic inflammatory response and consistently correlates with the magnitude of operative injury or illness severity 1, 3