SIRS Criteria Definition
Systemic Inflammatory Response Syndrome (SIRS) 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
SIRS requires the presence of at least 2 of the following 4 clinical parameters 1:
- Temperature abnormality: >38°C (>38.5°C in some definitions) or <36°C 1
- Tachycardia: Heart rate >90 beats/min 1, 3
- Tachypnea: Respiratory rate >20 breaths/min, or PaCO2 <32 mmHg (4.3 kPa) 1
- White blood cell abnormality: WBC count >12,000/mm³, or <4,000/mm³ (4 G/L), or >10% immature (band) forms 1
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 even chronic salicylate intoxication. 1, 4, 5, 6
Key Clinical Points:
- SIRS is not synonymous with sepsis—sepsis specifically requires SIRS plus a proven or suspected infection 1, 3, 2
- The presence of SIRS indicates systemic inflammation and serves as an early warning system for potential clinical deterioration 3, 7, 6
- Each additional SIRS criterion increases mortality risk linearly (odds ratio 1.13 per criterion), without a specific threshold effect at 2 criteria 8
Important Clinical Caveats
In the postoperative period, SIRS criteria must be interpreted carefully—during the first 48 hours after surgery, SIRS may result from surgical stress and cardiopulmonary bypass rather than infection, requiring more stringent diagnostic criteria for sepsis during this timeframe. 1
Approximately 12% of patients with infection and organ failure (severe sepsis) may not meet SIRS criteria yet still have similar mortality rates to SIRS-positive patients, highlighting that absence of SIRS does not exclude serious infection. 8
The number and type of positive SIRS criteria matter—patients with 4 positive criteria have significantly more surgical interventions, intensive care requirements, and fatal outcomes compared to those with 2-3 criteria. 7 The respiratory rate is the most influential factor, followed by white cell count and heart rate/temperature. 7
Current Clinical Relevance
SIRS remains widely used in clinical practice and validated classification systems, including the IWGDF/IDSA classification for diabetic foot infections (2024) and multiple sepsis management guidelines, despite newer scoring systems like qSOFA and SOFA being developed. 1, 3