What are the SIRS (Systemic Inflammatory Response Syndrome) criteria?

SIRS Criteria

SIRS (Systemic Inflammatory Response Syndrome) is defined by the presence of at least 2 of the following 4 criteria: 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

The Four SIRS Criteria

Temperature abnormality:

• Core temperature >38°C (fever) or <36°C (hypothermia) 2, 1

Tachycardia:

• Heart rate >90 beats per minute 2, 1

Tachypnea:

• Respiratory rate >20 breaths per minute, OR
• PaCO2 <32 mmHg (4.3 kPa) 2, 1

White blood cell abnormality:

• WBC count >12,000/mm³ (>12 G/L), OR
• WBC count <4,000/mm³ (<4 G/L), OR

• 10% immature (band) forms present 2, 1

Clinical Context and Interpretation

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 injuries. 1, 3

SIRS is not synonymous with sepsis—sepsis requires SIRS plus a proven or suspected infection. 1 The presence of SIRS indicates systemic inflammation and serves as an early warning system for potential clinical deterioration. 1

Prognostic Significance

Mortality increases progressively with the number of SIRS criteria met: mortality rises from 7% with SIRS alone to 16% with sepsis, 20% with severe sepsis, and 46% with septic shock. 4 Each additional SIRS criterion increases mortality risk in a stepwise fashion. 1, 4

Individual SIRS criteria are not equivalent in their mortality associations: mortality varies from 10.6% for respiratory rate criterion to 15.8% for heart rate criterion, and from 10.1% for high temperature to 14.4% for low temperature. 5 When any two SIRS criteria are present, hospital mortality can range from 11.5% to 30.8% depending on which specific combination is met. 5

Important Clinical Caveats

In postoperative patients, SIRS criteria must be interpreted cautiously because SIRS may result from surgical stress and cardiopulmonary bypass rather than infection. 1 This is a common pitfall where clinicians may overdiagnose infection in the immediate postoperative period.

SIRS has high sensitivity but low specificity for bacteremia: 95% of patients with clinically important bacteremia fulfill SIRS criteria, but the positive predictive value of SIRS for predicting bacteremia is only 7%. 6 However, patients who do not fulfill SIRS criteria when blood cultures are taken are at very low risk of bacteremia (comprising 45% of the study population with only 3/121 having bacteremia). 6

SIRS remains widely used in validated clinical classification systems, including the IWGDF/IDSA classification for diabetic foot infections (where ≥2 SIRS criteria define severe/grade 4 infection) and multiple sepsis management guidelines. 2, 1