How it's calculated
Four criteria — temperature, heart rate, respiratory rate (or PaCO₂), and white cell count — are each scored present/absent. Two or more present criteria is considered SIRS-positive.
SIRS criteria
| Criterion | Threshold |
|---|---|
| Temperature | > 38°C or < 36°C |
| Heart rate | > 90 bpm |
| Respiratory rate | > 20/min, or PaCO₂ < 32 mmHg |
| White blood cell count | > 12,000/mm³, < 4,000/mm³, or > 10% bands |
Clinical use
- SIRS ≥2 criteria with a suspected or confirmed source of infection was the older (Sepsis-2) definition of sepsis, before the Sepsis-3 consensus shifted emphasis to qSOFA/SOFA-based organ dysfunction criteria.
- SIRS criteria are highly sensitive but not specific for infection — many non-infectious conditions (trauma, pancreatitis, major surgery) also trigger a SIRS response.
- Despite the shift to Sepsis-3 definitions, SIRS remains widely used clinically as a simple first-pass screening tool, particularly outside specialist critical care settings.
- A patient can have severe sepsis with organ dysfunction but a SIRS score below 2 (particularly in elderly or immunocompromised patients with a blunted inflammatory response) — clinical suspicion should not be dismissed based on SIRS alone.
Frequently asked questions
What is SIRS used for?
SIRS (Systemic Inflammatory Response Syndrome) criteria screen for a significant inflammatory response in the body — historically, SIRS plus a suspected infection source defined sepsis under the older Sepsis-2 definitions.
Has SIRS been replaced by qSOFA?
The Sepsis-3 consensus (2016) shifted emphasis toward qSOFA and the full SOFA score, which focus on organ dysfunction rather than inflammatory response, but SIRS criteria remain widely used clinically as a simple screening tool.
Can someone have a serious infection without meeting SIRS criteria?
Yes — particularly elderly or immunocompromised patients can have a blunted inflammatory response and may not meet 2 SIRS criteria despite having a serious, even life-threatening infection, so clinical suspicion should never be dismissed based on SIRS status alone.
Does meeting SIRS criteria always mean infection is present?
No — many non-infectious conditions, including trauma, burns, pancreatitis, and major surgery, can also trigger a SIRS response, which is why SIRS alone (without a suspected infection source) does not diagnose sepsis.
References
- Bone RC, et al. Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. Chest. 1992;101:1644-1655.
- Singer M, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA. 2016;315:801-810.
- Kaukonen KM, et al. Systemic inflammatory response syndrome criteria in defining severe sepsis. N Engl J Med. 2015;372:1629-1638.