Critical Care
SIRS Criteria
Systemic inflammatory response syndrome.
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Background
The systemic inflammatory response syndrome (SIRS) criteria identify a generalised inflammatory state that may be caused by infection or by non-infectious insults such as trauma, pancreatitis or burns. They were defined at a 1991 consensus conference and comprise four variables: temperature above 38°C or below 36°C, heart rate above 90/min, respiratory rate above 20/min (or pCO₂ below 4.3 kPa), and a white cell count above 12 or below 4 ×10⁹/L (or more than 10% immature forms). Meeting two or more criteria denotes SIRS. Historically, SIRS plus suspected infection defined sepsis.
Interpreting the result
Fewer than two criteria means SIRS is not met; two or more constitute a positive SIRS response. In the presence of suspected or confirmed infection, this combination was previously used to diagnose sepsis. However, the Sepsis-3 definitions (2016) replaced this framework, because SIRS is both very common and non-specific — many unwell patients meet it without being septic, and some septic patients do not. It is now best used as a sensitive early flag that prompts assessment rather than as a diagnostic rule.
Advice
With suspected infection, consider sepsis and assess with qSOFA / full clinical picture; SIRS alone is non-specific.
Worked example
A patient with suspected pneumonia has a temperature of 38.5°C, heart rate 110/min and respiratory rate 24/min: three SIRS criteria are met. This meets the SIRS threshold and, with a suspected source of infection, should prompt sepsis screening and further assessment with tools such as qSOFA.
Pearls / pitfalls
- SIRS is non-specific — common after surgery, exercise or anxiety — so a positive result does not by itself mean infection or sepsis.
- Sepsis-3 no longer uses SIRS to define sepsis; assess organ dysfunction (e.g. qSOFA or full SOFA) and treat on clinical suspicion.
- Some genuinely septic patients, particularly the elderly or immunosuppressed, never meet SIRS criteria, so a negative screen is reassuring but not exclusionary.
- Beta-blockers, antipyretics and chronic disease can blunt the vital-sign and temperature responses, masking SIRS.
Evidence & validation
Defined by Bone et al. at the 1991 ACCP/SCCM consensus conference; subsequently superseded for sepsis diagnosis by the Sepsis-3 (Singer et al., 2016) definitions, which use organ dysfunction (SOFA/qSOFA) rather than SIRS.
Frequently asked questions
Does meeting SIRS mean a patient has sepsis?
No. SIRS is a non-specific inflammatory response that can occur without infection, such as after trauma or surgery. Sepsis requires suspected infection together with organ dysfunction under current definitions.
Why was SIRS replaced for diagnosing sepsis?
SIRS is too common and non-specific, flagging many non-septic patients while missing some septic ones. The Sepsis-3 definitions instead base sepsis on infection plus organ dysfunction measured by SOFA or qSOFA.
Is SIRS still useful at all?
Yes — as a quick, sensitive screen it can prompt earlier assessment for infection. It should trigger further evaluation rather than serve as a final diagnosis.
Can a septic patient have a normal SIRS screen?
Yes. Elderly, frail or immunosuppressed patients may not mount the expected vital-sign or white-cell changes. A negative screen does not exclude serious infection.
What counts as a positive SIRS result?
Two or more of the four criteria — temperature, heart rate, respiratory rate (or pCO₂) and white-cell count — constitute SIRS. With suspected infection this should prompt sepsis assessment.
References
- Bone RC, Balk RA, Cerra FB, et al. Definitions for sepsis and organ failure (ACCP/SCCM Consensus Conference). Chest. 1992;101(6):1644–1655.
- Singer M, Deutschman CS, Seymour CW, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA. 2016;315(8):801–810.
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