SIRS Criteria — Complete Clinical Guide
Systemic Inflammatory Response Syndrome (SIRS) was defined by the 1991 ACCP/SCCM Consensus Conference as a non-specific clinical syndrome representing the body's systemic response to a wide variety of insults — both infectious and non-infectious. Two or more of the four SIRS criteria constitute a positive SIRS response. The key clinical insight is that SIRS is a physiological state of heightened inflammation and immune activation, not a diagnosis in itself.
SIRS can be triggered by infection (leading to sepsis under the old definition), trauma, burns, pancreatitis, surgery, ischaemia, malignancy, autoimmune disease, or any severe physiological stress. The diagnosis of sepsis in the Sepsis-2 framework required SIRS plus a suspected or confirmed infectious source. This was the standard clinical framework from 1991 until 2016.
The Four SIRS Criteria Explained
- Temperature: Core temperature >38.3°C (100.9°F) reflects pyrogenic cytokine activity (IL-1, IL-6, TNF-α). Hypothermia <36°C occurs in severe sepsis, gram-negative bacteraemia, overwhelming infection in the immunocompromised or elderly, and is associated with worse prognosis than fever
- Heart Rate >90 bpm: Tachycardia reflects increased sympathetic drive from pain, fever, hypovolaemia, or mediator release. In patients on beta-blockers or with pacemakers, heart rate may not reflect the underlying inflammatory state — clinical judgement required
- Respiratory Rate >20 or PaCO₂ <32 mmHg: Tachypnoea and hyperventilation reflect respiratory compensation for metabolic acidosis, direct lung involvement, hypoxaemia, or pain. PaCO₂ <32 mmHg on ABG confirms respiratory alkalosis from hyperventilation
- WBC >12,000 or <4,000/µL or >10% bands: Leukocytosis reflects bone marrow response to infection/inflammation. Leukopenia occurs in overwhelming infection, viral sepsis, or immunocompromised states and is a red flag. Bandemia (>10% immature neutrophils/bands) represents a left-shift indicating bone marrow recruitment of immature cells
Sepsis-2 vs Sepsis-3 — The 2016 Paradigm Shift
| Feature | Sepsis-2 (1991) | Sepsis-3 (2016) |
|---|---|---|
| Sepsis definition | SIRS (≥2 criteria) + suspected infection | Life-threatening organ dysfunction from dysregulated host response to infection (SOFA ≥2) |
| Severe sepsis | Sepsis + organ dysfunction | Term abandoned — now just "sepsis" |
| Septic shock | Sepsis + hypotension despite fluids | Sepsis + vasopressors to maintain MAP ≥65 + lactate >2 mmol/L despite adequate resuscitation |
| Screening tool | SIRS criteria | qSOFA (bedside), SOFA (ICU) |
| Clinical use | Still widely used in ED triage and general wards | Standard for ICU and research |
Why SIRS Is Still Clinically Useful Despite Sepsis-3
Despite the Sepsis-3 redefinition, SIRS criteria remain valuable in clinical practice for several reasons. They are simple, require no laboratory tests for the basic 3 clinical criteria (temperature, HR, RR), and can be assessed at the bedside within seconds. They have high sensitivity (though low specificity) for detecting patients who need closer evaluation. SIRS criteria are still embedded in many hospital triage systems, electronic health record sepsis alerts, and nursing assessment protocols worldwide.
The Sepsis-3 criticism of SIRS is primarily its low specificity — up to 30% of general medical ward patients and virtually all post-operative patients meet SIRS criteria without having infection-related organ dysfunction. However, in the emergency department and acute care context, SIRS remains a useful first-pass screening tool when combined with clinical assessment.
Surviving Sepsis Campaign — Hour-1 Bundle (SSC 2018/2021)
For patients meeting sepsis or septic shock criteria, the SSC Hour-1 Bundle recommends the following actions within 60 minutes of recognition:
- Measure lactate level — if >2 mmol/L, repeat within 2 hours to assess response
- Blood cultures before antibiotics — do not delay antibiotics for cultures
- Broad-spectrum antibiotics — administer within 1 hour of sepsis recognition. For septic shock, within 1 hour. Each hour of delay increases mortality by ~7%
- IV crystalloid resuscitation — 30 mL/kg IV crystalloid (normal saline or Hartmann's) for MAP <65 or lactate ≥4 mmol/L. Administer within first 3 hours
- Vasopressors — noradrenaline (norepinephrine) is first-line if MAP <65 despite fluid resuscitation. Target MAP ≥65 mmHg. Add vasopressin 0.03 units/min if noradrenaline dose escalating
Non-Infectious Causes of SIRS — Important Differentials
- Acute pancreatitis — one of the most common non-infectious causes; SIRS score correlates with severity
- Major trauma and burns — cytokine release from tissue injury
- Post-operative state — surgical stress response peaks at 24–48 hours
- Acute myocardial infarction — inflammatory response to myocardial necrosis
- Pulmonary embolism — reperfusion and platelet activation
- Autoimmune disease flare — SLE, vasculitis, adult Still disease
- Drug reactions — drug fever, neuroleptic malignant syndrome, serotonin syndrome
- Malignancy — tumour fever, lymphoma B-symptoms
- Transfusion reactions — TRALI, haemolytic transfusion reactions