1Systemic Inflammatory Response Syndrome (SIRS) — Clinical Guide
Systemic Inflammatory Response Syndrome (SIRS) was defined by the 1991 ACCP/SCCM Consensus Conference as a non-specific physiological state reflecting generalized systemic inflammation. Meeting two or more of the four vital sign and leukocyte parameters establishes positive SIRS.
Crucially, SIRS is not a disease or diagnosis by itself; it represents the body's generalized immune and metabolic reaction to diverse clinical insults, both infectious (sepsis) and non-infectious (trauma, burns, pancreatitis, major surgery, ischemia).
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) represents a left-shift indicating bone marrow recruitment of immature cells.
2Sepsis-2 vs. Sepsis-3 Paradigm Shift
| Clinical Concept | Sepsis-2 Framework (1991) | Sepsis-3 Framework (2016) |
|---|---|---|
| Sepsis Definition | SIRS (≥ 2 criteria) + Suspected or confirmed infection. | Life-threatening organ dysfunction caused by a dysregulated host response to infection (SOFA ≥ 2). |
| Severe Sepsis | Sepsis + acute sepsis-induced organ dysfunction. | Term abandoned (subsumed under "Sepsis"). |
| Septic Shock | Sepsis-induced hypotension refractory to fluid resuscitation. | Sepsis + persistent hypotension requiring vasopressors to maintain MAP ≥ 65 mmHg AND serum lactate > 2.0 mmol/L despite fluids. |
| Screening Tool | SIRS Criteria | qSOFA (Bedside) / SOFA (ICU) |
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.
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
4About SIRS Criteria and Sepsis Definitions
The Systemic Inflammatory Response Syndrome (SIRS) criteria were introduced by Bone et al. at the 1991 ACCP/SCCM Consensus Conference as a framework for identifying patients with a generalised inflammatory response, whether infectious or non-infectious. SIRS represented the first standardised definition of sepsis and enabled clinical research and trial enrolment for nearly two decades.
However, the limitations of SIRS became increasingly apparent: its high sensitivity (~97%) came at the cost of very poor specificity — SIRS criteria are met by the majority of ICU patients regardless of infection, and by many ward patients post-surgery, with pancreatitis, burns, or autoimmune disease. The 2016 Sepsis-3 consensus replaced SIRS with organ dysfunction (acute SOFA score increase ≥2) as the defining criterion for sepsis, better identifying the subset of infected patients with life-threatening organ failure and high mortality.
Despite its replacement in formal definitions, SIRS retains practical value in resource-limited settings. In district hospitals and primary care centres without rapid SOFA component testing, SIRS criteria plus clinical suspicion of infection remain a reasonable trigger for blood cultures, empirical antibiotics, and patient escalation. SIRS is also still embedded in scoring systems (BISAP for pancreatitis) and paediatric sepsis definitions. The clinical priority remains early recognition of sick patients — any scoring system is a tool, not a substitute for clinical judgment.
5Surviving Sepsis Campaign (SSC) Hour-1 Bundle
- 1. Measure Lactate: Re-measure within 2–4 hours if initial lactate > 2.0 mmol/L.
- 2. Blood Cultures: Obtain at least 2 sets prior to antibiotic administration (do not delay antibiotics > 45 minutes).
- 3. Broad-Spectrum Antibiotics: Initiate IV antimicrobials within 1 hour of recognition.
- 4. Fluid Resuscitation: Infuse 30 mL/kg balanced crystalloids for hypotension (MAP < 65) or lactate ≥ 4.0 mmol/L.
- 5. Vasopressors: Start norepinephrine if MAP remains < 65 mmHg despite initial fluid challenge.