What are the diagnostic criteria for systemic inflammatory response syndrome (SIRS) and the initial management?

Systemic Inflammatory Response Syndrome (SIRS): Diagnostic Criteria and Initial Management

Diagnostic Criteria

SIRS is diagnosed when a patient meets at least 2 of the following 4 clinical criteria: 1, 2

• Temperature: >38°C or <36°C 1, 2
• Heart rate: >90 beats/min 1, 2
• Respiratory rate: >20 breaths/min OR PaCO₂ <32 mmHg 1, 2
• White blood cell count: >12,000/mm³ OR <4,000/mm³ OR >10% immature (band) forms 1, 2

These criteria can be assessed at the bedside without requiring cultures or pathological specimens, making SIRS a clinical diagnosis rather than a laboratory-confirmed one. 1

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Immediate Diagnostic Workup

When ≥2 SIRS criteria are met, obtain the following tests without delay: 2

Core Laboratory Panel

• Complete blood count with differential (to confirm WBC abnormalities and assess band forms) 2
• C-reactive protein (the prototypical SIRS biomarker that correlates with inflammation magnitude) 1, 2
• Procalcitonin 2
• Blood cultures 2
• Serum lactate (to detect tissue hypoperfusion) 2
• Creatine phosphokinase 2

Metabolic and Organ Function Assessment

• Urea, creatinine, and electrolytes (renal function) 2
• Liver function tests and lactate dehydrogenase 2
• Arterial or venous blood gas if oxygen saturation <92% on room air 2

Site-Specific Cultures

• Urine culture when urinary involvement suspected 2
• Expectorated sputum (not nasopharyngeal aspirate) for respiratory symptoms 2
• Stool culture for gastrointestinal symptoms 2

Imaging

• Chest radiograph to screen for pneumonia or pulmonary pathology 2
• Contrast-enhanced CT when intra-abdominal pathology or bowel strangulation suspected 2

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Critical Assessment for Organ Dysfunction

The presence of ANY organ dysfunction defines severe sepsis and mandates immediate escalation of care, regardless of how many SIRS criteria are met. 1

Specific Organ Dysfunction Markers to Evaluate Immediately:

• Hemodynamic: Systolic BP <90 mmHg or mean arterial pressure <65 mmHg 1
• Tissue perfusion: Lactate >2 mmol/L, oliguria, or mottled skin 1
• Respiratory: Requirement for supplemental oxygen due to hypoxemia 1
• Renal: New-onset oliguria or rising creatinine 1
• Mental status: Altered consciousness or confusion 1

Do not wait for hypotension to develop—organ dysfunction without hypotension still meets criteria for severe sepsis and requires emergency department transfer. 1

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Initial Management

Identify and Treat the Underlying Cause

SIRS is not a final diagnosis but a clinical expression requiring a thorough search for the precipitating cause. 1

Common triggers to evaluate: 1

• Infectious: Sepsis, pneumonia, urinary tract infection, diabetic foot infection
• Tissue injury: Postoperative state, trauma, hematoma, venous thrombosis
• Cardiovascular: Myocardial infarction, pulmonary infarction
• Other: Pancreatitis, transplant rejection, subarachnoid hemorrhage

Fluid Management

Maintain near-zero fluid and electrolyte balance in surgical patients—this reduces complications by 59% and shortens hospital stay by 3.4 days. 1

Avoid fluid overload, which worsens outcomes in SIRS patients. 1

Vasopressor Support

For hypotension in septic shock, use norepinephrine as first-line vasopressor to maintain mean arterial pressure ≥65 mmHg. 1

Norepinephrine is more efficacious than dopamine and causes less tachycardia and arrhythmia. 1

Antibiotic Decisions

For skin and soft tissue infections with SIRS: 1

• Administer antibiotics based on SIRS presence
• Use an antibiotic active against MRSA for carbuncles or abscesses with SIRS

For surgical site infections with SIRS: 1

• Perform wound opening and evacuation of infected material
• Give systemic antibiotics only when temperature ≥38.5°C OR pulse ≥100 beats/min

For intra-abdominal infections with SIRS: 1

• Perform step-up diagnostic approach
• Source control timing is critical—late or incomplete procedures severely worsen outcomes

Source Control

For intra-abdominal infections, timely and complete source control is critical—delayed or incomplete procedures dramatically worsen mortality. 1

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Monitoring and Prognostic Assessment

Temporal Assessment

Assess for resolution of SIRS criteria within 48 hours—persistence beyond this timeframe indicates high risk of death. 1

• Transient SIRS (<48 hours): 8% mortality 1
• Persistent SIRS (>48 hours): 25.4% mortality 1
• No SIRS: 0.7% mortality 1

Every attempt should be made to restore normality as soon as possible, as SIRS precedes organ failure. 1

Biomarker Monitoring

C-reactive protein consistently correlates with the magnitude of operative injury or illness severity and can be used to monitor the inflammatory response trajectory. 1

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Common Pitfalls and Caveats

Postoperative SIRS

In the first 48–72 hours after surgery, SIRS criteria may result from surgical stress rather than infection. 2, 3

• The "trauma of surgery" universally triggers inflammatory mediators identical to SIRS 3
• Surgical site infections rarely occur within the first 48 hours postoperatively 3
• Do NOT reflexively prescribe antibiotics or open the incision based solely on fever and tachycardia during this early period 3

Consider antibiotics postoperatively only if: 3

• Temperature >38.5°C
• Heart rate >110 bpm
• Erythema extending >5 cm beyond wound margins

SIRS vs. Sepsis

SIRS is NOT synonymous with sepsis. 2

• SIRS = ≥2 clinical criteria from any cause (infectious or non-infectious) 1, 2
• Sepsis = SIRS + proven or suspected infection 2
• Septic shock = Sepsis + vasopressor requirement to maintain MAP ≥65 mmHg + lactate >2 mmol/L 1

Diabetic Foot Infections

In diabetic foot infections, the presence of ≥2 SIRS criteria automatically classifies the infection as severe (Grade 4) and mandates aggressive therapy. 1

Necrotizing Infections

Urgent surgical consultation is warranted if signs of necrotizing infection develop: 3

• Rapidly progressive erythema
• Skin necrosis
• Crepitus
• Severe pain out of proportion to physical findings

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Multidisciplinary Care

Complex cases may require specialists in hepatology, critical care, infectious disease, and nephrology. 1

For conditions like aneurysmal subarachnoid hemorrhage with SIRS, frequent neurological assessment using Glasgow Coma Scale or NIH Stroke Scale is recommended. 1