SIRS

1. Definitive diagnosis requires clinical identification of infection in a patient who also meets the clinical criteria for the Systemic Inflammatory Response Syndrome (SIRS). According to a revised consensus conference definition in 2001, SIRS is defined by the presence of 2 or more criteria from a collection of clinical signs and laboratory investigations as follows: [Surviving Sepsis Campaign: evaluation for severe sepsis screening tool] (external link)

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Temperature >38.3°C (101°F) or <36.0°C (96.8°F)
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Tachycardia >90 bpm
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Tachypnoea >20 breaths/minute
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PCO2 <4.3 kPa (32 mmHg)
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Hyperglycaemia (blood glucose >6.66 mmol/L [120 mg/dL]) in absence of diabetes mellitus
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Acutely altered mental status
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WBC count >12×10^9/L (12,000/microlitre) or <4×10^9/L (4000/microlitre), or normal WBC count with >10% immature forms.
2. Sepsis: when SIRS is present in an individual patient and the cause is thought likely to be an infection, sepsis is present.
3. Severe sepsis: present when sepsis leads to dysfunction of 1 or more organ systems, and includes the subset septic shock. Organ dysfunction variables are:
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Arterial hypoxaemia (PaO2/FiO2 ratio <300) with new pulmonary infiltrates
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A new or increased oxygen requirement to maintain SpO2 >90%
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Acute oliguria (urine output <0.5 mL/kg/hour for at least 2 hours)
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Serum creatinine >176.8 micromol/L (2.0 mg/dL)
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Coagulation abnormalities (INR >1.5 or aPTT >60 seconds)
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Thrombocytopenia (platelets <100 × 10^9/L [100,000/microlitre])
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Hyperbilirubinaemia (total bilirubin >68.42 micromol/L [4 mg/dL])
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Arterial hypotension (systolic BP <90 mmHg, mean BP <65 mmHg, or reduction in systolic BP >40 mmHg from baseline)
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Serum lactate >2 mmol/L.
4. Septic shock is defined as:
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Arterial hypotension (systolic BP <90 mmHg, mean BP <65 mmHg, or reduction in systolic BP >40 mmHg from baseline) persisting for at least 1 hour, despite adequate fluid resuscitation, or
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Serum lactate >4 mmol/L after adequate fluid resuscitation.
The use of vasopressor agents to correct hypotension does not exclude shock.Acute physiology and chronic health evaluation II score (APACHE II)[72]

The APACHE score is commonly used to establish illness severity in the ICU and predict the risk of death. [APACHE II calculator] (external link)
 There is a high risk of death if the score is ≥25.
There are several other models that have been developed for use in the ICU, including APACHE III, Mortality in Emergency Department Sepsis score, Simplified Acute Physiology Score, Sepsis-related Organ Failure Assessment, and Mortality Probability Model II.