SIRS (presence of 2 or more of the following criteria)

• Fever (core temperature > 38.3 C or 101.0 F) or hypothermia (core temperature < 36 C or 96.8 F)
• Heart rate > 90 beats per minute
• Respiratory rate > 20 breaths/min or PaCO2 < 32 or need for mechanical ventilation for an acute respiratory process
• WBC > 12,000/mm3, < 4,000/mm3, or bands > 10%

Sepsis

• SIRS and suspected or confirmed infection
• NEW Definition (Society of Critical Care Medicine and the European Society of Intensive Care Medicine)
  
• Evidence of infection -AND- life-threatening organ dysfunction (acute change of > 2 points in the SOFA score)
• SOFA Score / qSOFA Score
• Does NOT use SIRS
  

Severe Sepsis

• Sepsis and at least one NEW organ dysfunction
• Mental status change
  
• SBP < 90 mmHg, MAP < 65 mmHg for at least one hour despite adequate fluid resuscitation (20 ml/kg saline) or use of vasopressors
  
• Lactate > 4 mmol/L
• Urine output < 0.5 ml/kg/hr after adequate fluid resuscitation or rise in creatinine > 0.5 mg/dL over baseline
• PaO2/FiO2 ratio < 300 or requiring > 4 liters oxygen via nasal cannula to maintain SpO2 >  90%
• Platelets < 100,000/mm3, INR > 1.5, PTT > 60s
  

Septic shock

• Vasopressor required to maintain MAP > 65 mm Hg and serum lactate level > 2 mmol/L in the absence of hypovolemia
  

Sources

• Overall most common source is pneumonia
• Most common source in patients over age 64 is UTI
• Consider fungal in immunocompromised patient or patient with many comorbidities
• Consider adrenal insufficiency (Addison's, long-term steroid use) in patients with refractory hypotension