This is not a clinic

Sepsis and the SIRS Criteria

“I can’t define obscenity, but I know it when I see it.”


In The Prince, Book III Niccolol Machiavelli wrote, “Hectic fever at its inception is difficult to recognize but easy to treat. Left untended, it becomes easy to recognize but difficult to treat.” Five hundred years later, in the early 1990’s, researchers and clinicians working with sepsis found themselves with many great advances in sepsis treatment (including hand-washing and antibiotics) but with the same underlying observational difficulty. There was a plethora of words to describe sepsis and its manifestations, but there was no standardization of definition for these words between research studies. “Infected”, “bacteremia”, “sepsis”, “septic shock”, “septic syndrome”, and “septicemia” were all used without standardization. 1 And there was no terminology to describe the recently discovered common inflammatory pathways that could be caused by both infectious and non-infectious etiologies. Roger Bone, Dean of Rush Medical College, echoed Machiavelli when he quoted Justice Potter Stewart in an editorial to Chest, “I can’t define obscenity, but I know it when I see it.” 1


And so in 1992 a ACCP-SCCM Consensus Conference laid out a new set of definitions for researchers and clinicians dealing with sepsis. 2

Systemic Inflammatory Response Syndrome

            Possibly the most important definition that came out of the conference was that of systemic inflammatory response syndrome (SIRS). SIRS represents a state of widespread inflammation. It does not itself represent infection. A clear line was drawn between SIRS and infection because there are many non-infectious causes of SIRS including pancreatitis, ischemia, trauma, hemorrhagic shock and immunologically mediated organ injury. 2 The best representation of this is actually in a Venn diagram from the original article which clearly shows how SIRS and infection overlap, but also how they make both exist separate from each other. 2 This separation of SIRS and infection promoted research on SIRS as a common pathway of injury separate from a specific cause.

            So what are the SIRS criteria?

The SIRS Criteria

  1. A temperature > 38.0 ۫C or < 36.0 ۫C
  2. A heart rate > 90 BPM
  3. A respiratory rate > 20 BPM or PaCO2 less than 32 mm Hg
  4. And WBC count > 12,000/cu mm, < 4,000/cu mm, or with >10% bands


I remember the criteria by thinking of their placement on a progress note. The SIRS criteria are your vitals without blood pressure and O2 saturation, plus the white blood cell count.
This begs the question. Why aren’t blood pressure and oxygen saturation included like the other vital signs? A drop in blood pressure is a sign of progression from SIRS to shock and is included in the upcoming definitions. To explain why oxygen saturation isn’t part of the definition of SIRS it is necessary to remember that oxygenation and ventilation are separate entities. SIRS primarily results in changes in ventilation (respiratory rate and PaCO2) due to acid-base disorders such as metabolic acidosis from increased lactate levels. Oxygenation is unaffected by the SIRS response, although it can be decreased by the process causing the SIRS (i.e. a pneumonia causing SIRS could both affect oxygenation and ventilation).

SIRS is the hardest concept conceptually. The remainder of the following definitions simply build upon it.


As stated above, SIRS can come in two flavors, infectious and non-infectious. Sepsis is simply SIRS caused by a known infection.

Severe Sepsis

Severe sepsis is sepsis with organ dysfunction, hypoperfusion abnormality or sepsis-induced hypotension. Patients who would fall into this category would be those with organ function tests elevated above baseline, elevated lactate, acid-base disorders, altered mental status, oliguria or blood pressure changes meeting the criteria below.

Sepsis-induced Hypotension

Sepsis-induced hypotension is a systolic blood pressure < 90 mm Hg or a systolic blood pressure ≥ 40 mm Hg less than baseline. A patient whose systolic blood pressure is chronically 200 mm Hg may well be experiencing hypoperfusion at a “normal” blood pressure of 120/80..

Septic Shock

Septic shock is severe sepsis and sepsis-induced hypotension that is unresponsive to “adequate” fluid resuscitation.

Multiple Organ Dysfunction (MODS)

MODS is the “presence of altered organ function in an acutely ill patient such that homeostasis cannot be maintained without intervention.” Lactic acidosis from organ hypoperfusion or changes in cellular metabolism are an example of MODS. MODS may occur either by primary organ injury (such as trauma or chemical insult) or secondary to the SIRS response.

Sensitivity & Specificity of the SIRS Criteria


SIRS is a definition. And so to ask what its sensitivity and specificity is enters the realm of asking how often a tree is really a tree, or how often obscenity is obscene. There are really two important questions for the clinician regarding the SIRS criteria. How often are the SIRS criteria met even though the patient is lacking a systemic inflammatory response? And how often does positive SIRS criteria in a patient represent an underlying infectious etiology?

How often are the SIRS criteria met even though the patient is lacking a systemic inflammatory response?

Clinical judgement.

And how often does positive SIRS criteria in a patient represent an underlying infectious etiology?


SIRS is a marker of inflammation, not infection.

But the definition was created to improve the speed of diagnosis and the implementation of treatment. Does it successfully do this?

SIRS was made to be a sensitive but nonspecific definition. Casting the note wide so as to include many types of patients. Two broad categories exist: infectious SIRS and non-infectious SIRS.


The SIRS to identify infected patients in the emergency room. SIRS plus suspected infection. How often were these patients actually infected? Although it is never said, what they are really asking is how often does SIRS plus suspected infection equal a patient that is proven septic.

734 enrolled with suspected infection
503 met SIRS criteria (68.5%)
657 were discharged with a clinical diagnosis of infection (89.4%)
276 were discharged with a microbial confirmation of infection



(3) Looking at SIRS the wrong way. Should we be trying to make it more specific?
How often is infection found in a SIRS patient, with the end result being sepsis.

Factors Complicating Use

Can you become septic without meeting SIRS?

½ of bacteremic patients do not mount a febrile response


Future Directions

Markers to increase specificity of SIRS for sepsis


  1. Editorial 1
  2. Consensus statement 2
  3. 2003 sirs in the ed 3



Garbage Can

How sepsis is treated in modern medicine is a story of definition. Sepsis is a difficult to define entity and exists in a large gray area. In the early 1990’s researchers and practionners struggled with this. Can a patient be septic if they are not bacteremic? How does one define sepsis in a research trial? New criteria was set forth in 1992 simultaneously in Chest and Critical Care Medicine by an ACCP/SCCM consensus conference with the goal of standardizing definitions for the broad range of manifestations of sepsis(1,2).

Match the following states to their appropriate patient:








Septic Shock


Septic Syndrome



Difficult? This is the state that clinicians had found themselves in. And clear definitions were needed to sort out the mess.





Eliminated from usage

Systemic Inflammatory Response Syndrome

  • Temp > 38 or < 36
  • Heart rate > 90
  • Tachypnea > 20 or hyperventilation with a PaCO2 less than 32
  • WBC count >12000 or less than 4000 or the presence of more than 10% bands

Can be the result of many non-infectious states (2)

  • Pancreatitis
  • Ischemia
  • Trauma
  • Hemorrhagic shock
  • Immune-related organ injury


SIRS + known infection

Severe Sepsis
Sepsis + organ dysfunction, hypoperfusion or hypotension (oliguria, lactic acidosis, AMS)
Sepsis induced hypotension is defined as a systolic blood pressure of < 90 mmHg or a MAP < 70 mm Hg or a SBP decrease > 40 mm Hg or < 2 SD below normal for age in absence of other causes of hypotension

Septic Shock
Severe Sepsis + sepsis induced hypotension despite adequate fluid resuscitation

Sepsis induced hypotension <90 or 40 below baseline


SIRS Criteria
Last Updated June 28, 2010