What are the Systemic Inflammatory Response Syndrome (SIRS) criteria?

From the Guidelines

SIRS criteria are met when a patient presents with at least two of the following: temperature >38°C or <36°C, heart rate >90 beats per minute, respiratory rate >20 breaths per minute or PaCO2 <32 mmHg, and white blood cell count >12,000/mm³, <4,000/mm³, or >10% immature bands, as defined by the American Critical Care Medicine consensus 1. The SIRS criteria are used to identify patients with potential systemic inflammation that may progress to sepsis.

• Temperature >38°C (100.4°F) or <36°C (96.8°F)
• Heart rate >90 beats per minute
• Respiratory rate >20 breaths per minute or PaCO2 <32 mmHg
• White blood cell count >12,000/mm³, <4,000/mm³, or >10% immature bands These criteria help clinicians identify patients who may have an inflammatory response to infection or other insults like trauma, burns, or pancreatitis, as described in the study published in Clinical Nutrition 1. While SIRS criteria have been widely used since the 1990s, newer scoring systems like qSOFA (quick Sequential Organ Failure Assessment) and the Sepsis-3 definitions have somewhat replaced SIRS for sepsis identification specifically, however, the definition provided by 1 is the most recent and highest quality study. The absence of a source of an infective focus is also an important consideration in the diagnosis of SIRS, as noted in the study 1. In clinical practice, it is essential to consider these criteria as part of a comprehensive assessment to identify patients who may need closer monitoring or further evaluation for potential infection or inflammatory conditions, as supported by the evidence from 1.

From the Research

Definition and Criteria of SIRS

• The Systemic Inflammatory Response Syndrome (SIRS) criteria were established to define the clinical expression of the action of complex intrinsic mediators of the acute phase reaction 2.
• SIRS can be precipitated by events such as infection, trauma, pancreatitis, and surgery, and can compromise the function of various organ systems resulting in Multiple Organ Dysfunction Syndrome (MODS) 2.
• The consensus definition of severe sepsis requires suspected or proven infection, organ failure, and signs that meet two or more criteria for SIRS 3.

Association of SIRS Criteria with Mortality

• Different individual and combinations of SIRS criteria were associated with marked differences in hospital mortality, implying that individual SIRS criteria are not equivalent or interchangeable 4.
• Mortality increased linearly with each additional SIRS criterion, without any transitional increase in risk at a threshold of two SIRS criteria 3.
• The need for two or more SIRS criteria to define severe sepsis excluded one in eight otherwise similar patients with infection, organ failure, and substantial mortality 3.

Limitations and Relevance of SIRS Criteria

• The systemic inflammatory response syndrome has low specificity to identify infected patients at risk of worsening to severe sepsis or shock 5.
• A score estimating the risk of worsening sepsis, using objectively defined criteria for SIRS, could be used by physicians to stratify patients for clinical management and to test new interventions 5.
• Clinicians should learn to identify SIRS in their patients at an early stage to determine the underlying cause and treatment before the SIRS progresses to a more severe form 2.

Historical Context and Development of SIRS

• The concept of SIRS was developed in 1991 as a set of clinical parameters to aid in the early identification of potential candidates to enter into clinical trials to evaluate new treatments for sepsis 6.
• The definition of SIRS was built upon a foundation of basic clinical and laboratory abnormalities that were readily available in almost all clinical settings 6.