Necrotizing Fasciitis: Likelihood Ratios

Introduction: An extremely deadly soft tissue infection involving the deep fascia. This may be caused by Group A Streptococcus or gas-forming organisms such as Clostridium perfringens.

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The likelihood ratios of findings for Necrotizing Fasciitis are listed below. See the left navigation bar to change the display.

Pre-Test Probability (Prevalence): %.	Post-Test Probability (Predictive Value): %.	Switch to display mode.
Note that calculating probabilities from more than one** finding is inherently inaccurate because findings are not independent. (For example, using two positive findings that share a common pathogenesis is likely to overestimate the true probability.)

Good Positive and Negative Likelihood Ratios
Finding	Result	+LR	-LR
[ + ] MRI	+ 0 -	7.1	0
But based on a more recent paper, sensitivity and specificity are likely lower. Exact values will depend on which imaging findings are considered. Study: AJR Am J Roentgenol. 1998 Mar;170(3):615-20. PMID: 9490940 Radiology. 2011 Jun;259(3):816-24. PMID: 21406630
[ + ] CT Scan	+ 0 -	5.3	0
Versus non-necrotizing soft tissue infections. (Gold standard - surgery and pathology.) From the article: Four elements consistent with NSTI were sought in particular: (1) asymmetrical and diffuse areas of soft tissue inflammation and ischemia, (2) muscle necrosis, (3) gas across tissue planes, and (4) fluid collections. The test (CT) result was considered positive for NSTI if it identified inflamed and ischemic or necrotic soft tissues with or without gas and fluid collections. It was considered negative if no such findings were reported. Study: Arch Surg. 2010 May;145(5):452-5. PMID: 20479343
[ + ] Leukocytosis or Hyponatremia	+ 0 -	3.8	0.1
admission WBC > 15.4 x 10(9)/L or serum sodium [Na] < 135 mmol/L Study: J Am Coll Surg. 2000 Sep;191(3):227-31. PMID: 10989895
[ + ] Laboratory Risk Indicator in Necrotising Fasciitis	+ 0 -	2.4	0.3
LRINEC >= 6 http://www.mdcalc.com/lrinec-score-for-necrotizing-soft-tissue-infection/ Study: Anaesth Intensive Care. 2009 Jul;37(4):588-92. PMID: 19681416 Crit Care Med. 2004 Jul;32(7):1535-41. PMID: 15241098