Investigations

• blood count ^c  
• urea and electrolytes ^c  

Note:

• Sensitivity of a white cell count > 10 x 10⁹/l for cellulitis: 50% (95% CI: 32% to 68%) ^c

Why?

A normal white cell count and serum sodium make necrotising fasciitis less likely

Patient	Target Disorder and Reference Standard	Diagnostic Test	LR+ (95% CI)	Post-test Probability	LR- (95% CI)	Post-test Probability
admitted to hospital with soft tissue infection c (pre-test probability:9%)	necrotising fasciitis (surgery, follow-up)	WCC > 15.4 x 109/l or Na < 135 mmol/lor both	3.7 (3.0 to 4.6)	26%	0.13 (0.044 to 0.38)	1.2%