A systematic review to identify clinical features that have value in confirming or excluding the possibility of serious infection in children presenting to ambulatory care settings resulted in the calculation of likelihood ratios. Clinical features with a positive likelihood ratio of more than 5.0 were deemed red flags (ie, warning signs for serious infection); features with a negative likelihood ratio of less than 0.2 were deemed rule-out signs.
The features identified in several studies as red flags were :

• Cyanosis (+LR range 2.66-52.20)
• Rapid breathing (+LR 1.26-9.78)
• Poor peripheral perfusion (+LR 2.39-38.80)
• Petechial rash (+LR 6.18-83.70)]

In one primary care study the following were identified as strong red flags:

• Parental concern (+LR 14.40, 95% CI 9.30-22.10)
• Clinician instinct (+LR 23.50, 95 % CI 16.80-32.70)

Temperature of 40 degrees C or more had value as a red flag in settings with a low prevalence of serious infection.
What about ruling out serious illness?
Unfortunately, no single clinical feature had rule-out value but some combinations can be used to exclude the possibility of serious infection-for example, pneumonia is very unlikely (-LR 0.07, 95% CI 0.01-0.46) if the child is not short of breath and there is no parental concern.
An accompanying editorial sums up the challenge of paediatric emergency medicine in a nutshell:
“What is clear is that in 2010 we do not know how to effectively recognise or rule out severe disease in ill children and what is more, we do not even have a cohesive national or even global research strategy to address this problem.”
Diagnostic value of clinical features at presentation to identify serious infection in children in developed countries: a systematic review.
Lancet. 2010 Mar 6;375(9717):834-45