Kocher criteria for a child with a painful hip, suspected to have septic arthritis:
1) non-weight-bearing on affect side
2) sedimentation rate greater than 40 mm/hr
3) fever
4) WBC >12,000

Criteria met and probability child has septic arthritis
4/4 -- 99%
3/4 -- 93%
2/4 -- 40%
1/4 --  3%

See:
Kocher etal. Validation of a Clinical Prediction Rule for the Differentiation Between Septic Arthritis and Transient Synovitis of the Hip in Children. The Journal of Bone and Joint Surgery (American) 86:1629-1635 (2004).
http://www.ejbjs.org/cgi/content/abstract/86/8/1629