One of the great examples of a validated decision rule in pediatrics, the Kocher criteria allows you to assess the risk of septic arthritis of the hip in a child. Differentiating transient synovitis (nee toxic synovitis) from septic arthritis is something that I guarantee you will do if you treat children in the ED.
See the tool here at MD Calc, the direct link is – https://www.mdcalc.com/kocher-criteria-septic-arthritis –
Recall the the key criteria are:
- Elevated WBC (>12,000)
- Elevated ESR >40 (or CRP>2)
- Inability to bear weight
The combination of different features is spelled out in the paper – which you should read since it guides our practice – but in summary the likelihood of septic arthritis based on number of predictors was:
- 0 – <0.2%
- 1 – 3%
- 2 – 40%
- 3 – 93.1%
- 4 – 99.6%
In a child with an acutely painful hip that you are concerned could be septic arthritis obtain a CBC and ESR. If you have a low or high suspicion an ultrasound is not necessary – if there’s an effusion and its equivocal Ortho will need to tap the hip anyway – so it doesn’t matter really.
This rule is for the hip and is not validated for use in other joints like the knee. You can extrapolate the results to the knee – but know where the evidence ends and your suppositions begin.
It is also important to note that the original prediction rule utilized ESR. CRP was not widely available yet. It does rise quicker, and ultimately a CRP >2.0 mg/dL could replace ESR in the prediction rule. Overall the performance between the two is similar.
References / Key Articles
Kocher MS, Zurakowski D, Kasser JR. Differentiating between septic arthritis and transient synovitis of the hip in children: an evidence-based clinical prediction algorithm. J Bone Joint Surg Am. 1999 Dec;81(12):1662-70. doi: 10.2106/00004623-199912000-00002. PMID: 10608376.
Kocher MS, Mandiga R, Zurakowski D, Barnewolt C, Kasser JR. Validation of a clinical prediction rule for the differentiation between septic arthritis and transient synovitis of the hip in children. J Bone Joint Surg Am. 2004 Aug;86(8):1629-35. doi: 10.2106/00004623-200408000-00005. PMID: 15292409.