What it is
The Kocher criteria help distinguish septic arthritis of the hip from transient (toxic) synovitis in a child presenting with hip pain, limp or refusal to bear weight. Four independent predictors generate a probability that the joint is septic — a condition that needs urgent aspiration and washout to prevent joint destruction.
The four predictors
Each is present (1) or absent (0):
- Non-weight-bearing on the affected side
- Fever >38.5°C
- ESR >40 mm/hr
- Serum WBC >12,000/µL
Probability by number of predictors
The toolkit uses Kocher’s original derivation probabilities:
| Predictors | Probability of septic arthritis |
|---|---|
| 0 | <0.2% |
| 1 | 3% |
| 2 | 40% |
| 3 | 93% |
| 4 | 99% |
The 2004 validation cohort showed lower probabilities for the same counts (e.g. ~93% became ~93% at 4, but 3 predictors fell to ~83%, 2 to ~62%), and later external series lower still — so treat the high-end figures as cohort-specific, not absolute.
When to use it
In the acutely limping or non-weight-bearing child after trauma is excluded, to decide how aggressively to pursue septic arthritis — ultrasound for effusion, inflammatory markers, and orthopaedic referral for aspiration. It is most useful at the extremes (0 predictors reassuring, 3–4 alarming).
Worked example
A 6-year-old refuses to bear weight (1), temperature 39.0°C (1), ESR 55 mm/hr (1), WBC 11,000/µL (0).
3/4 predictors → ~93% probability (original table). Urgent orthopaedic review and hip aspiration are warranted; do not wait out a “viral synovitis”.
Pitfalls
- The score does not rule out septic arthritis. A septic-looking child, or a hot irreducible joint, needs orthopaedic review and aspiration regardless of score.
- CRP is not in the original four but is a strong fifth predictor in later work (CRP >20 mg/L); incorporate it.
- Validation cohorts give lower probabilities than the 1999 figures — counsel and consent on the conservative end.
- Early septic arthritis and partially-treated cases can present with few predictors; re-examine and repeat markers if the picture evolves.
- Designed for the hip — do not transplant the cutoffs to other joints uncritically.
Run it: Kocher Criteria (Septic Hip)
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.