Irritable hip versus septic arthritis — Rapid

Synopsis

Differentiate self-limiting transient synovitis from destructive paediatric hip infection without using a prediction score or ultrasound effusion to provide false reassurance.

Transient synovitis usually affects a well child aged about three to eight after viral illness, with mild restriction and ability to bear some weight.
Septic arthritis is more likely with fever, toxic appearance, complete non-weight-bearing, severe passive-movement pain and raised CRP, but none is an absolute rule.
Kocher or Caird criteria support probability and escalation; they must never be used as a discharge rule or substitute for repeated clinical judgement.
First-line tests when infection is plausible are FBC, CRP, ESR, blood cultures, hip radiographs and urgent ultrasound for an effusion.
Ultrasound is sensitive for joint fluid but cannot determine sterility; image-guided aspiration with microscopy and culture is the diagnostic reference step.
If sepsis or serious infection is likely, obtain cultures rapidly, start empirical intravenous antibiotics and arrange urgent arthroscopic or open drainage without waiting for every result.
First-line treatment for a confidently low-risk irritable hip is rest from painful activity, weight-based analgesia and planned review within 24 to 48 hours.
Failure to improve promptly, symptoms beyond seven days or recurrence requires repeat examination and imaging for Perthes, infection and other pathology.

Key red flags

Fever, tachycardia, toxic appearance or reduced perfusion with an irritable hip indicates sepsis risk and needs emergency escalation.

Complete refusal to bear weight or pseudoparalysis is more concerning than a mild antalgic limp and should not be labelled transient synovitis casually.

Severe pain through tiny passive movements, a hip held flexed and externally rotated or progressive night pain suggests intra-articular pressure or infection.

Normal early CRP, white count or temperature does not exclude infection in an infant, immunocompromised child or early presentation.

An ultrasound effusion confirms fluid but cannot distinguish sterile synovitis from pus; aspiration and culture answer the microbiological question.

Symptoms lasting beyond a week, recurrence or persistent restricted abduction requires reassessment for Perthes disease, inflammatory arthritis, tumour or osteomyelitis.

Investigation priorities

First-line FBC, CRP, ESR and blood culturesFirst stepFirst line

Estimate inflammatory burden, detect bacteraemia and screen for cytopenia or malignant alternatives.

First-line AP and lateral hip radiographsFirst line

Find SUFE, Perthes, fracture, destructive bone change and late joint-space abnormality.

Management branches

High riskTreat possible septic arthritis

The child is febrile, unwell, unable to bear weight or has severe passive hip pain or raised inflammation.

Assess ABCDE and sepsis risk, obtain IV access, blood cultures and urgent paediatric, orthopaedic and microbiology input.
Keep fasting and non-weight-bearing and obtain radiographs and ultrasound without delaying treatment.
Aspirate the hip urgently when feasible and send complete microbiology, then drain arthroscopically or openly as indicated.
Start empirical IV antibiotics promptly in sepsis and tailor to culture, age, host and local resistance.

Key medicines

Empirical intravenous flucloxacillin for presumed MSSAFor a child older than one month, a commonly used severe bone-and-joint infection regimen is 50 mg/kg intravenously every six hours, up to 2 g per dose, verified against current BNFC and local microbiology policy.Modify for immediate penicillin allergy, MRSA risk, age, renal or hepatic impairment and local epidemiology; broaden cover for young or vulnerable hosts only with specialist advice.

Ibuprofen for low-risk transient synovitisChoose the current BNFC oral dose band from the child's age and weight, prescribe briefly, and record formulation strength, volume and interval.Avoid use with dehydration, kidney disease, gastrointestinal bleeding risk or NSAID-sensitive asthma. Improvement after one dose does not exclude infection; escalate fever, worsening pain or complete non-weight-bearing immediately.

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