Careful history and examination will give you most of the information, but investigations may be helpful to localise / identify pathology.

Age is key to the differential diagnosis – the very young (developmental dysplasia), school-age child (Perthes disease or irritable hip / transient synovitis) and in the older child (Slipped Upper [Capital] Femoral Epiphysis). At all ages, consider red flag conditions (sepsis, malignancy, non-accidental injury). Rheumatic fever and chronic low grade infection such as TB will also need to be considered in at risk populations.

Transient synovitis of the hip (irritable hip) should be considered in an otherwise well child with an acute onset of limp. Septic arthritis must be considered in the unwell child, febrile child or with raised white cell count. Rheumatic fever must be considered in a school-aged child of Maori or Pacific descent with evidence of a preceding streptococcal infection.

Perthes disease presents in school-age children, often with painless limp although there can be intermittent hip pain. It is important to consider Perthes in the differential diagnosis of knee pain in a school-aged child.

Slipped Upper [Capital] Femoral Epiphysis should always be considered in overweight teenagers with new onset of hip or knee pain. X-rays should be requested (with frog-leg views) and urgent orthopaedic review may be required. This may be more common in children with Down’s syndrome or hypothyroidism or hypogonadism.

Red flag features in the history include fever, weight loss, night pain and lethargy. Ask about trauma and consider non-accidental injury.

A feverish child with a limp, or a non-weight bearing child, needs urgent (same day) assessment by paediatrics or orthopaedics. Rheumatic fever should be considered if there is a new cardiac murmur and evidence of previous streptococcal infection.

Consider referred pain from the spine, groin or pelvis and pathology may not always arise from joints (e.g., hernia, testes, pelvic abscess, appendicitis).

Ask about morning stiffness and joint swelling – consider inflammatory causes such as Juvenile Idiopathic Arthritis (JIA) - isolated hip involvement is unusual with JIA but can occur in enthesitis related arthritis.

Children with hypermobility may get recurrent hip pain, particularly with activities that push the hips through an increased range of movement (e.g., gymnastics, breaststroke). Physiotherapy advice may be useful.