Childhood obesity is a major and growing health concern in the UK. Around 1 in 7 children aged 2-15 are living with obesity, which can lead to serious health problems and early death later in life. Some children develop obesity very early, before the age of 5. It is now recognised genes (the biological instructions that make up an individual) can play an important role alongside lifestyle and environment. Certain rare single-gene conditions ("monogenic obesity") can strongly influence a child's weight.

The NHS already offers genetic testing for children with severe early-onset obesity, but it is not known how useful these tests are in everyday clinical practice. This study will help answer that question.

The DECODE study will look back at information already collected from children aged 2-18 who attended specialist Complications of Excess Weight (CEW) clinics in England between 2021 and 2025. These clinics support children with severe obesity and related health problems. The study will include children whose obesity started before age 5 and who have already had one or both NHS genetic tests: the R149 obesity gene panel or a comparative genomic hybridisation (CGH) array (a test that looks for missing or extra pieces of DNA).

The aim is to find out how often these tests detect a genetic cause of obesity ("diagnostic yield") and whether certain clinical features-such as developmental delay, neurodivergence, short stature or different eating behaviours -help predict a positive result.

No new tests or visits are required for this study. Only anonymised information from medical records will be used. Around 500-800 children from up to ten hospitals are expected to be included.

The findings will help the NHS understand who benefits most from genetic testing and how results can guide treatment, support families, and shape future services.

Inclusion Criteria:

• Aged 2-18 years old
• Early onset of obesity (before 5 years old)
• Obesity (BMI SDS ≥3/ ≥99.6th percentile)
• Attended a Complications of Excess Weight (CEW) clinic appointment between 2021 and 2025 (inclusive) and had a genetic investigation (R149 and/or CGH) array

Exclusion Criteria:

• Onset of obesity after 5 years of age
• History of chemotherapy, radiotherapy, antipsychotics and steroid use (possible iatrogenic causes of obesity)
• Known diagnosis of craniopharyngioma or hypothalamic tumour

Participants will be identified from paediatric tier 3 complications of excess weight (CEW) clinic services in England.