Study exposures and outcomes

Recorded exposures included patient demographics measured at the index appointment of a screening programme: age (months), sex, height (metres), weight (kg), socioeconomic group and nationality. Age and sex referenced BMI (body mass index for age z-score (BMIz)) was calculated according to the WHO 2007 guidance, and children were categorised as ‘underweight’, ‘normal weight’, ‘overweight’ or ‘obese’.¹⁹ These weight status categories represent the number of SD away from the mean: underweight <−2, normal weight −2 to +2, overweight >+2 and obesity >+3. Biologically implausible values of BMIz were excluded according to WHO Expert Committee 1995 guidance, and implausible values in growth trajectory data up to the age of 10 years were identified using a conditional growth percentile model.²⁰ Socioeconomic group was measured using the MEDEA (Mortalidad en áreas pequeñas Españolas y Desigualdades Socioeconómicas y Ambientales) deprivation index calculated using five census-based socioeconomic indicators: unemployment, manual occupation, temporary employment, illiterate adults and school leavers before 16 years of age. Group 1 represents the least deprivation and group 5 the most deprivation. MEDEA categories also include rural communities and unclassified urban communities.

Outcomes were ascertained from diagnoses recorded in primary care paediatric records during visits to contributing health centres between 1 January 2006 and 31 December 2016 in the Catalan universal healthcare system. Prespecified ICD-10 codes were used to identify first episodes of back pain after 4 years of age: lumbar spine pain (M54.1, M54.3, M54.4 or M54.5), thoracic back pain (M54, M54.6, M54.8 or M54.9) or cervical spine pain (M54.2).