Small-airway function at seven years old tracked with COPD at fifty-three

Design

Prospective birth-to-midlife cohort with group-based trajectory modelling (Tasmanian Longitudinal Health Study)

Population

2,314 people recruited at age 7 from Tasmanian schools and followed to mean ages 13, 18, 45, 50 and 53

Primary outcome

FEF 25-75% trajectories from age 7 to 53 and their association with COPD by age 53

Effect

Persistently low adjusted OR 112 (95% CI 42-304); early normal with reduced growth and rapid decline 41 (15-112); early below average with reduced growth 3.1 (1.0-9.2)

Most life-course lung function work has used FEV1, a large-airway measure. This Tasmanian cohort used mean forced expiratory flow between 25% and 75% of vital capacity, a crude proxy for mid-to-small-airway function, measured first at age 7 and then repeatedly to a mean age of 53 in 2,314 people recruited from Tasmanian schools in 1968.

Six trajectories emerged. Three were associated with COPD by age 53: persistently low (adjusted odds ratio 112, 95% CI 42-304), early normal with reduced growth and rapid decline (41, 15-112), and early below average with reduced growth (3.1, 1.0-9.2). The factors associated with landing in an impaired trajectory were childhood ones - no breastfeeding, asthma or wheeze, allergic rhinitis, pneumonia or pleurisy - plus maternal asthma and parental smoking before conception. The persistently high trajectory was associated with parental preconception microbial exposures, a hygiene-hypothesis finding that should be treated as interesting rather than established.

The odds ratios are enormous and their confidence intervals are correspondingly enormous, which is what happens when a small number of people in an extreme trajectory group develop the outcome. This is an observational cohort and causality cannot be inferred, as the authors say.

What it supports is a shift in where COPD prevention is aimed. If a substantial share of midlife airflow obstruction is set by small-airway function established in early childhood, then childhood asthma control, pneumonia prevention and household smoke exposure are COPD interventions, not just paediatric ones. That framing matters more in India than in Tasmania, where childhood pneumonia and biomass smoke exposure remain common and are already modifiable.

• Treat childhood wheeze, pneumonia and household smoke exposure as long-term lung outcomes, not episodes
• Ask about childhood respiratory illness when a patient presents with unexplained midlife obstruction
• Do not use FEF 25-75% as a diagnostic test in an individual - it is noisy and was used here for trajectories
• Counsel on parental smoking before conception as well as during pregnancy
• Read the enormous odds ratios as direction, not magnitude

The statistics, in plain English

An adjusted odds ratio of 112 with an interval from 42 to 304 is not a precise estimate of anything; it tells you the association is strong and that very few people sit in that trajectory group, which is why the interval spans a sevenfold range. The lowest-risk of the three impaired trajectories, 3.1 with an interval of 1.0 to 9.2, only just excludes 1.0 and should be read as borderline. This is a single cohort born in one place in one era, and the preconception exposures were recalled by parents decades later, which is a further source of error.

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