<sec><title>Objectives</title>Functional status assessments of activities of daily living may improve prognostic precision during initial diagnostic evaluations in young and middle-aged adults with cancer. However, the association between pretreatment functional status and survival in these patients is poorly understood. This study aimed to evaluate the prognostic value of functional status in young and middle-aged patients with cancer.

</sec><sec><title>Design</title>Multicentre retrospective cohort study.

</sec><sec><title>Setting</title>We used a cancer registry from Osaka Prefecture, Japan. The data were linked to administrative claims data from 35 hospitals in the same prefecture.

</sec><sec><title>Participants</title>Patients aged 18–69 years who received new diagnoses of gastric, colorectal or lung cancer between 2010 and 2014.

</sec><sec><title>Main outcome measure</title>Cox proportional hazards models of 5-year all-cause mortality were developed to examine the prognostic impact of pretreatment functional status, which was categorised into three levels of functional disability (none, moderate and severe) based on Barthel Index scores. The models controlled for age, sex, comorbidities, cancer stage and tumour histology.

</sec><sec><title>Results</title>We analysed 12 134 patients. Higher mortality risks were significantly associated with moderate functional disability (adjusted HR 1.44 (95% CI 1.18 to 1.75), 1.35 (95% CI 1.08 to 1.68) and 1.74 (95% CI 1.50 to 2.03) in patients with gastric, colorectal and lung cancer, respectively) and severe functional disability (adjusted HR 3.56 (95% CI 2.81 to 4.51), 2.37 (95% CI 1.89 to 2.95) and 2.34 (95% CI 2.00 to 2.75) in patients with gastric, colorectal and lung cancer, respectively).

</sec><sec><title>Conclusion</title>Accounting for functional status at cancer diagnosis may improve the prediction of survival time in young and middle-aged adults with cancer. Functional status has potential applications in survival predictions and risk adjustments when analysing outcomes in patients with cancer.

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