Mini Oral Australian Epidemiology Association ASM 2018

A quantitative bias analysis to estimate measurement error-related attenuation of the association between self-reported physical activity and colorectal cancer risk (#72)

Shahid Mahmood 1 , Nga Nguyen 2 , Julie Bassett 2 , Robert MacInnis 2 , Amalia Karahalios 1 , Neville Owen 1 , Fiona Bruinsma 2 , Roger Milne 2 , Graham Giles 2 , Dallas English 2 , Brigid Lynch 2
  1. Centre for Epidemiology and Biostatistics, Melbourne School of Population and Global Health , The University of Melbourne, Melbourne, Victoria, Australia
  2. Cancer Epidemiology Centre, Cancer Council Victoria, Melbourne, Victoria, Australia

Background

Self-reported physical activity is inaccurate, yet few investigators attempt to adjust for measurement error when estimating risks for health outcomes. We estimated what the association between self-reported physical activity and colorectal cancer risk would be if physical activity had been assessed using accelerometry instead.

Methods

Altogether, 235 Australian adults completed a telephone-administered International Physical Activity Questionnaire (IPAQ), and wore an accelerometer (Actigraph GTX3+) for seven days. We calculated a validity coefficient and an attenuation factor using a structural equation model adjusted for age, sex, education and body mass index. The attenuation factor was applied to data from Melbourne Collaborative Cohort Study (MCCS) to compute bias-adjusted hazard ratios (HR) and 95% confidence intervals (CI). 

Results

Average daily minutes of physical activity from the IPAQ-short were considerably higher than accelerometer-measured duration (55 versus 32 minutes). The validity coefficient (0.32; 95% CI: 0.20-0.43) and attenuation factor (0.20; 95% CI: 0.12-0.28) were low, suggesting substantial measurement error from the IPAQ-short. To quantify the attenuation of risk due to measurement error, we estimated the HRs for colorectal cancer risk for high (75th percentile; 411 minutes/week) versus low (25th percentile; 62 minutes/week) levels of self-reported physical activity before (HR=0.95; 95% CI: 0.87-1.05) and after bias adjustment (HR=0.78; 95% CI: 0.47-1.28). 

Conclusions

Over-estimation of physical activity by the IPAQ-short substantially attenuates the association between physical activity and colorectal cancer risk, suggesting that the protective effect of physical activity has been previously under-estimated.