RESUMEN
PURPOSE: Establishing accurate estimates of physical activity at baseline is essential for interventions assessing the potential benefits of exercise in adults with cancer. This study compares self-reported physical activity with independent data from activity trackers in women with early breast cancer (BC) recruited into a "walking" intervention during chemotherapy. METHODS: Baseline (pre-intervention) questions inquired about self-reported physical activity-number of walking days/week and minutes/day-in women who were initiating chemotherapy for Stage I-III BC. Activity trackers measured steps per day during the first full week of chemotherapy. Weighted Kappa statistic and Pearson correlation coefficients were used to evaluate agreement and association between self-reported and objectively tracked physical activity levels, respectively. Univariate analyses were conducted to identify variables that may influence congruence between the two measures. RESULTS: In a sample of 161 women, 77% were white, with mean age 56 years. Agreement between self-reported and objectively tracked physical activity was "fair" (kappa coefficient = 0.31), with most patients (59%) over-reporting their physical activity levels. There was weak correlation between the two measures (r = 0.24); however, correlation was strong in participants who were not married (r = 0.53) and/or living alone (r = 0.69). CONCLUSIONS: Objective methods for assessing physical activity (activity trackers, accelerometers) should be used as a complement to self-reported measures to establish credible activity levels for intervention studies seeking to increase physical activity and/or measure the impact of increased physical activity in women with breast cancer.