Impact of Lookback Duration on the Performance of a Claims-Based Frailty Proxy in Women With Stage I-III Breast Cancer.

Background: Frailty is an important prognostic indicator in older women with breast cancer. The Faurot frailty index, a validated claims-based frailty proxy measure, uses healthcare billing codes during a user-specified ascertainment window to predict frailty. We assessed how the duration of frailty ascertainment affected the ability of the Faurot frailty index to predict one-year mortality in women with stage I-II breast cancer.

Methods: We included 128 857 women (66+ years) with stage I-III breast cancer in the SEER-Medicare database (2003-2019). The Faurot frailty index was calculated using 3-, 6-, 8-, and 12-month ascertainment windows prior to diagnosis or using all-available lookback. Associations between the Faurot frailty index using each window and one-year all-cause mortality were estimated using Kaplan-Meier curves. Discrimination of one-year mortality risk was assessed using C-statistics.

Results: Five percent of women died during the year following diagnosis. Higher Faurot scores were associated with increased mortality risk for all frailty ascertainment windows. Differences in one-year mortality risk for women with high vs. low Faurot frailty scores were reduced when using all-available lookback (16% vs. 2%, difference = 15%, 95% CI 0.14-0.15) compared to shorter windows (e.g., 8 months: 25% vs. 2%, difference = 23%, 95% CI 0.22-0.24). C-statistics ranged from 0.758 (all-available lookback) to 0.770 (12 months) and were robust in subgroups defined by age, race, ethnicity, region, stage, and cancer subtype.

Conclusions: The Faurot frailty index performed well across 3- to 12-month frailty ascertainment windows in women with breast cancer. Researchers should employ this index to address confounding by frailty in studies of cancer populations.