Quantifying comorbidity in a disease-specific cohort: Adaptation of the Total Illness Burden Index to prostate cancer

Objectives. Numerous studies have demonstrated the importance of comorbid i llness when analyzing medical outcomes. The purpose of this study was to ad apt a generic comorbidity index, the Total Illness Burden Index (TIBI), for use in men with prostate cancer, and to evaluate the usefulness of the new instrument in adjusting for the impact of comorbidity on functional outcom es in a prostate cancer cohort. Methods. The TIBI uses patients' self-report of symptoms and diagnoses to d etermine not only the presence but also the severity of comorbidities in ea ch of 16 body system domains. To create the TIBI-P (prostate cancer modific ation), some domains were added and others were modified according to clini cal criteria. The TIBI-P was completed by 1658 men with prostate cancer fol lowed up longitudinally in 29 urology practices in the United States. TIBI- P scores were calculated for each patient and analyzed with scores on the S F-36 quality-of-life questionnaire and with patient report of days confined to bed. Results. After adjusting for age and income, lower SF-36 scale scores and i ncreases in confinement to bed were associated with a greater burden of com orbid illness as measured by the TIBI-P, independent of the extent of prost ate cancer. The TIBI-P explained 24% of the variance in the SF-36 physical functioning domain score. Conclusions. The TIBI-P is a powerful measure of the impact of comorbid ill ness on the quality of life and functioning among patients with prostate ca ncer. This index may prove valuable in research on clinical and economic ou tcomes of prostate cancer. (C) 1999, Elsevier Science Inc.