Quantifying comorbidity in a disease-specific cohort: adaptation of the total illness burden index to prostate cancer

Numerous studies have demonstrated the importance of comorbid illness when analyzing medical outcomes. The purpose of this study was to adapt 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 outcomes in a prostate cancer cohort.

The TIBI uses patients' self-report of symptoms and diagnoses to determine not only the presence but also the severity of comorbidities in each of 16 body system domains. To create the TIBI-P (prostate cancer modification), some domains were added and others were modified according to clinical criteria. The TIBI-P was completed by 1638 men with prostate cancer followed up longitudinally in 29 urology practices in the United States. TIBI-P scores were calculated for each patient and analyzed with scores on the SF-36 quality-of-life questionnaire and with patient report of days confined to bed.

After adjusting for age and income, lower SF-36 scale scores and increases in confinement to bed were associated with a greater burden of comorbid illness as measured by the TIBI-P, independent of the extent of prostate cancer. The TIBI-P explained 24% of the variance in the SF-36 physical functioning domain score.

The TIBI-P is a powerful measure of the impact of comorbid illness on the quality of life and functioning among patients with prostate cancer. This index may prove valuable in research on clinical and economic outcomes of prostate cancer.