The impact of competing risks on the observed rate of chronic hepatitis C progression

Background & aims In previous studies about the natural history of chronic hepatitis C (CHC), age at the time of infection correlated with the rate at which hepatic fibrosis progresses. The presence of a competing risk, namely higher mortality from natural causes, may contribute to this observation. A simulation experiment was conducted to measure the magnitude of the effect of competing risks on the observed rate of fibrosis progression of CHC. Methods A computer-based probabilistic model was created in which fibrosis of CHC progressed from stage 0 to 4 (cirrhosis) in 20-year-old and 50-year-old male and female cohorts. The rate of fibrosis progression was randomly assigned to each simulated individual from a distribution common to all age- and sex-specific cohorts. The cohorts also experienced mortality from natural causes according to the 2000 census data. Results The observed median time to reach cirrhosis for the 50-year-old cohorts was 20.4 ± 0.2 years compared with 29.7 ± 0.2 for the 20-year-old cohorts (P < 0.01). The median time to reach cirrhosis in men was 24.2 ± 0.6 years compared with 25.9 ± 0.6 in women (P = 0.01). Overall, the observed rate of progression was slowest among young women. Similarly, accelerating mortality from natural causes, simulating the impact of comorbid conditions that shorten survival, reduced the observed time to reach cirrhosis. Conclusions Even if the underlying rate of fibrosis progression in CHC was held constant, the time to reach cirrhosis will be observed to be substantially shorter in subjects with a higher competing mortality.