Abstract
BackgroundIncidence density ratios (IDRs) are frequently used to account for varying follow-up times when comparing the risks of adverse events in two treatment groups. The validity of the IDR as approximation of the hazard ratio (HR) is unknown in the situation of differential average follow up by treatment group and non-constant hazard functions. Thus, the use of the IDR when individual patient data are not available might be questionable.MethodsA simulation study was performed using various survival-time distributions with increasing and decreasing hazard functions and various situations of differential follow up by treatment group. HRs and IDRs were estimated from the simulated survival times and compared with the true HR. A rule of thumb was derived to decide in which data situations the IDR can be used as approximation of the HR.ResultsThe results show that the validity of the IDR depends on the survival-time distribution, the difference between the average follow-up durations, the baseline risk, and the sample size. For non-constant hazard functions, the IDR is only an adequate approximation of the HR if the average follow-up durations of the groups are equal and the baseline risk is not larger than 25%. In the case of large differences in the average follow-up durations between the groups and non-constant hazard functions, the IDR represents no valid approximation of the HR.ConclusionsThe proposed rule of thumb allows the use of the IDR as approximation of the HR in specific data situations, when it is not possible to estimate the HR by means of adequate survival-time methods because the required individual patient data are not available. However, in general, adequate survival-time methods should be used to analyze adverse events rather than the simple IDR.
Highlights
Incidence density ratios (IDRs) are frequently used to account for varying follow-up times when comparing the risks of adverse events in two treatment groups
This means that even in the case of non-constant hazard functions but a constant hazard ratio (HR), the IDR - independent of the effect size and the sample size - can be used as approximation to the HR if the average follow-up durations in both groups are equal and the Baseline risk (BLR) is not larger than 25%
The IDR should not be used for relative average follow-up durations in the control group below 50%, because in general the IDR represents no valid approximation of the HR and the meaning of the IDR is unclear
Summary
Incidence density ratios (IDRs) are frequently used to account for varying follow-up times when comparing the risks of adverse events in two treatment groups. It is questionable whether the use of the IDR is adequate in this data situation because the median follow-up duration was 14.8 months in the abiraterone group but only 9.3 months in the control group The reason for this large difference was the discontinuation of treatment after disease progression with stopping of the monitoring of adverse events 30 days later. Access to the individual patient data is not available in the assessment of dossiers or publications with aggregate-level data In this situation, a decision has to be made on the situations in which the IDR can or cannot be used as adequate approximation for the HR
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