This is more important in studies using the HDPS, as the inclusion of more than 500 covariables in the calculation of the HDPS likely leads to a good discrimination of patients in both groups.
In our main HDPS analysis, we trimmed the cohort using the approach by Crump et al.
If covariables are included in the HDPS that are only weakly associated with the outcome but strongly with the exposure, the precision in the effect estimates might be decreased (Brookhart et al.
The distribution of the HDPS is displayed in Figure 1.
The results of the HDPS analyses changed only slightly, when 100, 200, or 1,000 covariables instead of 500 were selected for the calculation of the HDPS (Figures A4-A6 in the Appendix SA2).
In this study, we applied HDPS methods to control for residual confounding due to disease severity, which was presumed to have affected a recently conducted study on the comparison of the mortality risk after elective PCI between the in- and outpatient setting, in which a lower mortality risk was observed in the outpatient setting compared to the inpatient setting.
Figure 2: Hazard Ratio of Death in the Outpatient versus Inpatient Setting in the Adjusted, PS, and HDPS Analyses HR LCL UCL Adjusted analysis 0.
In our study, trimming led to the exclusion of patients with a low HDPS in the inpatient setting.
Trimming of patients guarantees that the same range of the HDPS and thus a comparable profile in the covariables included in the calculation of the HDPS is present in patients with inpatient as well as outpatient PCIs.
However, although the HDPS adjusts for hundreds of covariables and may adjust for proxies of residual confounders, further unmeasured confounders completely unrelated to the information in the underlying database can never be ruled out.
The implementation of the HDPS in an IPT-weighted analysis might also lead to biased results, if individuals with high weights dominate the analysis.
Besides issues regarding generalizability and unbiasedness of the results, the additional benefit of the HDPS compared to conventional methods is further not consistently reported in the literature.