Statins and lower mortality in rheumatic diseases: An effect of immortal time bias?

OBJECTIVE

Randomized controlled trials of the effectiveness of statins on rheumatic disease outcomes have shown limited or no benefit. On the other hand, observational studies have reported remarkable effectiveness of statins at reducing mortality in patients with rheumatic diseases. We evaluated these observational studies for the presence of immortal time bias, which tends to exaggerate the effectiveness of drugs by creating a survival advantage for exposed patients.

METHODS

We searched PubMed for observational studies investigating the impact of statins in patients with common rheumatic diseases, including rheumatoid arthritis, osteoarthritis, lupus, ankylosing spondylitis, gout and systemic autoimmune diseases. Studies were included if estimates for all-cause mortality among statin users compared to non-users were reported. We evaluated each study for the presence of immortal time bias and estimated the impact of the bias on the published results.

RESULTS

We found eight observational studies investigating the impact of statins on mortality among patients with rheumatic diseases. Four studies were affected by the classical variant of immortal time bias, while the others introduced immortal time into the comparator via random-calendar date assignment. The studies with the classical form of immortal time bias, which tends to exaggerate drug effectiveness, reported protective effects of statins on mortality ranging from 13% to 57% reductions. In contrast, immortal time bias through random-calendar date assignment, which tends to play down the effectiveness by introducing immortal time in the comparator, reported 16% to 37% reductions in mortality.

CONCLUSION

Bias in observational studies may explain the discrepancy in findings with randomized controlled trials on the effectiveness of statins in patients with rheumatic diseases. Future observational studies will need to rely on incident and prevalent new-user designs that emulate randomized trials and avoid immortal time bias.

Does Metformin Reduce Cancer Risks? Methodologic Considerations

The substantial burden of cancer and diabetes and the association between the two conditions has been a motivation for researchers to look for targeted strategies that can simultaneously affect both diseases and reduce their overlapping burden. In the absence of randomized clinical trials, researchers have taken advantage of the availability and richness of administrative databases and electronic medical records to investigate the effects of drugs on cancer risk among diabetic individuals. The majority of these studies suggest that metformin could potentially reduce cancer risk. However, the validity of this purported reduction in cancer risk is limited by several methodological flaws either in the study design or in the analysis. Whether metformin use decreases cancer risk relies heavily on the availability of valid data sources with complete information on confounders, accurate assessment of drug use, appropriate study design, and robust analytical techniques. The majority of the observational studies assessing the association between metformin and cancer risk suffer from methodological shortcomings and efforts to address these issues have been incomplete. Future investigations on the association between metformin and cancer risk should clearly address the methodological issues due to confounding by indication, prevalent user bias, and time-related biases. Although the proposed strategies do not guarantee a bias-free estimate for the association between metformin and cancer, they will reduce synthesis of and reporting of erroneous results.