Table 2.

Limitations of clinical studies with ESRD patients

Variable	Potential Bias
Patient selection	Healthier patients perform better in clinical trials; the sickest patients are often excluded from randomized clinical trials; patients new to dialysis (incident versus prevalent patients)
Geographical disparities	Randomized clinical trials may enroll patients from a specific geographical location (e.g., Japan) or single study site (lacking diversity in race/ethnicity), generating results that may not be extrapolated to the United States patient population; observational studies from clinical settings outside the United States (e.g., Australia) may not translate to United States clinical settings
Clinical intervention	Improved patient outcomes can arise from uncontrolled for health care provider or facility-level practices rather than a treatment variable; study participants respond positively to personal attention and clinical care
Analytic confounders	Unit of analysis or comparison (e.g., patient, clinic, or hospital level); person versus person-time calculations; complexity of statistical tools (e.g., marginal structural models); role of competing events (e.g., dead patients cannot be hospitalized)