The purpose of this study is to collect historical occurrences of risk factors that are potentially associated with the development of anti-erythropoietin (EPO) antibody positive pure red cell aplasia (PRCA) in participants with chronic kidney disease who have been recently treated with epoetin alfa (EPREX).

This is a multicenter (study conducted at multiple sites), case-control (study that compare individuals with a disease or condition [cases] to a group of individuals without the disease or condition [controls] to determine the possible factor which increased disease incidence), retrospective (a study in which the participants are identified and then followed backward, as time passes) study. Retrospective risk factor data will be collected for control participants matched to the subset of participants in Protocol EPO-IMU-301 identified as having chronic kidney disease and anti-EPO antibody positive PRCA that began while the participant was receiving treatment with EPREX (index participants). For each index participant, up to 4 matched non-PRCA control participants with chronic kidney disease will be enrolled in this study. Approximately 600 control participants will be enrolled in this study. Control participants will be selected from the same site as the index participant and the data will be collected from the date closest to the reference date (loss of efficacy [drop in hemoglobin of greater than 2 g/dL/month] was first seen) that the control participant satisfies all study inclusion and exclusion criteria. The optional pharmacogenomic part (testing for polymorphisms and haploid types of the erythropoietin gene) will be recorded for the control participants who will sign the pharmacogenomics part of the study. No drug administration or treatment will be mandated by this study. Safety evaluation will include assessment of adverse events.

Inclusion Criteria:

• History of anemia due to chronic kidney disease
• Pure red cell aplasia (PRCA) associated with erythropoietin-alpha (EPO) treatment
• Treatment with EPO for a minimum of 2 months occurring within more or less 3 months of the reference date (date of loss of efficacy [drop in hemoglobin of greater than 2 g/dL/month] was first observed)

Exclusion criteria

• History of and information related to past exposure to EPO not available
• History of PRCA or anti-EPO antibody positive status before or after the reference date

Participants having chronic kidney disease and an immune-mediated cause of pure red cell aplasia indicated by the presence of anti-erythropoietin antibodies in their serum at the time of loss of efficacy