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Comment & Response
March 26, 2020

Double Counting of Patients in Meta-analyses of Observational Studies

Author Affiliations
  • 1Division of Hematology, Faculty of Medicine, University of British Columbia, Vancouver, British Columbia, Canada
JAMA Oncol. Published online March 26, 2020. doi:10.1001/jamaoncol.2020.0167

To the Editor We read with interest the systematic review and meta-analysis by Gagelmann and colleagues.1 This is an important synthesis of the literature on the emerging topic of haploidentical donor transplantation using posttransplant cyclophosphamide for graft-vs-host disease prophylaxis. One of the purported strengths of this study is the large number of patients (N = 22 974) included in the meta-analysis; however, we wish to point out that there were likely fewer unique patients, as some patients appear to be reported in more than 1 of the 30 included studies. For example, the 2016 single-center retrospective review by Bashey and coauthors2 (N = 475 patients with hematologic cancers; time horizon, February 2005 to February 2014) appears to include almost all the patients included in the 2013 retrospective review at the same center by Bashey and colleagues3 (N = 271 patients with hematologic cancers; time horizon, February 2005 to October 2010), except for 14 patients in the 2013 paper who received matched unrelated donor transplant with a donor matched at 9 of 10 HLA loci (all matched unrelated donors in the 2016 paper were matched at 10 of 10 loci). Similarly, the 2019 retrospective review of Center for International Blood and Marrow Transplant Research data by Rashidi and coauthors4 examining outcomes after matched sibling vs haploidentical donor transplants for patients with acute myeloid leukemia (n = 336 haploidentical transplant recipients; time horizon, 2005 to 2015) appears to include all patients with acute myeloid leukemia who underwent haploidentical donor transplant and were included in the 2016 single-center retrospective review by Rashidi and colleagues5 that compared outcomes vs matched unrelated donor transplants (n = 52 haploidentical transplant recipients; time horizon, January 2010 to August 2015). In these 2 examples, we identified 309 patients who appear to have been counted twice in the meta-analysis, and we also identified several other examples.

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