A retrospective study of nearly 1.5 million patients over the course of 44 years showed no evidence to support the claim that neurodegenerative disorders can be transmitted between individuals through blood transfusions.

“Given that the neurodegenerative diseases on the dementia spectrum are relatively common, the misfolded proteins in affected persons have a wide tissue distribution, and the diseases may have a protracted prediagnostic course, potential transmission through transfusion could have important public health implications,” explained study authors led by Gustaf Edgren, MD, PhD, of the Karolinska Institute in Stockholm.

The retrospective cohort study analyzed data on 1,465,845 patients listed in nationwide transfusion registers, all of whom had undergone transfusions between 1968 and 2012 in Sweden, or during 1981-2012 in Denmark. The investigators looked for transfusions in which the donor had a diagnosis – either before or after the transfusion – of Alzheimer’s disease, Parkinson’s disease, or amyotrophic lateral sclerosis, and compared them with transfusions involving two healthy control groups. Creutzfeldt-Jakob disease was not included in the analysis (Ann Intern Med. 2016 Jun 28. doi: 10.7326/M15-2421).

All transfusion recipients were followed from 180 days after the transfusion date in order to minimize the risk of including a patient who had a neurodegenerative disorder that was present but not registered at the time of the transfusion. Follow-ups lasted until either the first diagnosis of a neurodegenerative disorder, death, emigration, or the end of the trial period on Dec. 31, 2012.

If a donor had received a diagnosis of a neurodegenerative disorder within 20 years of the transfusion, all recipients from said donor were considered to be exposed. “The maximum latency of 20 years was used as a compromise between allowing a long preclinical phase – during which donors may still be infectious – and avoiding classifying clearly healthy donors as potentially infectious,” the authors wrote.

Only 42,254 (2.9%) of subjects were deemed to be exposed to a neurodegenerative disorder based on their blood transfusion donor. However, there was no evidence of any increased risk of dementia of any type from exposure to blood from affected donors in any of the recipients over the course of the follow-up period (hazard ratio, 1.04; 95% confidence interval, 0.99-1.09), and similar results were obtained individually for Alzheimer’s disease and Parkinson’s disease. As a control measure, the investigators also tested for hepatitis C transmission before and after transfusions, and were “as expected, readily able to detect transmission of viral hepatitis,” despite finding no traces of neurodegenerative conditions.

“Despite accumulating scientific support for horizontal transmissibility of a range of disorders in the neurodegenerative spectrum through various methods of inoculation in animal models and of variant Creutzfeldt-Jakob disease from human and animal model data, this study provides evidence against blood transfusion as an important route of transmission of neurodegenerative diseases between humans,” the authors concluded.

The authors also added that “given that the study was based on the entire computerized blood banking history in two countries with several decades of follow-up, more comprehensive data are unlikely to be available in the foreseeable future.”

Funding for the study was provided by the Swedish Research Council, the Swedish Heart-Lung Foundation, the Swedish Society for Medical Research, and the Danish Council for Independent Research. One coauthor reported receiving a grant from the Danish Council for Independent Research, and another reported receiving grants from the Swedish Research Council and the Swedish Heart-Lung Foundation during the conduct of the study. No other authors reported any relevant financial disclosures.