Platelet contamination causes large variation as well as overestimation of mitochondrial DNA content of peripheral blood mononuclear cells

Background: Alterations in the copy number of mitochondrial DNA (mtDNA) play a role in the pathogenesis of mitochondrial diseases and other many common diseases. Recently, the copy number of leukocyte mtDNA has been considered to serve as a biomarker to monitor or chase such diseases. Therefore, reproducible mtDNA measurement is required. Methods: Peripheral blood mononuclear cells were prepared by a density-based method. The mtDNA/cell was measured by quantitative realtime polymerase chain reaction. Results: The degree of platelet contamination varied to a large extent among preparations. The mtDNA copy numbers per mononuclear cell were 269 +/- 51 and 146 +/- 14 in the samples before and after the platelet depletion, respectively. Conclusion: A density-based mononuclear cell preparation causes heavy platelet contamination. The platelet depletion from a sample is particularly important for comparing the mtDNA contents between different dates or between different patients.