Practical problems in detecting abnormal mitochondrial function and genomes.

Mitochondrial respiratory chain dysfunction causes a wide range of primary diseases in adults and children, with highly variable organ involvement. Diagnosis involves weighing evidence from a number of sources, including the clinical presentation, metabolic measurements in vivo, imaging studies, analysis of respiratory chain function or enzyme activities in vitro, studies of mitochondrial morphology after biopsy, and mitochondrial (mt) DNA mutation analysis. Irrespective of the category of the information, it can be difficult to determine whether abnormal results are due to primary defects of the respiratory chain or to practical problems that complicate the diagnostic methodology. This review describes six sources of such problems: genetic complexity, tissue and temporal variation, methodological limitations, secondary effects, logistical issues, and questions of interpretation. When these issues are all addressed, a reliable categorization of the diagnosis as definite, probable, or possible respiratory chain defect becomes possible.