Is mitochondrial disease difficult to diagnose? How is a diagnosis reached?

Often it is. Mitochondrial diseases can present from any organ system at any age. Diagnosis can be made by specific clinical criteria (symptoms, brain MRI imaging, muscle biopsy, elevated lactate in blood) or, by finding a genetic mutation either in mitochondrial dna or one of the hundreds of other genes that cause mitochondrial disease. Latter is a more accurate diagnosis.