The swinging pendulum of biomarkers in mitochondrial disease: The role of FGF21.

Mitochondrial diseases are clinically and genetically heterogeneous metabolic disorders that often present a substantial diagnostic challenge even for experienced clinicians.1 Although diagnostic clues are sought using widely available clinical chemistry tests (e.g., elevated serum lactate, pyruvate, or creatine kinase levels), patients are frequently subjected to invasive investigations including a muscle biopsy. Even then, a definitive diagnosis may not be forthcoming and consequently the identification of noninvasive diagnostic biomarkers has the potential to streamline the diagnostic process for both clinicians and patients.