Screening for mitochondrial cytopathies: the sub-anaerobic threshold exercise test (SATET).

A simple test is described for identifying patients with abnormalities of muscle energy metabolism secondary to mitochondrial dysfunction, based on the venous lactate response to exercise at 90% of predicted work rate at the anaerobic threshold. The test was standardised for age, weight and sex of subjects, and was abnormal in all cases of mitochondrial cytopathy tested, with a false positive rate of 7% in a control population. The test was abnormal in two cases of mitochondrial disease in which muscle biopsy was normal or showed only non-specific changes.