As it is under debate if determination of
lactate during cycle ergometry (
lactate stress testing, LST) under a continuous, unadjusted, low workload is a valuable diagnostic tool for
mitochondrial myopathy (
MMP), the present study aimed to investigate how sensitive the LST is in a large cohort of patients with indications for
MMP (
MMP patients). Serum
lactate was determined once before, three times during, and once after a 15-min, constant 30 W-workload on a bicycle ergometer in 115 healthy controls, 166 patients with
neurological disorders other than
MMP, and 291
MMP patients. Serum
lactate's upper reference limit at rest, 5, 10, 15 min after starting, and 15 min after finishing the exercise was 2.0, 2.0, 2.1, 2.0 and 1.7 mmol/l, respectively. Resting
lactate was increased in 75
MMP patients (26%). The specificity of resting
lactate determination was 84%. The sensitivity of the LST was 66% and the specificity 84%. Among the 192
MMP patients with abnormal LST, 120 (63%) had a normal resting
lactate. The LST is abnormal in two thirds of the
MMP patients. The sensitivity of the LST is higher than that of resting
lactate determination. The LST is a simple and cheap but effective and reliable screening method for detecting the impaired oxidative metabolism in
MMP.