Background: Mitochondrial disorders are characterized by an accumulation of lactate and an insufficient oxygen extraction from blood during exercise. Therefore, both parameters (lactate and oxygen saturation) can be used as screening tests in mitochondrial disorders. However, conflicting results regarding sensitivities and specifities of both tests have been reported. Method: We examined 27 patients with genetically defined mitochondrial disorders (single deletions n=15,multiple deletions n = 5, A3243G mutation n = 7), patients with other neuromuscular disorders, and healthy controls. In the first test subjects performed intermittent isometric handgrip exercise (0.5 Hz)at 80 % (3 minutes) and 30 % (3 and 15 minutes) of maximal contraction force (MCF). Oxygen saturation and partial pressure in cubital venous blood from the exercising arm were measured. In the second test subjects underwent cycle ergometry at 30 W for 15 minutes. Venous lactate at rest, during and 15 minutes postexercise was determined. Result: Both tests showed specificities of 92- 96% . Sensitivities for changes of venous oxygen partial pressure and oxygen saturation ranged from 21- 26% at 80% MCF for 3 minutes to 47- 58% at 30% MCF for 15 minutes. Sensitivities for venous resting, peak, and post- exercise lactate was 33% , 58% , and 67% , respectively. The degree of deoxygenation, however,was independent of the intensity and duration of the applied forces. Oxygen desaturation and lactate increase in patients with mitochondrial disorders were not different in patients with and without clinical symptoms of myopathy. There were significant correlations between the heteroplasmy and both the degree of oxygen desaturation and lactate increase in patients with single deletions. In patients who performed both protocols (n = 16) a combination of both tests increased sensitivity up to 87% . Conclusion: Oxygen desaturation in forearm exercise tests and lactate increase in cycle ergometry tests show a high specifity but only moderate sensitivity. Combination of the two screening test clearly increases the sensitivity. Background: Mitochondrial disorders are characterized by an accumulation of lactate and an insufficiency oxygen extraction from blood during exercise. Therefore, both parameters (lactate and oxygen saturation) can be used as screening tests in mitochondrial disorders. However, conflicting results regarding sensitivities and specifities of Both tests have been reported. Methods: We examined 27 patients with genetically defined mitochondrial disorders (single deletions n = 15, multiple deletions n = 5, A3243G mutation n = 7), patients with other neuromuscular disorders, and healthy controls. In the first Oxygen saturation and partial pressure in cubital venous blood from the exercising arm were measured. The test subjects performed intermittent isometric handgrip exercise (0.5 Hz) at 80% (3 minutes) and 30% (3 and 15 minutes) of maximal contracting force . In the second test subjects underwent cycle ergometry at 30 W for 15 minutes. Venous lactate at rest, during and 15 minutes postexercise was determined. Both tests showed specificities of 92-96%. Sensitivities for changes of venous oxygen partial pressure and oxygen saturation ranged from 21- 26% at 80% MCF for 3 minutes to 47-58% at 30% MCF for 15 minutes. Sensitivities for venous resting, peak, and post-exercise lactate was 33%, 58%, and 67%, respectively. The degree of deoxygenation, however, was independent of the intensity and duration of the applied forces. Oxygen desaturation and lactate increase in patients with mitochondrial disorders were not different in patients with and without clinical symptoms of myopathy. There were significant correlations between the heteroplasmy and both the degree the oxygen of desaturation and lactate increase in patients with single deletions. = 16) a combination of both tests increased sensitivity up to 87%. Conclusion: Oxygen desaturation in forearm exercise tests and lactate increase in cycle ergometry tests show a high spellcifity but only moderate sensitivity. Combination of the two screening test clearly increases the sensitivity.