Background:The nonischemic forearm exercise test(NIFET)has been shown to be as effective as the classic ischemic forearm exercise test(IFET)in the diagnosis of patients with Mc Ardle disease.Recently,the lactate increase normalized to the mechanical energy production in NIFET was suggested to have a intermediate sensitivity and satisfactory specifity for the screening of mitochondrial disorders.Methods:NIFET at 80%maximal contraction force(MCF)was performed in normal controls(n = 41),patients with mitochondrial disorders(n = 15)and other myopathies(diseased controls,n = 20).26 healthy volunteers also underwent IFET at 80%MCF.The ratio of lactate increase and workload was defined as specific lactate production(mmol x s/N x l).Results:In normal controls there was no significant different lactate increase during NIFET and IFET.The workload performed showed only a weak significant positive correlation with the lactate increase in the NIFET in normal controls(r 2 = 0.20)but not in IFET and NIFET with patients.A moderate negative correlation of specific lactate production and the absolute workload was found in all groups and in both protocols(r2 = 0.22-0.34).The specific lactate production was highest in patients with other myopathies,intermediate in patients with mitochondrial disorders and lowest in normal controls.NIFET showed a sensitivity of only 20 %and a specifity of 95%for normal controls,but only 75 %for diseased controls.Conclusion:The specific lactate production during NIFET is neither sufficently specific nor sensitive for the diagnosis of mitochondrial disorders.Increased specific lactate production during rest-to-work transition period might be caused by increased acetyl group deficits. Background: The nonischemic forearm exercise test (NIFET) has been shown to be as effective as the classic ischemic forearm exercise test (IFET) in the diagnosis of patients with Mc Ardle disease. Recently, the lactate increase normalized to the mechanical energy production in NIFET was suggested to have an intermediate sensitivity and matching specifity for the screening of mitochondrial disorders. Methods: NIFET at 80% maximal contraction force (MCF) was performed in normal controls (n = 41), patients with mitochondrial disorders (n = 15) and other myopathies (diseased controls, n = 20) .26 healthy volunteers also underwent IFET at 80% MCF. The ratio of lactate increase and workload was defined as specific lactate production (mmol xs / N xl). Results: In normal controls there was no significant different lactate increase during NIFET and IFET. the work load showed showed a weak significant positive correlation with the lactate increase in the NIFET in normal controls (r 2 = 0.20) but not in IFET and NIFET with patients. A moderate negative correlation of specific lactate production and the absolute workload was found in all groups and in both protocols (r2 = 0.22-0.34). The specific lactate production was highest in patients with other myopathies, intermediate in patients with mitochondrial disorders and lowest in normal controls. NIFET showed a sensitivity of only 20% and a specifity of 95% for normal controls, but only 75% for diseased controls. Confc: The specific lactate production during NIFET is not sufficently specific nor sensitive for the diagnosis of mitochondrial disorders. Increased specific lactate production during rest-to-work transition period might be caused by increased acetyl group deficits.