[en] BACKGROUND: Although the blood lactate-to-pyruvate (L:P) molar ratio is used to distinguish between pyruvate dehydrogenase deficiency (PDH-D) and other causes of congenital lactic acidosis (CLA), its diagnostic accuracy for differentiating between these 2 types of CLA has not been evaluated formally. METHODS: We conducted a retrospective study of all patients followed for mitochondrial diseases between 1985 and 2005 in a tertiary care pediatric hospital. RESULTS: At the recommended cut point of approximately 25, individual median L:P ratio demonstrated low sensitivity and specificity (77% and 91%, respectively) for differentiating between patients with enzymatically proven PDH-D (n = 11) and those with mitochondrial disease but normal pyruvate dehydrogenase (PDH) activity (non-PDH; n = 35). We observed a strong positive association between L:P ratio and blood lactate in non-PDH CLA, whereas this association was weak in PDH-D CLA. Consequently, patient classification based on median L:P ratio showed improved diagnostic accuracy at higher lactate concentrations: for lactate <2.5 mmol/L the area under the ROC curve was not statistically different from 0.5 (P = 0.3), whereas it was statistically different for lactate >2.5 mmol/L. In the 2.5 to 5.0 mmol/L lactate category, the sensitivity and specificity at an optimal cut point of 18.4 were 93% (95% CI, 77%-99%) and 71% (95% CI, 20%-96%), respectively; for lactate >5.0 mmol/L, with an optimal cut point of 25.8, sensitivity and specificity were 96% (95% CI, 77%-99%) and 100% (95% CI, 59%-100%), respectively. CONCLUSION: Usefulness of the L:P ratio for differentiating non-PDH and PDH-D types of CLA increases at higher lactate concentrations.