Postpartal the urinary excretion of methylmalonic acid increased markedly accompanied by a decline of the renal function, but improved partially in the following months. This is the first report of pregnancy in a woman with Cobalamin-Bdeficiency. A close observation is necessary to ensure an optimal care during and after pregnancy in patients with inborn metabolic diseases. Newborn screening for propionic (PA) and methylmalonic acidemias (MMA) based on measuring propionylcarnitine (C3) is neither sensitive nor specific. Due to a significant overlap in C3 concentration between affected and unaffected individuals, ratios such as C3/C2 were introduced to improve the newborn screening algorithms. However, the false positive rate of C3 disorders remains significant. While methylcitrate (MCA) is a specific marker for C3 disorders, it is not detectable by the current MS/MS screening method. To overcome this, we developed a simple and specific LC-MS/MS method for MCA in DBS samples. The method is based on derivatization with DAABD-AE which was undertaken to improve the poor MS properties of MCA. No separate extraction step was required and derivatization was achieved by incubating a 3.2 mm disc with DAABD-AE at 60:C for 45 min. MCA peak eluted at 2 min and the injection to injection time was 10 min. The method was successfully applied for the analysis of DBS samples from established PA and MMA patients (n=10) as well as controls (n=310). We anticipate that the implementation of this method will improve the screening process for PA and MMA by reducing the false positive rate and increasing the positive predictive value. Methylmalonic acidurias (MMA) constitute an important group of inherited metabolic disorders involving neurological deficits. Recent studies have shown an increased reactive oxygen species (ROS) production and apoptotic cell rate in fibroblasts especially from cblB patients with isolated MMA and cblC with combined MMA and homocystinuria. To gain insight into the pathophysiology of these disorders, the goal of this study was to examine mitochondrial dysfunction as a putative disease mechanism. An altered mitochondrial morphology of a grain-like structure was presented mainly in those fibroblasts with an increased ROS content (cblB and cblC), suggesting that ROS might lead to fission and a grain-like structure of the mitochondrial reticulum. Decreased oxygen consumption rate (OCR) was found in patients´fibroblasts. When cells were respiring in glucose free-medium supplemented with galactose, basal and maximal OCR increased in patients and control fibroblasts. Permeabilized fibroblasts from cblB patients compared to patients from other groups and controls, revealed that in vitro mitochondria display an enhanced Ca2+ uptake velocity and premature opening of the permeability transition pore which has been described to provoke the release of mitochondrial proapoptotic factors. These studies using cell models establish that mitochondrial dysfunction is directly or indirectly involved in these cobalamin metabolism defects. Background: Patients with propionic (PAemia) and methylmalonic (MMAemia) acidemias present severe crises of metabolic decompensation in the neonatal period, in which the levels of propionic (PA) and Lmethylmalonic (MMA) acids, respectively, can be as high as 2.5-5 mM. Treatment is constituted of a low-protein diet supplemented with Lcarnitine. Recently, some works have suggested that lipid and protein oxidative damage may be involved in the pathophysiology of these diseases, but DNA damage has not been fully investigated.