Elevated levels of methylmalonic acid (MMA) are caused by inherited defects of enzymes involved in MMA metabolism. Methylmalonic Acidurias (MMAs) are a group of metabolic diseases of propionate catabolism characterized by gastrointestinal and neurometabolic manifestations resulting from a deficiency in the function of methylmalonyl-CoA mutase, methylmalonyl-CoA epimerase, and cobalamin metabolism. Increased levels propionylcarnitine (C3) and/or ratios by MS/MS analysis of NBS, DBS samples are suggestive of propionic acidemia or MMAs. C3 elevation is not considered a specific marker for these diseases, resulting in high false-positive rates. Using analyte ratios increases specificity, but still cannot solve the diagnostic problem. Second tier tests for confirmation of primary NBS results and differential diagnosis become important at this point. LC-MS/MS analysis allows quantification of more specific biomarkers of metabolic disease.