Abstract: The present invention relates to the determination of the presence of methylmalonic acid in biologic samples including the steps of methylmalonic extraction from the sample; derivatization of methylmalonic acid and use of mass spectrometry with negative mode atmospheric pressure chemical ionization to determine the presence of methymalonic acid through the formation of an ion of mass to charge ratio (m/z) 477. An additional objective of the present invention concerns diagnosis kits for determination of presence and quantification of methylmalonic acid based on the method mentioned before.