Methylmalonic acidemia is a common organic acidemia, mainly due to the defect of methylmalonyl coenzyme A or its coenzyme adenosylcobalamin and methylcobalamin, resulting in an increase of methylmalonic acid, methylcitrate, homocysteine, 3-hydroxypropionic acid and malonylcarnitine in patients, and a large amount of organic acids are excreted by urine. The diagnosis can be made by tandem mass spectrometry and gas chromatography mass spectrometry. Since a variety of factors can lead to an increase in blood malonylcarnitine or a mild increase in urinary methylmalonic acid, the disease cannot be easily diagnosed in patients with mildly abnormal blood tandem mass spectrometry or urinary gas chromatography mass spectrometry results, and repeated testing or genetic testing is required to diagnose or rule out the disease. 1, urine gas chromatography mass spectrometry detection: gas chromatography mass spectrometry detection of methylmalonic acid and methylcitric acid levels in urine, as a semi-quantitative method, the results are the ratio of the ionic strength of methylmalonic acid or methylcitric acid to the ionic strength of the internal standard 17-alkanoic acid, the specific detection methods refer to the literature. 2, blood tandem mass spectrometry detection: tandem mass spectrometry was used to detect the levels of free carnitine, acetylcarnitine, propionylcarnitine and methionine in blood, and to calculate the ratio of C3 to C2. The specimens were collected by the dried blood filter paper method, finger blood collection or venous blood sampling, dried on special blood filter paper and sent for testing. The specific test method was referred to the methods reported in the literature. Diagnosis and typing of methylmalonic acidemia: (1) clinical manifestations, such as recurrent vomiting, lethargy, convulsions, dyskinesia and metabolic acidosis; (2) increased urinary methylmalonic acid and methylcitric acid levels by gas chromatography mass spectrometry; (3) increased blood C3 levels and C3/C2 by tandem mass spectrometry; (4) exclusion of secondary factors such as vitamin B12 deficiency. In some patients, the diagnosis was confirmed by mutation analysis. The basis of typing: MMA patients with increased serum homocysteine levels are classified as methylmalonic acid combined with homocysteinemia (hereafter referred to as combined MMA), while those with normal serum homocysteine levels are classified as simple MMA.