(1) clinical manifestations and general biochemical abnormalities, such as recurrent vomiting, lethargy, convulsions, developmental delay or regression, anemia and metabolic acidosis; (2) blood propionylcarnitine (C3) >4 μmol/L, propionylcarnitine to acetylcarnitine (C2) ratio (C3/C2) >0.25 by tandem mass spectrometry (3) urinary methylmalonic acid greater than 3.6 and/or methylcitrate greater than 1.1 by gas chromatography mass spectrometry (urinary result is the ratio to the internal standard 17 alkanoic acid); (4) blood homocysteine level greater than 14 μmol/L; (5) exclusion of vitamin B12 and folic acid deficiency. Patients from newborn screening may have no clinical manifestations. The diagnosis is confirmed in some patients by genetic mutation testing. Treatment: rehydration and acid correction in the acute phase; high-calorie, low-protein diet with natural protein intake controlled at 0.8-1.2 g/(kg?d), along with special milk powder or protein powder treatment without isoleucine, valine, threonine and methionine, and methionine supplementation (200 mg/d) for methionine deficient patients; L-carnitine 100-200 mg/(kg?d) Intravenous or oral; vitamin B12, 1 mg/d, once daily, intramuscularly, usually for 5 d. Stable treatment includes intramuscular or subcutaneous injections of vitamin B12 (cyanocobalamin or hydroxocobalamin, 1 mg/d, once or twice weekly), oral betaine (0.5-3 g/d, up to 6-9 g/d in older patients), levocarnitine [50-100 mg/(kg?d) The children were given methionine (200 mg/d), folic acid (10-30 mg/d), vitamin B6 (10-30 mg/d), and if necessary, methionine (200 mg/d); diet restriction was usually not required, but some patients with recurrent disease reduced natural protein intake [1-1.5 g/(kg?d)], and special milk powder or protein powder [1-3 g/(kg?d)] was given to ensure nutrition. Parents changed vitamin B12 to oral (3-6 mg/d) on their own during the stabilization period of the children, and the rest of the treatment remained unchanged. Hydroxocobalamin treatment was used, so some patients received cyanocobalamin and hydroxocobalamin successively by intramuscular or subcutaneous injection.