Methylmalonic acid, a malonic acid derivative, is a vital intermediate in the metabolism of fat and proteins and plays a key role in energy metabolism. Elevated methylmalonic acid levels can be caused by an inborn error of metabolism leading to methylmalonic aciduria or indicate early vitamin B₁₂ deficiency. This internal standard is suitable for quantitation of methylmalonic acid levels by LC/MS or GC/MS in newborn screening and clinical testing applications to test for methylmalonic acidemia and diagnose vitamin B₁₂ deficiency.