Is there an "optimal" range for methylmalonic acid (MMA)?

No. Methylmalonic acid (MMA) is a toxic compound that accumulates in the body when levels of vitamin B12 are deficient. Elevated levels of MMA are primarily caused by vitamin B12 deficiency. Thus, MMA is a highly selective metabolic (and functional) marker for vitamin B12 deficiency. Together, blood levels of vitamin B12 and MMA provide a more accurate measurement of cobalamin status than does either biomarker alone. Levels of MMA decrease quickly after treatment with vitamin B12. Therefore, serial measurement of MMA may be used to verify adequate replacement of vitamin B12. For these reasons, there is no "optimal level" for MMA as the compound should not accumulate in the blood stream if vitamin B12 status is optimal and functional.