I run MMA in urine because it can be normalized to creatinine which research points that it's better than blood
There isn’t a direct conversion from urine MMA results (umol/mmol creatinine) to a plasma value (umol/L or nmol/L). Plasma MMA can build up and be falsely elevated in renal failure in which case urinary MMA is preferred.
Methylmalonic acid (MMA) is the metabolic byproduct that builds up when there is not enough vitamin B12/cobalamin to facilitate the intracellular conversion of L-methylmalonyl coenzyme A to succinyl CoA (Testing.com). The MMA accumulates and is excreted by the kidneys. If kidney function declines, then more MMA can build up in the blood and confound serum results.
One prospective study of 127 inpatients found that even mild renal impairment (eGFR below 100) may influence plasma MMA levels, confirming that renal function must be taken into account when assessing MMA (Supakul 2020).
Evaluating CBC, holotranscobalamin, and homocysteine levels can also contribute to a comprehensive assessment of vitamin B12 status.
References
Testing.com. Methylmalonic acid.
Supakul, Sopak et al. “Diagnostic Performances of Urinary Methylmalonic Acid/Creatinine Ratio in Vitamin B12 Deficiency.” Journal of clinical medicine vol. 9,8 2335. 22 Jul. 2020, doi:10.3390/jcm9082335