In the last X Fact Is There a Better Blood Test for B12 Deficiency Other Than a B12 Level, it was explained how MMA will accumulate in the cell when there is not enough B12 to meet its energy needs and can be measured in blood as an indicator of B12 deficiency. But, the MMA level in blood is rapidly filtered through the kidneys and concentrated in the urine up to 40 times as much as in the blood and therefore, even slight elevations above normal can be more easily detected. This concentration effect and the fact that MMA is much more stable in urine than blood (up to three weeks in a urine sample at room temperature) makes it a much more reliable and sensitive test than measuring blood MMA.

One more point. Because kidney filtration can vary from one time to another within the same individual, or between people with different levels of kidney function, the amount of MMA measured in urine always has to be compared to creatinine (a waste product) measured in the same sample. Because creatinine is a reliable indicator of kidney function and filtration, the MMA/creatinine ratio can correct for any decrease in kidney function or high filtration rates, that can give falsely elevated results. This is why the test is reported as micrograms MMA per milligrams creatinine (mcg MMA/mg creatinine) and a value above 3.8mcg MMA/mg creatinine indicates vitamin B12 deficiency and a value below 3.8 indicates adequate cellular B12 status. Simple as that.