Methylmalonic acid

Upper limits of 250 to 380 nmol/L are in use and some laboratories print umol/L; it rises with age and with reduced kidney function; use the range your lab printed.

Methylmalonic acid is a substance that builds up in the blood when cells do not have enough vitamin B12 to process it, because one of the enzymes that clears it depends on B12. It is therefore a marker of B12 shortage at the level of the cells, rather than a measure of the vitamin itself. It is not affected by folate.

Doctors request it when the total or active B12 sits in a grey zone and it is unclear whether the cells are short of B12, particularly in people with nerve symptoms. It is a specialist test sent to a few laboratories.

• Vitamin B12 shortage at the level of the cells
• Reduced kidney function, because it is cleared by the kidneys
• Older age
• Dehydration
• Some rare inherited metabolic conditions
• Overgrowth of bacteria in the small intestine

• B12 injections or supplements, within days
• There are no medical causes of a low methylmalonic acid; a low value is not of any importance

• It rises with age and with even mild reductions in kidney function in the general population, without any B12 shortage
• Dehydration on the day raises it
• Different laboratory methods and units make results hard to compare

No fasting needed. Best taken before any B12 supplements are started, and read with the B12 result and a kidney function test.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.