uMMA - 'Catch-22' or Catch-all?
The main reason doctors don't order a uMMA (urinary MMA) test for B12 deficiency is because they don't know about it and it is not performed by most clinical laboratories. One could also say that it is not widely available, because doctors don't ask for it - a proverbial 'Catch-22'. This is a pity, because the test has been shown to be especially useful when the B12 blood level test falls in the low-normal range.
For decades, the threshold for diagnosing deficiency from a blood B12 test was 200 ng/L (148 pmol/L). In a recently published study, 490 hospital patients were tested for vitamin B12 status and nearly 30% were found to have a blood B12 level between 201-350 ng/L - i.e. on the low end of a very wide range of what is considered normal (200 - 950 ng/L). If the blood B12 level was used as the sole determinant of their B12 status, the test would be reported as normal. But when taking into account blood MMA and homocysteine levels, 69 out of the 127 with low-normal levels were found to be B12 deficient.
In other words, over half (54.3%) of the patients who would be told they have a normal result on their blood B12 test alone, WERE in fact B12 deficient. The uMMA test confirmed the results and is more accurate than blood MMA or homocysteine levels when individuals have even very slight decrease in kidney function - which is quite common. Is uMMA a 'Catch-All' test for B12 deficiency? No, but it is very useful when the diagnosis is in doubt or when B12 blood levels are in the low-normal range.