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Overview :
Vitamin B12 (cobalamin) deficiency may be difficult to
identify from serum cobalamin levels alone. In patients biochemically deficient
for cobalamin, methylmalonic acid (MMA) is increased in the serum and urine, but
normalizes after treatment with vitamin B12. MMA testing can be expensive and
may not be readily available but can help identify patients with vitamin B12
deficiency before irreversible neurological damage has occurred.
With vitamin B12 deficiency, the conversion of
L-methylmalonyl-Co-A to succinyl-Co-A is reduced, and is instead metabolized to
methylmalonic acid. For a discussion on the biochemistry involved, refer to Elin
and Winter (2001).
Specimen: Serum levels are a better indicator of status than
erythrocyte levels (Tietz).
|
Serum Cobalamin |
Interpretation |
|
< 100
pg/mL |
vitamin B12 deficient |
|
100 – 299
pg/mL |
Perform serum methylmalonic acid testing (see
below). |
|
>= 300
pg/mL |
not vitamin B12 deficient |
where:
• Holleland
et al used RIA (Diagnostic Product Corp) with the lower limit to the reference
range is given as 170 pmol/L (230 pg/mL). The upper limit for the reference
limit was 700 pmol/L (949 pg/mL). This seems to give a mean of 435 pmol/L (590
pg/mL) and an SD of 130 pmol/L (144 pg/mL; assuming the lower limit was – 2.5 SD
below the mean).
• It would
appear that patients in the low-normal reference range for serum cobalamin may
be biochemically deficient, and that this group is the most likely to benefit
from the serum MMA testing.
• Using 1.5
or 2 SD below the mean for the serum cobalamin level as the point for MMA
testing may be a reasonable approach.
If the patient shows :
(1) a low or low-normal serum cobalamin level AND
(2) an increased serum or urine methylmalonic acid is present
AND
(3) the
serum MMA level returns to the normal range after cobalamin therapy,
then
vitamin B12 deficiency was present.
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