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).
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Serum Cobalamin
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Interpretation
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< 100 pg/mL
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vitamin B12 deficient
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100 – 299 pg/mL
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Perform serum methylmalonic acid testing (see below).
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>= 300 pg/mL
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not vitamin B12 deficient
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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.
