Maybe this will clarify for you:
Quote:
http://www.aafp.org/afp/20030301/979.html
From the American Family Physician:
In a large study10 of 406 patients with known vitamin B12 deficiency, 98.4 percent had elevated serum methylmalonic acid levels, and 95.9 percent had elevated serum homocysteine levels (defined as three standard deviations above the mean). Only one patient out of 406 had normal levels of both metabolites, resulting in a sensitivity of 99.8 percent when methylmalonic acid and homocysteine levels are used for diagnosis. Interestingly, 28 percent of the patients in this study had normal hematocrit levels, and 17 percent had normal mean corpuscular volumes.
In another study13 of patients with known pernicious anemia who had not received maintenance vitamin B12 injections for months to years, the rise of methylmalonic acid and homocysteine levels was found to precede the decrease in serum vitamin B12 and the decline in hematocrit. This finding suggests that methylmalonic acid and homocysteine levels can be early markers for tissue vitamin B12 deficiency, even before hematologic manifestations occur
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[bolding added by rose]