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Table 2: Typical Stages in the Development of a Vitamin B12 Deficiency.

StageManifestationComment
ICirculating serum B12 levels depletedPatients are typically asymptomatic and can remain in this stage for several years.

II

Cellular stores of B12 are depletedPatients can remain asymptomatic. This stage can also continue for several years.

III

Evidence of bio-chemical deficiency via increases in serum homocysteine and methylmalonic acidVitamin B12 is required for the conversion of these compounds.
IVClinical signs and symptoms apparentThe spectrum of clinical manifestations is broad and the sequence of symptom development varies markedly.

Table developed by Herbert, 1994

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