Vitamin B12 (Cobalamin) Test

The Vitamin B12 Test measures total serum cobalamin, an essential cofactor for methionine synthase in the methylation cycle and for methylmalonyl-CoA mutase in propionate metabolism. Deficiency causes megaloblastic anaemia and a spectrum of neurological disease including peripheral neuropathy, subacute combined degeneration of the spinal cord, and cognitive impairment. It is ordered in the assessment of macrocytic anaemia, unexplained neuropathy or paraesthesiae, cognitive decline, glossitis, and fatigue, and to monitor patients at elevated risk of deficiency. Serum B12 is the appropriate first-line test, but practitioners should understand where it fails, because the consequences of a missed diagnosis are neurological and may be irreversible. The test measures total circulating cobalamin, of which 70 to 80% is bound to haptocorrin and metabolically unavailable; only the holotranscobalamin fraction reaches tissues. This creates two distinct problems. The first is the indeterminate zone: results between roughly 200 and 350 pg/mL are common and frequently accompany genuine tissue deficiency, so a result in this band should not be treated as reassuring in a symptomatic patient. The second is that serum B12 can be normal or frankly elevated in a deficient patient, because hepatic disease, myeloproliferative disorders, and recent oral or injected supplementation all raise total cobalamin without altering tissue availability. Methylmalonic acid resolves both situations: it accumulates specifically when cobalamin-dependent enzyme function fails, and it is the appropriate second-line test whenever the serum result and the clinical picture disagree. Homocysteine rises in both B12 and folate deficiency and is therefore less specific. One clinical point deserves emphasis because the resulting harm is both serious and preventable. Neurological manifestations of B12 deficiency can precede haematological ones, so a normal full blood count and a normal mean corpuscular volume do not exclude deficiency in a patient with compatible neurological symptoms. Relatedly, folate supplementation can correct the megaloblastic anaemia of B12 deficiency while the neurological damage continues to progress unchecked — which is why B12 status must be established before folate is given to a patient with macrocytic anaemia, and why high folate intake can mask the haematological signal that would otherwise have prompted investigation.