Normal serum vitamin B12 in pernicious anemia: a diagnostic pitfall in a resource-limited setting—a case report

Pernicious anemia remains an important cause of severe vitamin B12 deficiency, yet its diagnosis is increasingly complicated by the widespread reliance on competitive-binding immunoassays that quantify total, rather than biologically active, cobalamin. We report a 33-year-old black Tanzanian man presenting with classical clinical, dermatological, hematological, and morphological features of advanced cobalamin deficiency, including profound macrocytic anemia, pancytopenia, reticulocytopenia, hypersegmented neutrophils, markedly elevated LDH, neurological impairment, and striking palmar hyperpigmentation, despite a serum vitamin B12 concentration within the local reference range. Subsequent detection of intrinsic factor and parietal cell antibodies strongly supported a diagnosis of autoimmune pernicious anemia, while endoscopic and histopathological findings were compatible with, but not diagnostic of, early autoimmune gastritis, in the absence of overt glandular atrophy. This case underscores the diagnostic pitfalls posed by serum B12 assays, particularly in resource-limited settings where confirmatory biomarkers such as methylmalonic acid, homocysteine, or holotranscobalamin are inaccessible. Clinicians must therefore interpret serum B12 cautiously and integrate clinical, morphological, and autoimmune findings when evaluating suspected pernicious anemia.