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Background: Cobalamin attached to transcobalamin II (TC II), known as holo-TC II, is the active cobalamin fraction taken up by tissues. Holo-TC II is also the form in which absorbed cobalamin enters the circulation from the ileum. Therefore, holo-TC II has been proposed variously as a marker of cobalamin adequacy, cobalamin absorption, or both, including even its advocacy as a surrogate Schilling test. Such claims carry conflicting diagnostic implications because metabolic adequacy and absorption are not identical., Objective: The objective was to examine metabolic and absorptive influences on holo-TC II., Design: Treated patients with pernicious anemia (PA), who have abnormal absorption but a normal metabolic status, were chosen as the model to differentiate between the effects of the 2 cobalamin-related characteristics. Serum holo-TC II and indexes of cobalamin metabolism in 23 treated patients were compared with those of 6 untreated PA patients (abnormal absorption and metabolic status) and 33 control subjects (normal absorption and metabolic status)., Results: Holo-TC II, which correlated directly with cobalamin and inversely with homocysteine, was significantly higher in treated PA patients in metabolic remission than in untreated PA patients (74 +/- 59 compared with 9 +/- 6 pmol/L) and was significantly lower than in control subjects (105 +/- 58 pmol/L), although the latter difference was small and the values overlapped greatly., Conclusions: Metabolic cobalamin status is a major determinant of serum holo-TC II. Absorption status may have mild influence as well, although other explanations remain possible. Serum holo-TC II cannot be used clinically to diagnose cobalamin malabsorption because of overlap with normal values. The influences on holo-TC II are complex and require careful analysis.

Background: Before the advent of highly active antiretroviral therapy (HAART), 20% and 10% of HIV-infected patients had low vitamin B-12 and red blood cell folate (RBCF) concentrations, respectively. However, few patients had real vitamin B-12 deficiency., Objective: We evaluated the prevalence of low vitamin B-12 and RBCF concentrations in HIV-infected patients receiving HAART and the usefulness of serum homocysteine (sHcy) for differentiating patients with deficiency from those with harmlessly low vitamin B-12., Design: The prevalence of low vitamin B-12 and RBCF was evaluated in 126 HIV-infected patients receiving HAART. Moreover, sHcy concentrations were evaluated in 40 HIV-infected patients with low vitamin B-12 and in 37 HIV-infected patients with low RBCF and were compared with those in 128 HIV-infected patients with normal vitamin B-12 and RBCF. sHcy was used to monitor treatment with vitamin B-12 and folic acid in 28 patients (24 with low vitamin B-12 and RBCF and 4 with hyperhomocysteinemia but normal vitamin B-12 and RBCF)., Results: The prevalence of low vitamin B-12 was significantly lower in patients receiving HAART than in previously studied patients who did not receive HAART (8.7% compared with 27%). Nine of the 40 patients (22.5%) with low vitamin B-12 (< or = 200 pmol/L) had hyperhomocysteinemia (> 17.5 micromol homocysteine/L). Nineteen (51.4%) of the 37 patients with low RBCF (< or = 580 nmol/L, percentile 10) had hyperhomocysteinemia. Among the 9 patients with an RBCF concentration < or = 450 nmol/L (percentile 2.5), all had hyperhomocysteinemia. The treatment with vitamin B-12 and folic acid normalized sHcy concentrations., Conclusions: The prevalence of low vitamin B-12 decreased after the introduction of HAART. The study of sHcy is useful for detecting HIV-infected patients with low vitamin B-12 and real deficiency.