RIA for serum holo-transcobalamin:: Method evaluation in the clinical laboratory and reference interval

Background: Decreased serum holo-transcobalamin (holoTC) could be the earliest marker of cobalamin (Cbl) deficiency, but there has been no method suitable for routine use. We evaluated a new commercial holoTC RIA, determined reference values, and assessed holoTC concentrations in relation to other biochemical markers of Cbl deficiency. Methods: The reference population consisted of 303 individuals 22-88 years of age, without disease or medication affecting Cbl or homocysteine metabolism. In elderly individuals (greater than or equal to65 years), normal Cbl status was further confirmed by total homocysteine (tHcy <19 mumol/L) and methylmalonic acid (MMA; <0.28 mumol/L) concentrations within established reference intervals. HoloTC in Cbl deficiency was studied in a population of 107 elderly individuals with normal renal function. The Cbl deficiency was graded as potential (total Cbl less than or equal to150 pmol/L or tHcy greater than or equal to19 mumol/L), possible (total Cbl less than or equal to150 pmol/L and either tHcy greater than or equal to19 mumol/L or MMA greater than or equal to0.45 mumol/L), and probable (tHcy greater than or equal to19 mumol/L and MMA greater than or equal to0.45 mumol/L). Results: The intra- and between-assay imprecision (CV) for the holoTC RIA were 4-7% and 6-8%, respectively. A 95% central reference interval for serum holoTC was 37-171 pmol/L. All participants (n = 16) with probable Cbl deficiency, 86% of those with possible, and 30% of those with potential Cbl deficiency had holoTC below the reference limit (<37 pmol/L). The holoTC correlated with total CbI (r(s) = 0.80; P <0.0001) and inversely with MMA (r(s) = -0.52; P <0.0001). HoloTC concentrations were significantly (P = 0.01) higher in women than in men. Conclusions: The new holoTC RIA is precise and simple to perform. Low holoTC is found in individuals with biochemical signs of Cbl deficiency, but the sensitivity and specificity of low holoTC in diagnosis of Cbl deficiency need to be further evaluated. (C) 2003 American Association for Clinical Chemistry.