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RIA for Serum Holo-Transcobalamin: Method Evaluation in the Clinical Laboratory and Reference Interval

¹ Department of Clinical Chemistry, Turku University Central Hospital, FIN-20521 Turku, Finland.

² Härkätie Health Centre, FIN-21421 Lieto, Finland.

³ Department of Clinical Biochemistry, Aarhus University Hospital at Skejby, DK-8200 Aarhus N, Denmark.

⁴ Institute of Clinical Medicine, General Practice, University of Turku, FIN-20041 Turku, Finland.

⁵ Satakunta Central Hospital, FIN-28500 Pori, Finland.

^aAuthor for correspondence. Fax 358-2-2613920; e-mail saila.loikas{at}tyks.fi.

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 (65 years), normal Cbl status was further confirmed by total homocysteine (tHcy; <19 µmol/L) and methylmalonic acid (MMA; <0.28 µmol/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 150 pmol/L or tHcy 19 µmol/L), possible (total Cbl 150 pmol/L and either tHcy 19 µmol/L or MMA 0.45 µmol/L), and probable (tHcy 19 µmol/L and MMA 0.45 µmol/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 Cbl (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.

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