title: Measurement of total vitamin B12 and holotranscobalamin, singly and in combination, in screening for metabolic vitamin B12 deficiency.
abstract: BACKGROUND
  The standard screening test for vitamin B12 deficiency, measurement of total plasma vitamin B12, has limitations of sensitivity and specificity. Plasma vitamin B12 bound to transcobalamin (holoTC) is the fraction of total vitamin B12 available for tissue uptake and therefore has been proposed as a potentially useful alternative indicator of vitamin B12 status.
  
  
  METHODS
  We compared the diagnostic accuracy of total vitamin B12, holoTC, and a combination of both measures to screen for metabolic vitamin B12 deficiency in an elderly cohort (age > or = 60 years). Plasma methylmalonic acid and homocysteine were used as indicators of vitamin B12 deficiency.
  
  
  RESULTS
  Low total vitamin B12 (< 148 pmol/L) and low holoTC (< 35 pmol/L) were observed in 6.5% and 8.0%, and increased methylmalonic acid (> 350 nmol/L) and homocysteine (> 13 micromol/L) were observed in 12.1% and 17.0% of the study participants. In multiple regression models, holoTC explained 5%-6% more of the observed variance in methylmalonic acid and homocysteine than did total vitamin B12 (P < or = 0.004). ROC curve analysis indicated that total vitamin B12 and holoTC were essentially equivalent in their ability to discriminate persons with and without vitamin B12 deficiency. Individuals with low concentrations of both total vitamin B12 and holoTC had significantly higher concentrations of methylmalonic acid and homocysteine than did individuals with total vitamin B12 and/or holoTC within the reference intervals (P < 0.001).
  
  
  CONCLUSIONS
  HoloTC and total vitamin B12 have equal diagnostic accuracy in screening for metabolic vitamin B12 deficiency. Measurement of both holoTC and total vitamin B12 provides a better screen for vitamin B12 deficiency than either assay alone.
authors.affiliation: Department of Pathology and Laboratory Medicine, University of California, Davis, Sacramento, CA 95817, USA.
authors.forename: Joshua W
authors.initials: JW
authors.lastname: Miller
authors.forename: Marjorie G
authors.initials: MG
authors.lastname: Garrod
authors.forename: Alan L
authors.initials: AL
authors.lastname: Rockwood
authors.forename: Mark M
authors.initials: MM
authors.lastname: Kushnir
authors.forename: Lindsay H
authors.initials: LH
authors.lastname: Allen
authors.forename: Mary N
authors.initials: MN
authors.lastname: Haan
authors.forename: Ralph
authors.initials: R
authors.lastname: Green
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002140:California / Q000453:epidemiology; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008403:Mass Screening; D008875:Middle Aged; D014155:Transcobalamins / Q000032:analysis*; D014805:Vitamin B 12 / Q000097:blood*; D014806:Vitamin B 12 Deficiency / Q000097:blood / Q000175:diagnosis* / Q000453:epidemiology
chemical_list: D014155:Transcobalamins; D014805:Vitamin B 12