Determination of serum holotranscobalamin concentrations with the AxSYM active B(12) assay: cut-off point evaluation in the clinical laboratory.
BACKGROUND
A reliable early marker is required for diagnosis of cobalamin deficiency. We calculated an appropriate holotranscobalamin (HoloTC) cut-off point for identifying cobalamin deficiency using an immunoenzymatic assay.


METHODS
Determination of the cut-off threshold and correlation between HoloTC and the other diagnostic parameters routinely used for vitamin B(12) deficiency [total vitamin B(12) (tB(12)), folate, homocysteine] were measured in 250 routine blood specimens from 107 men (mean age 59.0+/-18.8 years) and 143 women (mean age 54.2+/-23.1 years). The inclusion criterion was serum tB(12) concentration <or=221 pmol/L.


RESULTS
Analytical performance results agreed with those reported by others. A weak correlation (R=0.42) was found between HoloTC and tB(12). A 40 pmol/L cut-off threshold was chosen for HoloTC and the associated sensitivity and specificity was 0.86 and 0.66, respectively. Out of 250 tested samples, 126 showed tB(12) concentrations 139-221 pmol/L (gray zone, GZ) and 124 had tB(12) concentrations <139 pmol/L (low, L). Values less than the cut-off for HoloTC were present in 68.2% and 37.9% of cases in the GZ and L group, respectively (p<0.01), and in 53.2% of subjects.


CONCLUSIONS
Our results confirmed the analytical reliability of the AxSYM HoloTC assay. The method is adequate for routine use and a cut-off threshold of 40 pmol/L is appropriate for assessing cobalamin deficiency in populations with reduced tB(12) values.
Bamonti
Fabrizia
F
Dipartimento Scienze Mediche, Università degli Studi di Milano, Ospedale Maggiore Policlinico, Mangiagalli e Regina Elena, Fondazione IRCCS, Milan, Italy. fabrizia.bamonti@unimi.it
Moscato
Giovanna Antonella
GA
Novembrino
Cristina
C
Gregori
Dario
D
Novi
Claudia
C
De Giuseppe
Rachele
R
Galli
Claudio
C
Uva
Valentina
V
Lonati
Silvia
S
Maiavacca
Rita
R
D000368:Aged; D015415:Biomarkers / Q000097:blood / Q000378:metabolism; D001774:Blood Chemical Analysis / Q000295:instrumentation*; D005260:Female; D005492:Folic Acid / Q000097:blood / Q000378:metabolism; D006710:Homocysteine / Q000097:blood / Q000378:metabolism; D006801:Humans; D007124:Immunoenzyme Techniques; D008297:Male; D008875:Middle Aged; D012680:Sensitivity and Specificity; D014155:Transcobalamins / Q000378:metabolism*; D014805:Vitamin B 12 / Q000097:blood* / Q000378:metabolism; D014806:Vitamin B 12 Deficiency / Q000097:blood* / Q000175:diagnosis / Q000378:metabolism; D014803:Vitamin B Complex / Q000097:blood / Q000378:metabolism
D015415:Biomarkers; D014155:Transcobalamins; D006710:Homocysteine; D014803:Vitamin B Complex; D005492:Folic Acid; D014805:Vitamin B 12