Biochemical Identification of Vitamin B12 Deficiency in a Medical Office

A vitamin B12 deficiency can be an underlying cause or a deteriorating factor in several diseases. Nevertheless, early identification of such a deficiency remains a problem. Holotranscobalamin (HTC) is presently considered to be the gold standard. We tested the predictive power of other B12 parameters by comparing them with HTC.

The blood of 77 patients from a medical office was tested for HTC, total B12 (CLIA [chemiluminescent immunoassay] and MTP [microbiological test with microtitre plates]), MMA (methylmalonic acid), HCY (homocysteine), and MCV (mean cell volume). The parameters were correlated and sensitivity, specificity, PPV (positive predictive value), NPV (negative predictive value), LR+ (positive likelihood ratio), and LR- (negative likelihood ratio) in comparison to HTC were determined. A ROC analysis was also performed.

At a cutoff value of 35 pmol/L for HTC, the total B12 CLIA (cutoff 211 ng/L) qualified 53% of individuals as having a B12 deficiency. The total B12 MTP (cutoff 288 ng/L) classified 71% as having a B12 deficiency. Specificity was similar in both cases (CLIA, 93%; MTP, 95%). With a cutoff value of 10 µmol/L for homocysteine, the best negative prediction was achieved. MVA has a low sensitivity (41%) and a high specificity (90%). Based on the ROC analysis, which indicated superiority of the B12-MTP, the reference levels of B12-CLIA and B12-MTP were raised to 304 and 368 ng/L, respectively. Thus, a probable B12 deficiency was identified in 94% of cases with either method and with a comparable specificity.

If total B12 is applied to identify B12 deficiency, the cutoff values should be elevated to 304 (B12-CLIA) and 368 ng/L (B12-MTP) to improve the predictive power. The negative-predictive power of HCY can be useful in daily routine. HTC has a broad grey area of uncertainty and MMA should only be applied as a confirmatory test.