Letters

Possible cause of false normal B-12 assays

BMJ 2006; 333 doi: http://dx.doi.org/10.1136/bmj.333.7569.654-c (Published 21 September 2006) Cite this as: BMJ 2006;333:654
  1. Malcolm S Hamilton, director (m.s.hamilton{at}talk21.com),
  2. Sheena Blackmore, deputy director,
  3. Anne Lee, scheme scientist
  1. United Kingdom External Quality Assessment Scheme, UKNEQAS Haematinics, Department of Haematology, Good Hope NHS Trust, Sutton Coldfield B77 7RR
  2. United Kingdom External Quality Assessment Scheme, UKNEQAS Haematinics, Department of Haematology, Good Hope NHS Trust, Sutton Coldfield B77 7RR

    EDITOR—In response to Devalia,1 we report a further eight cases of serum samples from patients with megaloblastic anaemia or subacute combined degeneration of the cord in whom current assays have failed to detect cobalamin deficiency. The cobalamin results were well within the manufacturers' reference range and not to do with cut-off point. The table shows results from two cases of megaloblastic anaemia, which subsequently responded to cobalamin treatment, in which pretreatment serum was available for analysis by several methods.

    Pretreatment serum cobalamin concentrations in two cases with false normal cobalamin results in current commercial assays

    View this table:

    Only two of the five methods found extremely low levels of cobalamin despite severe clinical deficiency. The Bayer centaur assay detected one of the two case samples as low. These competitive binding immunoassays are no boil and rely on alkaline hydrolysis and dithiothreitol or monothioglycerol treatment to release cobalamin from transcobalamin and denature intrinsic factor antibody.

    We postulate that intrinsic factor antibody present in the patient serum at high titre fails to be denatured by the alkaline hydrolysis and dithiothreitol treatment and persists into the binding stage, resulting in antibody interference rather than heterophilic antibody interference. The incidence is estimated at 1:3000 requests, and whether some assays are more vulnerable to this effect than others is not yet clear.

    Manufacturers are urged to test assays pre-release with high titre anti-intrinsic factor antibody serum samples. Clinicians are urged to be vigilant for these potentially dangerous “false normal” cobalamin results. Laboratories should not report cobalamin assays in isolation from intrinsic factor antibody and other haematological data. Pretreatment serum should be stored in any suspected cases for homocysteine and methylmalonic acid concentrations and for anti-intrinsic factor antibody titres to confirm that the megaloblastic anaemia or neuropathy is indeed due to cobalamin deficiency. A therapeutic trial of cobalamin will prevent delay in treatment and adverse clinical consequences. Such cases should be notified to the Medicines Health Care Related Products Agency (http://www.mhra.gov.uk) and UKNEQAS Haematinics (ukneqas.haematinics.org.uk).

    Footnotes

    • Competing interests None declared.

    References

    1. 1.