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Authors’ Response: Phantoms in the Assay Tube

Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota 55905

Address correspondence to: Stefan K. G. Grebe, M.D., Mayo Clinic, Department of Laboratory Medicine and Pathology, 200 1st Street SW, Rochester, Minnesota 55905. E-mail: grebes{at}mayo.edu.

Drs. Ward and Hickman are correct in stating that the heterophile interference rate of 0.03% reported in their paper (1) and referred to in our discussion (2) represents an estimation, rather than an actually observed figure. The major point, however, is that it indicates that in the particular TSH assay they have examined, the manufacturer’s blocking regimen seemed to work. Consequently, their paper has been widely cited as support for the effectiveness of modern heterophile blocking reagents, which is also the reason we have cited it.

Therefore, the relatively high interference rate observed in our study might indeed suggest that the Beckman thyroglobulin assay does not contain blocking reagents. However, this is not the case. Like most (or all) modern automated immunoassays, the Beckman thyroglobulin assay does contain blockers, but obviously, these are insufficient to avoid all heterophile interferences. The lesson is that, depending on the type of assay and the patient population, a blocking regimen that suffices in one particular assay may not do so in another. One should therefore not exclude the possibility of heterophile interferences in an immunoassay solely because a manufacturer has added blocking reagents, or because some other immunoassay, for an unrelated analyte, has been shown free of such interferences.

Received October 3, 2003.

References

1. Ward G, McKinnon L, Badrick T, Hickman PE 1997 Heterophilic antibodies remain a problem for the immunoassay laboratory. Am J Clin Pathol 108:417–421[Medline]
2. Preissner CM, O’Kane DJ, Singh RJ, Morris JC, Grebe SKG 2003 Phantoms in the assay tube: heterophile antibody interferences in serum thyroglobulin assays. J Clin Endocrinol Metab 88:3069–3074[Abstract/Free Full Text]

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