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Article: Rébecca Leboeuf, Marie-France Langlois, Marc Martin, Charaf E. Ahnadi, and Guy D. Fink "Hook-effect in calcitonin immunoradiometric assay in patients with metastatic medullary thyroid carcinoma: case report and review of the literature" J Clin Endocrinol Metab 2005; 0: jc.2005-1429v1 [Abstract]

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Calcitonin underestimation due to “high dose hook effect”

Nuno F Cunha, Fernando Rodrigues, Fátima Curado, Beatriz Campos and Frederico Valido   (8 March 2006)

Calcitonin underestimation due to “high dose hook effect” 8 March 2006
Nuno F Cunha, Biochemist Department of Clinical Pathology - Coimbra’s Portuguese Institute of Oncology (IPOC FG) - Portugal, Fernando Rodrigues, Fátima Curado, Beatriz Campos and Frederico Valido Send letter to journal: Re: Calcitonin underestimation due to “high dose hook effect” nfcunha{at}gmail.com Nuno F Cunha, et al.	We read with interest the paper by Leboeuf et al. (1) reporting a prozone “high dose hook effect” in a calcitonin IRMA assay and we would like to report a similar experience with a different assay. The case concerns a 68-year-old woman who presented with extensive bone and pulmonary metastases 10 years after resection of a medullary thyroid carcinoma (MTC). Two therapeutic 131I-mIBG doses were administered in April 2000 and February 2001. Serum calcitonin (hCT) and CEA concentrations were determined with a two step IRMA hCT kit (Cis biointernational) and an immunometric chemiluminescent assay (IMMULITE, Euro/DPC, Ltd), respectively. The values were consistently high (hCT 64,260 ng/L; CEA 460 ìg/L). During part of 2001, calcitonin measurements were performed with a different IRMA kit, the one- step IBL Calcitonin IRMA kit (IBL – Hamburg, Germany) with a linearity of 10–1000 ng/L. During this time, the calcitonin levels were found to be much lower than before, 322±206 ng/L (mean ± SD), with no significant changes in the CEA concentrations, raising some concern. Initially, it was hypothesized that the decreasing values of calcitonin could be due to the 131I-mIBG therapy, but the CEA concentration remained stable and there was no radiological evidence of tumor response. With this is mind, we started to question the linearity of the kit and after several dilutions (3) the presence of a “hook effect” was evident. As a consequence, the measuring method was changed to a two-step immunometric chemiluminescent assay (IMMULITE 2000, Euro/DPC, Ltd) calibrated to the WHO 2nd IRP 89/620 standard, with a sensitivity of 2 ng/L. Not surprisingly, the results obtained were as high as those with the initial IRMA method, with all sample results above 1,000 ng/L when analyzed with or without dilution. During this time, the measured hCT concentrations were 43,797±10,992 ng/L. Consequently, we agree with Leboeuf et al. that two step immunoassays are required to prevent the formation of antigen-signal antibody complexes (1) that lead to falsely low calcitonin results. Clinicians should be aware of this laboratory phenomenon, when evaluating patients with advanced MTC. If a “hook effect” is suspected, measurement of serial dilutions of calcitonin samples may prevent incorrect evaluation of therapeutic responses. References 1. Leboeuf R, Langlois MF, Martin M, Ahnadi C, Fink GD. 2006. Hook effect in calcitonin immunoradiometric assay in patients with metastatic medullary thyroid carcinoma: case report and review of the literature. J Clin Endocrinol Metab 91:361-364 2. Mukherjee JJ, Kaltsas GA, Islam N, Plowman PN, Foley R, Hilkmat J, Jenkins PJ, Chew SL, Monson JP, Besser GM, Grossman AB. 2001. Treatment of metastatic carcinoid tumors, phaeochromocytoma, paraganglioma and medullary carcinoma of the thyroid with 131I–meta-iodobenzylguanidine (131I-mIBG). Clin Endocrinol (Oxf) 55:47-60 3. Ooi DS, Escares EA. 1991. “High-Dose Hook Effect” in IRMA-Count PSA Assay of Prostate-Specific Antigen. [Letter] Clin Chem 37:771-772