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An unusual interference in parathormone assay caused by anti-goat IgG: a case report.

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Clin Chem Lab Med 2009;47(1):118 2009 by Walter de Gruyter • Berlin • New York. DOI 10.1515/CCLM.2009.015

2009/357

Article in press - uncorrected proof

Letter to the Editor

An unusual interference in parathormone assay caused

by anti-goat IgG: a case report

Etienne Cavalier*, Pierre Delanaye, Agne`s Carlisi, Jean-Paul Chapelle and Julien Collette Department of Clinical Chemistry, Centre Hospitalier Universitaire de Lie`ge, University of Lie`ge, Lie`ge, Belgium

Keywords: analytical interference; human anti-animal antibodies; parathormone.

We report the case of a 29-year-old woman, working as a nurse, who underwent a blood analysis to mon-itor hypothyroidism. In addition to the ‘‘classical’’ parameters, her physician ordered a parathormone (PTH) determination. The result of the PTH test was found to be )2000 pg/mL (normal range: 12–54 pg/mL) with the Liaison analyser (Diasorin, Stillwater, MN, USA). As recently published (1), we considered this result as ‘‘spurious’’ and we treated the sample with HBR 1 (Scantibodies, Santee, CA, USA) and RF-Absorbent (IBL, Hamburg, Germany) to remove a possible interference due to human animal anti-bodies (HAAA) and rheumatoid factor (RF), respec-tively. We also determined PTH with another method (Roche Elecsys, Mannheim, Germany). We observed that the result remained unchanged after HBR treat-ment, whereas PTH dramatically fell to 40 pg/mL after anti-RF treatment. With the Elecsys analyser, the PTH was also normal (24 pg/mL). We thus suspected an interference due to RF, but the patient neither suffered from rheumatoid arthritis (and other autoimmune dis-eases), nor was positive when we tested for RF (BN2, Siemens, Marburg, Germany). To explore the cause of this interference with the Liaison analyser, we then performed a chromatographic separation with a Superdex 200 HR 10/30 column (Amersham Biosci-ences, Piscataway, NJ, USA) and we concluded that there was PTH reactivity in the fraction that corre-sponded to the molecular weight of the IgGs. This confirmed the results of the RF-Absorbent treatment, which, indeed, is a treatment of the sample with anti-IgG antibody.

As the antibodies used for the PTH Liaison analyser are polyclonal goat antibodies (whereas Roche Elec-sys use monoclonal mouse antibodies), we suspected that there was anti-goat IgG present in the serum of the patient that could interfere with the assay. To con-firm this hypothesis, we incubated 400 mL of the patient’s serum with 40mL of goat serum. In this con-dition, we observed that PTH dropped from )2000 to 41 pg/mL.

Human anti-animal antibodies are a known cause of interferences in immunoassays (2). This patient had no obvious reason to present anti-goat antibodies (any animal contact, professional exposition, coeliac disease, drug, vaccination or blood transfusion). She did not eat goat’s cheese or drink milk. A possible cause could be a pregnancy 3 years prior, even if she was primiparous (3). Even if the manufacturers include in the inserts of the kits that this type of inter-ference remains possible, one should be careful when interpreting the results of any immunoassay. We must keep in mind the tragic outcomes that can be observed with these interferences (4). Indeed, these antibodies can give spurious results, leading to unnecessary cost-effective and stressful extra investigations.

Conflict of interest: None.

References

1. Cavalier E, Carlisi A, Chapelle JP, Delanaye P. False pos-itive PTH results: an easy strategy to test and detect ana-lytical interferences in routine practice. Clin Chim Acta 2008;387:150–2.

2. Kricka LJ. Human anti-animal antibody interferences in immunological assays. Clin Chem 1999;45:942–56. 3. Hawkins BR, Saueracker GC, Dawkins RL, Davey MG,

O’Connor KJ. Population study of heterophile antibodies. Vox Sang 1980;39:339–42.

4. Rotmensch S, Cole LA. False diagnosis and needless ther-apy of presumed malignant disease in women with false-positive human chorionic gonadotropin concentrations. Lancet 2000;355:712–5.

*Corresponding author: Dr Etienne Cavalier, Department of Clinical Chemistry, University Hospital of Liege, University of Liege, Domaine du Sart-Tilman, Baˆtiment B35,

4000 Liege, Belgium

Phone: q32-43667692, Fax: q32-43667691, E-mail: Etienne.cavalier@chu.ulg.ac.be

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