title: False positive in the measurement of thyroglobulin induced by rheumatoid factor.
abstract: BACKGROUND
  We report a case of interference in thyroglobulin (Tg) measurement in a woman with differentiated thyroid cancer and rheumatoid arthritis history. Due to discordant Tg in relation to TSH concentrations and negative images, we investigated possible interference in the measurement of Tg.
  
  
  METHODS
  During the follow-up we measured Tg by chemiluminescence (TgQL) using Immulite 2000 immunoassay system. To investigate possible interference in Tg measurement, we made serial dilutions, re-testing of Tg by an alternative method: electrochemiluminescence, Cobas 6000 analyzer (TgEQL), recovery test of Tg and polyethylene glycol (PEG) 6000 precipitation.
  
  
  RESULTS
  During the patient follow-up, the TgQL ranged between <0.3 and 16.1ng/ml. In the evaluated serum samples very high titers of rheumatoid factor (RF) were found. When RF titers were lowered post PEG precipitation, Tg QL concentrations became undetectable.
  
  
  CONCLUSIONS
  We describe an unusual case of interference in Tg assay due to RF. When disagreement among Tg concentrations, images and clinical features is observed, we suggest taking into account the evaluation of possible Tg interference to avoid unnecessary complementary exams and inappropriate treatment.
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.. Electronic address: gbastarita@intramed.net.ar.
authors.forename: G
authors.initials: G
authors.lastname: Astarita
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: S
authors.initials: S
authors.lastname: Gutiérrez
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: N
authors.initials: N
authors.lastname: Kogovsek
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: E
authors.initials: E
authors.lastname: Mormandi
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: P
authors.initials: P
authors.lastname: Otero
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: C
authors.initials: C
authors.lastname: Calabrese
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: G
authors.initials: G
authors.lastname: Alcaraz
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: A
authors.initials: A
authors.lastname: Vázquez
authors.affiliation: Endocrinology Division, Durand Hospital, Buenos Aires, Argentina.
authors.forename: M
authors.initials: M
authors.lastname: Abalovich
mesh_terms: D000328:Adult; D001774:Blood Chemical Analysis / Q000379:methods*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D007118:Immunoassay; D008163:Luminescent Measurements; D012217:Rheumatoid Factor / Q000276:immunology*; D013954:Thyroglobulin / Q000097:blood*; D013964:Thyroid Neoplasms / Q000097:blood / Q000276:immunology
chemical_list: D012217:Rheumatoid Factor; D013954:Thyroglobulin
keywords: Differentiated thyroid cancer; Heterophilic antibodies; Interference; Rheumatoid factor; Thyroglobulin