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Our study included samples from patients with RF-positive rheumatoid arthritis (RA) and controls (patients with RF-negative RA and psoriatic arthritis), included in an early arthritis-cohort. Reactivity to mouse IgG1, mouse IgG2a, rabbit IgG, bovine IgG, sheep\/goat IgG and human IgG was analysed using in-house interference assays. RF-positive sera with strong reactivity to mouse IgG1 were analysed in three commercial immunoassays. To reveal interference, results before and after addition of blocking aggregated murine IgG1 were compared. Samples from 124 RF-positive RA patients and 66 controls were tested. We found considerably stronger reactivity toward animal antibodies, particularly mouse IgG1 (73% vs. 12%) and rabbit IgG (81% vs. 6%), in sera from RF-positive RA-patients compared to controls (<jats:italic>p<\/jats:italic>\u2009&lt;\u20090.001). After selecting samples for testing in commercial assays, interference was revealed in 6\/30 sera in the Architect \u03b2-hCG assay, 7\/10 sera in the 27-plex cytokine assays, and in 2\/33 samples in the Elecsys Soluble Transferrin Receptor assay. Our study revealed considerable RF reactivity to animal antibodies used in immunoassays and RF was associated with falsely elevated results in immunoassays used in clinical care and research. Clinicians, laboratorians, researchers and assay manufacturers must be alert to the risk of falsely elevated test results in RF-positive RA patients.<\/jats:p>","DOI":"10.1007\/s00296-021-04865-9","type":"journal-article","created":{"date-parts":[[2021,5,4]],"date-time":"2021-05-04T06:03:02Z","timestamp":1620108182000},"page":"1657-1665","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":30,"title":["Rheumatoid factor and falsely elevated results in commercial immunoassays: data from an early arthritis cohort"],"prefix":"10.1007","volume":"41","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9896-0223","authenticated-orcid":false,"given":"Johanna E.","family":"Gehin","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6078-5108","authenticated-orcid":false,"given":"Rolf A.","family":"Klaasen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7777-0714","authenticated-orcid":false,"given":"Ellen S.","family":"Norli","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6854-1282","authenticated-orcid":false,"given":"David J.","family":"Warren","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2159-3696","authenticated-orcid":false,"given":"Silje W.","family":"Syversen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8506-725X","authenticated-orcid":false,"given":"Guro L.","family":"Goll","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5578-615X","authenticated-orcid":false,"given":"Trine","family":"Bj\u00f8ro","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8441-3093","authenticated-orcid":false,"given":"Tore K.","family":"Kvien","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0692-3271","authenticated-orcid":false,"given":"Maria D.","family":"Mjaavatten","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nils","family":"Bolstad","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2021,5,4]]},"reference":[{"issue":"7","key":"4865_CR1","first-page":"942","volume":"45","author":"LJ Kricka","year":"1999","unstructured":"Kricka LJ (1999) Human anti-animal antibody interferences in immunological assays. 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GLG Abbvie, Biogen, Boehringer Ingelheim, Orion Pharma, Eli Lilly, Novartis, Pfizer, MSD, Roche, UCB. TKK AbbVie, Amgen, Biogen, BMS, Celltrion, Egis, Eli Lilly, Ewapharma, Ewopahrma, Gilead, Hikma, MSD, Mylan, Novartis, Oktal, Pfizer, Roche, Sandoz, Sanofi, UCB. NB Takeda, Roche, Janssen, Novartis, The remaining authors: None declared. Disclosures listed above include fees for speaking and\/or consulting or research funding to the institution (Diakonhjemmet Hospital).","order":2,"name":"Ethics","group":{"name":"EthicsHeading","label":"Conflict of interest"}},{"value":"The study was approved by the Regional Ethics Committee of Eastern Norway.","order":3,"name":"Ethics","group":{"name":"EthicsHeading","label":"Ethics approval"}},{"value":"All patients provided written, informed consent before inclusion.","order":4,"name":"Ethics","group":{"name":"EthicsHeading","label":"Consent to participate"}},{"value":"Not applicable.","order":5,"name":"Ethics","group":{"name":"EthicsHeading","label":"Consent for publication"}}]}}