title: Weakly positive tests and chronologic variation of the QuantiFERON assay: a retrospective appraisal of usefulness.
abstract: OBJECTIVE
  QuantiFERON Tb Gold IT (QFT) is routinely used as diagnostic tool for detecting immune reactivity towards Mycobacterium tuberculosis. When diagnosing a latent infection weakly positive results and test reproducibility must be considered.
  
  
  OBJECTIVE
  To judge the frequency of useful information versus confusion gained by QFT in patients with immune-mediated diseases and in pulmonary outpatients.
  
  
  METHODS
  We retrospectively analysed 1485 QFT-tests performed on 855 patients with autoimmune diseases prior to initiating biologics and on 447 pulmonary outpatients with a suspected tuberculosis infection over a period of 16 months. In 161 cases the IGRA test was erroneously repeated. After 18 months clinical, radiographic and bacteriologic evidence for tuberculosis was collected.
  
  
  RESULTS
  Of all 1485 QFT-tests 247 (=16.63%) showed a positive result, 1186 (79.87%) were negative and 52 (3.50%) turned out to be indeterminate. Using the manufacturers cut-off value for the interferon-γ response, a cohort (n = 64) of weakly positive individuals (26% of all positives) was apparent. Repeated testing within ten weeks yielded negative reactivity in 69% of these patients. Of 25 IGRA positive patients who had a repeat test after one year without tuberculosis medication, 48% showed a reversion to a negative test. The frequency of indeterminate results of the IGRA was satisfying (3.5%).
  
  
  CONCLUSIONS
  In case of only slightly elevated immune reactivity quantitative reporting of positive QFT-test results and a repeat test strategy might be of advantage for the usefulness of the test.
authors: Division of Rheumatology and Immunology, Department of Internal Medicine, Medical University of Graz, Auenbruggerplatz 15, 8036 Graz, Austria. Electronic address: anjafelber@gmx.at.
authors: Anja
authors: A
authors: Felber
authors: Winfried
authors: W
authors: Graninger
mesh_terms: D000942:Antigens, Bacterial / Q000276:immunology; D001327:Autoimmune Diseases / Q000188:drug therapy / Q000276:immunology; D005189:False Positive Reactions; D006801:Humans; D007166:Immunosuppressive Agents / Q000627:therapeutic use; D007371:Interferon-gamma / Q000097:blood; D059425:Interferon-gamma Release Tests / Q000379:methods*; D055985:Latent Tuberculosis / Q000175:diagnosis* / Q000276:immunology; D015203:Reproducibility of Results; D012189:Retrospective Studies; D014397:Tuberculosis, Pulmonary / Q000175:diagnosis* / Q000276:immunology; D014409:Tumor Necrosis Factor-alpha / Q000037:antagonists & inhibitors
chemical_list: D000942:Antigens, Bacterial; D007166:Immunosuppressive Agents; D014409:Tumor Necrosis Factor-alpha; C072681:Mycobacterium tuberculosis antigens; D007371:Interferon-gamma
keywords: Anti-TNF-α-therapy; Immunosuppression; QuantiFERON; Test reproducibility