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Background: Accurate diagnosis of idiopathic pulmonary fibrosis (IPF) is essential to inform prognosis and treatment. In 2018, the ATS/ERS/JRS/ALAT and Fleischner Society released new diagnostic guidelines for usual interstitial pneumonitis (UIP)/IPF, adding Probable UIP as a CT category based on prior studies demonstrating this category had relatively high positive predictive value (PPV) for histopathologic UIP/Probable UIP. This study applies the 2018 ATS/ERS/JRS/ALAT and Fleischner Society guidelines to determine test characteristics of CT categories in academic clinical practice., Methods: CT and histopathology were evaluated by three thoracic radiologists and two thoracic pathologists. Comparison of consensus categorization by the 2018 ATS and Fleischner Society guidelines by CT and histopathology was performed., Results: Of patients with CT UIP, 87% (PPV, 95% CI: 60-98%) had histopathologic UIP with 97% (CI: 90-100%) specificity. Of patients with CT Probable UIP, 38% (PPV, CI: 14-68%) had histopathologic UIP and 46% (PPV, CI: 19-75%) had either histopathologic UIP or Probable UIP, with 88% (CI: 77-95%) specificity. Patients with CT Indeterminate and Alternative Diagnosis had histopathologic UIP in 27% (PPV, CI: 6-61%) and 21% (PPV, CI: 11-33%) of cases with specificities of 90% (CI: 80-96%) and 25% (CI: 16-37%). Interobserver variability (kappa) between radiologists ranged 0.32-0.81., Conclusions: CT UIP and Probable UIP have high specificity for histopathologic UIP, and CT UIP has high PPV for histopathologic UIP. PPV of CT Probable UIP was 46% for combined histopathologic UIP/Probable UIP. Our results indicate that additional studies are needed to further assess and refine the guideline criteria to improve classification performance.