Differentiating Usual Interstitial Pneumonia From Nonspecific Interstitial Pneumonia Using High-resolution Computed Tomography: The "Straight-edge Sign".

Purpose:The purpose of this article was to determine whether a novel finding on coronal computed tomography (CT) can help differentiate usual interstitial pneumonia (UIP) from nonspecific interstitial pneumonia (NSIP) in order to obviate lung biopsy.Materials and Methods:Two chest radiologists, blinded to clinical data, reviewed 3 preselected coronal images from CT scans, performed within 1 year of surgical lung biopsy (SLB), from 51 patients with biopsy-proven UIP and 15 with biopsy-proven NSIP. The 198 (66x3) images were anonymized and randomized. The radiologists assessed each coronal image for the presence or absence of the straight-edge sign (SES) on both the right and left sides, anecdotally thought to be more common in NSIP than in UIP. The SES was defined as reticulation isolated to the lung bases with sharp demarcation in the craniocaudal plane and without substantial extension along the lateral margins of the lungs. A validation cohort from a second medical center was also evaluated to reassess our findings.Results:The absence of a bilateral SES yielded a sensitivity, specificity, positive predictive value (PPV), and negative predictive value of 56.9%, 93.3%, 96.7%, and 38.9%, respectively, for UIP on SLB. The unilateral or bilateral absence of the SES yielded a sensitivity, specificity, PPV, and negative predictive value of 76.5%, 66.7%, 88.6%, and 45.5%, respectively, for UIP on SLB. For the 11 subjects with an overall CT pattern consistent with NSIP but a pathologic diagnosis of UIP, the SES was absent in 6 (54.5%) subjects. In the validation cohort, the SES was much more common in NSIP than in UIP (46.6% compared with 3.3%, respectively; P<0.001).Conclusion:The absence of the SES has a high PPV for biopsy-proven UIP. Bilateral absence of the SES has high specificity (93.3%) for biopsy-proven UIP. The SES may be useful for differentiating UIP from NSIP.