Straight-Edge Sign on High-Resolution CT
The straight-edge sign is a sharply demarcated border between fibrotic lung abnormality and relatively normal upper lung parenchyma seen on coronal CT images, and it is highly specific (94-96%) for connective tissue disease-related interstitial lung disease (CTD-ILD) rather than idiopathic pulmonary fibrosis (IPF), though its sensitivity is low (25-36%). 1, 2, 3
Definition and Radiologic Appearance
The straight-edge sign appears as a sharp demarcation of fibrotic abnormality on coronal CT images, creating a distinct horizontal line separating diseased lower lung zones from relatively preserved upper lung zones. 1 This contrasts with the more gradual transition typically seen in IPF, where fibrosis blends more smoothly into normal lung parenchyma. 2
Diagnostic Performance Characteristics
Specificity and Sensitivity
- Specificity ranges from 85% to 96% for differentiating CTD-ILD from IPF when a UIP pattern is present 2, 3, 4
- The highest specificity (95.7%) is achieved when the straight-edge sign is present bilaterally 3
- Sensitivity is relatively low at 25-36%, meaning its absence does not exclude CTD-ILD 2, 3, 4
Prevalence
- Present in 22-36% of CTD-ILD cases with UIP pattern 2, 3, 4
- Present in only 6-12% of IPF cases with UIP pattern 2, 3
- Interobserver agreement is excellent (kappa 0.81-0.87), making it a reliable sign when present 4
Clinical Context and Associated Signs
The straight-edge sign should be evaluated alongside two other CTD-ILD-associated signs on HRCT: 1
- Exuberant honeycombing sign: Honeycomb cysts occupying >70% of fibrotic lung portions 1
- Anterior upper lobe sign: Relative increase in fibrosis in the anterior upper lobes 1
When multiple signs are present together, specificity for CTD-ILD increases further, though sensitivity decreases. 4 The presence of any of these three signs should raise clinical suspicion for underlying CTD rather than IPF. 1, 2
Management Implications
Diagnostic Workup
When the straight-edge sign is identified in a patient with UIP pattern, a thorough workup for CTD should be pursued, including referral to rheumatology. 2 This includes:
- Comprehensive autoimmune serologies (ANA, rheumatoid factor, anti-CCP antibodies, myositis panel, scleroderma antibodies) 5
- Clinical evaluation for extrapulmonary CTD manifestations 5
- Multidisciplinary discussion integrating clinical, radiologic, and laboratory data 6, 5
Prognostic Significance
The straight-edge sign is associated with improved survival compared to typical IPF (p=0.0383), suggesting patients with this finding may represent a different phenotype with attenuated disease course. 3 This survival benefit appears most pronounced in patients initially classified as IPF who have the straight-edge sign. 3
Treatment Considerations
Recognition of CTD-ILD versus IPF fundamentally changes management:
- CTD-ILD: Immunomodulatory therapy (mycophenolate mofetil, rituximab, tocilizumab) may slow decline or improve lung function 7
- IPF: Antifibrotic therapy (nintedanib or pirfenidone) is first-line treatment 7
- Both conditions may benefit from antifibrotics in progressive fibrotic phenotypes 7
Critical Pitfalls to Avoid
Do not exclude CTD-ILD based on absence of the straight-edge sign alone, as the low sensitivity (25-36%) means most CTD-ILD cases will not demonstrate this finding. 2, 3, 4 The sign's value lies in its high specificity—when present, it strongly suggests CTD-ILD and mandates further workup. 2
Do not diagnose IPF without systematically excluding CTD, as missing CTD-ILD leads to inappropriate treatment selection and missed opportunities for immunomodulatory therapy. 5 Coronal reformatted images are essential for detecting the straight-edge sign, as it may not be apparent on axial images alone. 1
Always correlate imaging findings with clinical context, including detailed review of systems for CTD symptoms, family history of autoimmune disease, and comprehensive serologic testing before finalizing a diagnosis of IPF. 6, 5