Is interstitial lung disease (ILD) more evident on computed tomography (CT) scan or pulmonary function tests (PFTs)?

Interstitial Lung Disease is More Evident on CT Scan Than PFTs

High-resolution computed tomography (HRCT) of the chest is significantly more sensitive than pulmonary function tests (PFTs) for detecting interstitial lung disease (ILD), with HRCT showing a sensitivity of 100% compared to only 47.5% sensitivity for PFTs using FVC <80% as a threshold. 1

Diagnostic Accuracy Comparison

HRCT Advantages

• HRCT is the gold standard for ILD detection with near-perfect sensitivity (95.7-100%) 1
• Can detect subclinical ILD that may not be apparent on PFTs 1
• Provides critical information about:
  • Pattern of disease (UIP, NSIP, organizing pneumonia)
  • Extent of involvement
  • Distribution (peripheral vs. central, upper vs. lower lobe)
  • Specific features (honeycombing, reticulation, ground glass opacities)

PFT Limitations

• PFTs alone are insufficient for ILD detection in systemic autoimmune rheumatic diseases 1
• FVC <80% has only 47.5% sensitivity and 78.7% specificity for ILD 1
• Patients with ILD can have completely normal PFTs, especially early in disease 1
• Some patients cannot adequately perform PFTs due to cough or microstomia 1

Clinical Implications

Why HRCT is Superior

1. Earlier Detection: HRCT can identify ILD before physiologic impairment occurs 2
2. Pattern Recognition: HRCT can differentiate between various ILD patterns, which has prognostic and treatment implications 1
3. Quantification: Modern AI-based quantification of HRCT can provide objective assessment of disease extent 3
4. Comprehensive Assessment: HRCT can detect other pulmonary conditions that may coexist with ILD (pulmonary hypertension, emphysema, bronchiectasis) 1

Role of PFTs

While less sensitive for detection, PFTs remain valuable for:

• Assessing physiologic impairment
• Monitoring disease progression
• Evaluating response to therapy
• Providing complementary information to HRCT

Optimal Approach to ILD

The 2023 ACR/CHEST guidelines strongly support using both modalities together:

1. For Initial Diagnosis:

   • HRCT is essential as the primary diagnostic tool
   • PFTs provide baseline physiologic assessment

2. For Monitoring:

   • PFTs more frequently (every 3-12 months depending on disease)
   • HRCT when clinically indicated (not routinely) 1

Important Considerations

HRCT Technical Factors

• Thin slices (≤1.5 mm) are optimal for ILD assessment 1
• Inspiratory and expiratory images improve diagnostic yield 1
• CT angiogram studies may be inadequate for ILD assessment due to incomplete inspiration 1
• Low-dose protocols (20mA) with 3-5mm slice thickness can reduce radiation while maintaining diagnostic accuracy 4

Common Pitfalls

1. Relying solely on PFTs: May miss early or stable ILD
2. Misinterpreting CT angiograms: These are often performed with incomplete inspiration 1
3. Overreliance on chest radiography: Very low sensitivity (58-64%) for ILD 1
4. Failure to correlate findings: HRCT and PFTs provide complementary information and should be interpreted together

Conclusion

While both HRCT and PFTs are important in the evaluation of ILD, HRCT is clearly superior for detection and characterization of disease. The optimal approach involves using HRCT for diagnosis and PFTs for physiologic assessment and routine monitoring, with repeat HRCT when clinically indicated.