What is the meaning of diagnostic code R91.8 in relation to lung infiltrate?

Understanding Diagnostic Code R91.8: Lung Infiltrate

Diagnostic code R91.8 refers to "Other nonspecific abnormal findings on radiological examination of lung," which includes lung infiltrates identified on imaging studies. 1

Clinical Significance of Lung Infiltrates

Lung infiltrates represent abnormal opacities or densities visible on chest imaging that can indicate various underlying conditions:

Types of Lung Infiltrates

• Infectious causes:

  • Bacterial pneumonia (typically focal/localized infiltrates) 2
  • Fungal infections (particularly Aspergillus in immunocompromised patients) 2
  • Viral pneumonia (often diffuse pattern) 1
  • Pneumocystis pneumonia (especially in immunocompromised hosts) 2

• Non-infectious causes:

  • Pulmonary edema (cardiogenic or non-cardiogenic) 3
  • Pulmonary hemorrhage 3
  • Hypersensitivity pneumonitis 2
  • Malignancy-related infiltrates 4
  • Drug-induced lung injury 4
  • Aspiration (including lipoid pneumonia) 5

Radiographic Patterns

The pattern of infiltrates provides diagnostic clues:

• Mosaic attenuation: Patchwork of regions with varying density on inspiratory CT images 2
• Three-density pattern: Combination of normal-appearing lung, ground-glass opacity, and lucent lung areas - highly specific for fibrotic hypersensitivity pneumonitis 2
• Diffuse bilateral infiltrates: May indicate viral pneumonia, pulmonary edema, or diffuse alveolar damage 1
• Localized infiltrates: More commonly associated with bacterial or fungal infections 2

Diagnostic Approach for Lung Infiltrates

When lung infiltrates are identified under code R91.8, the following diagnostic approach is recommended:

1. High-resolution CT scan: More sensitive than conventional chest X-rays for characterizing infiltrates 2

   • Should be performed within 24 hours of clinical indication 2
   • Can be done without contrast in most cases 2

2. Bronchoalveolar lavage (BAL):

   • Should be performed when infiltrates are detected on CT 2
   • Target the bronchial segment supplying the area of radiographic abnormality 2
   • Requires standardized protocol with 100-300ml of normal saline 2
   • Optimal retrieval should be ≥30% of instilled volume 2

3. Microbiological testing:

   • BAL fluid should be processed for culture, microscopy, and PCR 2
   • Quantitative cultures with thresholds of 10^4 CFU/ml for BAL and 10^3 CFU/ml for protected specimen brush 2
   • Galactomannan testing in BAL fluid for suspected fungal infections 2

4. Cellular analysis of BAL fluid:

   • 15% lymphocytes suggests lymphocytic pattern

   • 3% neutrophils indicates neutrophilic pattern

   • 1% eosinophils suggests eosinophilic pattern

   • 0.5% mast cells indicates mastocytosis 2

Clinical Implications

The R91.8 code requires further clinical correlation to determine the specific etiology of lung infiltrates:

• In immunocompromised patients, infectious causes are most common (77% of cases), with bacterial (24%), fungal (17%), and viral (10%) etiologies predominating 3

• Non-resolving infiltrates (persisting >6 weeks despite appropriate therapy) warrant further investigation, including consideration of bronchoscopy 6

• Early intervention is critical for infectious causes, with significantly better outcomes when treatment changes are made within the first 7 days (29% mortality vs. 71% when changes are made later) 3

Management Considerations

Management depends on the underlying cause of the infiltrate:

• Empiric antimicrobial therapy may be necessary while awaiting diagnostic results 1

• Follow-up imaging should be performed 6-8 weeks after initiating treatment to ensure resolution 1

• Persistent infiltrates despite appropriate therapy may require additional diagnostic procedures, including tissue biopsy 6

Remember that while R91.8 is a nonspecific code indicating an abnormal radiological finding, it requires thorough investigation to determine the underlying cause and guide appropriate management.