What SR (Spirometric Ratio) range indicates abnormal Maximum Expiratory Flow (MEF) results?

Interpreting Abnormal MEF Values in Spirometric Ratio (SR)

Values below the 5th percentile of predicted MEF values indicate abnormal results when reported as SR (Spirometric Ratio). 1

Understanding MEF Values and Spirometric Ratio

Maximum Expiratory Flow (MEF) values are important parameters in pulmonary function testing that help assess airway function, particularly in the small airways. When reported as Spirometric Ratio (SR), they represent the relationship between measured flow rates and predicted values based on age, height, and gender.

Key MEF Parameters:

• MEF 75 (also called FEF 25): Flow rate when 75% of FVC remains in lungs
• MEF 50 (also called FEF 50): Flow rate at 50% of FVC
• MEF 25 (also called FEF 75): Flow rate when 25% of FVC remains in lungs
• MEF 25-75% (also called FEF 25-75%): Average flow rate between 25% and 75% of FVC

Interpretation Algorithm for MEF Values

1. Normal Range: MEF values within the 5th-95th percentile of predicted values 1
2. Abnormal Range: MEF values below the 5th percentile of predicted values 1
3. Context: Always interpret MEF values alongside FEV1 and FEV1/VC ratio

Clinical Significance of Abnormal MEF Values

Early Detection of Obstruction

• MEF values, particularly MEF 75, can detect small airway obstruction before changes in FEV1 become apparent 2
• A proportionally greater reduction in MEF 75 or MEF 25-75% than in FEV1 suggests small airway disease 2

Pattern Recognition

• Concave shape on flow-volume curve with reduced MEF values suggests airflow obstruction 2
• When FEV1 and FEV1/VC are within normal range, abnormal MEF values have limited clinical significance 2
• In borderline FEV1/VC cases, abnormal MEF values may suggest early airway obstruction 2

Important Caveats and Pitfalls

1. High Variability: MEF values have higher variability than FEV1 and FVC, requiring cautious interpretation 1

2. False Positives: Examining multiple parameters simultaneously increases the likelihood of finding abnormal values even in healthy subjects 2

   • When examining only FEV1, FVC, and FEV1/FVC ratio, 10% of healthy subjects show at least one abnormal test
   • When examining 14 different spirometric measurements, 24% of healthy subjects show at least one abnormal test

3. Limited Specificity: Mid-range flow measurements are not specific for small airway disease in individual patients 2

4. Context Dependency: MEF values should never be interpreted in isolation but always alongside FEV1 and FEV1/VC 2

Special Considerations

• MEF values are particularly useful in children with cystic fibrosis for monitoring disease progression 2
• MEF values can show significant changes after bronchodilator administration, potentially indicating airway hyperresponsiveness 1
• The ratio of maximum expiratory to inspiratory flow at 50% VC (MEF50/MIF50) can help identify upper airway obstruction when >1 3

Remember that while MEF values provide valuable information about airway function, particularly in the small airways, they should always be interpreted as part of a comprehensive pulmonary function assessment that includes FEV1 and FEV1/VC ratio.