Should the lower limit of normal (LLN) be used for FEV1 (Forced Expiratory Volume in 1 second)/FVC (Forced Vital Capacity) ratio in Pulmonary Function Tests (PFTs) for Chronic Obstructive Pulmonary Disease (COPD) diagnosis?

Using Lower Limit of Normal vs. Fixed Ratio for FEV1/FVC in COPD Diagnosis

The lower limit of normal (LLN) should be used instead of the fixed ratio of 0.70 for FEV1/FVC when diagnosing COPD, as it provides more accurate classification, especially at the extremes of age. 1

Current Diagnostic Approaches

The diagnosis of COPD requires confirmation of airflow limitation that is not fully reversible via spirometry, along with a history of exposure to risk factors such as smoking. There are two main approaches to defining airflow limitation:

• Fixed ratio approach: Uses a post-bronchodilator FEV1/FVC ratio <0.70 1
• Lower limit of normal (LLN) approach: Uses a post-bronchodilator FEV1/FVC below the 5th percentile of the predicted value for age, sex, and height 1

Problems with the Fixed Ratio Approach

The fixed ratio of 0.70 has significant limitations:

• Overdiagnoses COPD in elderly patients (>70 years) 1, 2
• Underdiagnoses COPD in younger adults (<45 years) 1, 2
• Results in approximately 7-8% discordant diagnoses compared to LLN, with age being the strongest predictor of discordance 2
• Up to 16% of subjects over 74 years of age may be misclassified when using the fixed ratio compared to LLN 2

Advantages of the LLN Approach

The LLN approach offers several benefits:

• Accounts for normal age-related changes in lung function 1
• Reduces false-positive diagnoses in the elderly 1
• Provides more accurate classification across all age groups 2
• Aligns with ATS/ERS recommendations for interpreting lung function tests 1

Clinical Implications of Diagnostic Criteria

The choice between fixed ratio and LLN has important clinical consequences:

• Using the fixed ratio alone may lead to overdiagnosis of COPD and inappropriate treatment 3
• Patients classified as having COPD by both fixed ratio and LLN show stronger associations with adverse clinical outcomes than those identified by fixed ratio only 3
• The strongest associations with clinical outcomes occur when both a low FEV1/FVC ratio and a low FEV1 (<80% predicted) are present 3

Considerations for Optimal Diagnosis

For the most accurate COPD diagnosis:

• Consider using vital capacity (VC) rather than forced vital capacity (FVC) when calculating the ratio, as FEV1/VC may identify more obstructive patterns than FEV1/FVC 1, 4
• Use the highest value of slow vital capacity (SVC) or FVC when available, as this approach identifies more cases with clinically significant obstruction and air trapping 4
• Be aware that patients identified as having COPD when using VC (but not FVC) tend to have more obstruction and air trapping 4

Recent Research Findings

Despite the theoretical advantages of LLN, some recent research presents contrasting findings:

• A 2019 study from the NHLBI Pooled Cohorts Study found that the fixed threshold of 0.70 provided discrimination of COPD-related hospitalization and mortality that was not significantly different or was more accurate than the LLN 5
• This study suggests the fixed ratio may still be clinically relevant for identifying individuals at risk of clinically significant COPD 5

Expert Recommendations

The ATS/ERS task force recommends:

• Studies comparing outcomes among individuals diagnosed with COPD using the fixed ratio versus the LLN 1
• Further research to determine which approach better predicts clinically important outcomes 1
• Consideration of additional parameters beyond the FEV1/FVC ratio, such as FEV1% predicted and residual volume/total lung capacity ratio (RV/TLC) to improve diagnostic accuracy 6

Practical Approach to Diagnosis

For optimal COPD diagnosis in clinical practice:

1. Use the LLN of FEV1/FVC as the primary criterion for airflow obstruction, especially in patients at the extremes of age 1
2. Consider additional lung function parameters, particularly FEV1% predicted 6, 3
3. Assess for symptoms and exposure history to risk factors 1
4. In borderline cases, consider additional tests such as residual volume/total lung capacity ratio to detect air trapping 6
5. Remember that spirometry alone is not sufficient for COPD diagnosis and should be combined with clinical assessment 1