Interpretation of Lung Function Parameters

Forced Expiratory Flow (FEF)

The European Respiratory Society suggests that a low FEF, particularly FEF25-75%, can indicate small airway obstruction, but its clinical significance is limited when FEV1 and FEV1/VC are within the normal range 1, 2
FEF25-75% has a greater variability compared to FEV1, resulting in a wider range of predicted values 3, 4
In patients with cystic fibrosis, FEF such as MEF 25-75% can be more sensitive 5, 6

The European Respiratory Society recommends that PEF should not be relied upon as a substitute for FEV1 in assessing the degree of airway obstruction, especially in severe emphysema 3, 4
PEF can be useful for monitoring lung function at home and documenting diurnal variations 3, 4

The European Respiratory Society defines an obstructive pattern as a decrease in the FEV1/VC ratio, with a normal or increased TLC, and lower than expected flows throughout the volume range 1, 2, 7
Special attention is required when FEV1 and FVC decrease together with a normal or near-normal FEV1/FVC ratio 1, 2, 7

The European Respiratory Society advises against relying solely on PEF for diagnosis or assessment of airway obstruction severity - always confirm with complete spirometry including FEV1 3, 4
Measurement of FEF in addition to FEV1 and FEV1/VC is only considered after determining the presence and clinical significance of obstructive disorder using baseline values 5, 6

The European Respiratory Society states that FEF25-75 and similar mid-expiratory flow measures do not provide more useful information in clinical practice than FEV1 and VC, as they have greater intrasubject and intersubject variability with a larger range of predicted values 8, 9, 10
FEF25-75 should not replace FEV1 and FEV1/FVC as primary measures for diagnosing or assessing airway obstruction severity, due to its limited clinical utility when FEV1 and FEV1/FVC are normal 8, 9

The European Respiratory Society recommends using post-bronchodilator FEV1 % predicted to classify COPD severity, with mild (≥70%), moderate (50-69%), and severe (<50%) classifications 8, 9
A bronchodilator trial may be considered in symptomatic patients with reduced FEF25-75, as it may indicate reversible small airway dysfunction even when conventional spirometry appears normal 11

In osteogenesis imperfecta patients, FEF25-75 measurement is specifically recommended to detect comorbid asthma or COPD, as many have normal FEV1 but abnormal FEF25-75 11

An increase in expiratory flows after bronchodilator administration reflects bronchodilation and improved airway patency, as recommended by the European Respiratory Society, with a strength of evidence based on multiple studies 12, 13
Increased partial flows often correlate better with symptom improvement than FEV1 changes alone, according to the European Respiratory Society, with a moderate level of evidence 12, 13
The improvement in flow parameters within the tidal breathing range may explain decreased dyspnea even when FEV1 and FVC show minimal changes, as noted by the European Respiratory Society, with a low level of evidence 12, 13

Higher expiratory flows indicate reduced airway resistance and improved airway caliber, as stated by the European Respiratory Society, with a high level of evidence 12, 13
In obstructive disease, increased flows after intervention suggest reversible airway narrowing rather than fixed obstruction, according to the European Respiratory Society, with a moderate level of evidence 12
The increase may reflect decreased bronchial smooth muscle tone, reduced airway wall edema, or decreased mucus plugging, as suggested by the European Respiratory Society, with a low level of evidence 12

An isolated increase in FVC (>12% of control and >200 mL) not due to increased expiratory time after bronchodilator is a sign of bronchodilation, as recommended by the European Respiratory Society, with a high level of evidence 13
Improvement in lung function parameters within the tidal breathing range, such as increased partial flows, may explain symptom improvement even without significant FEV1/FVC changes, according to the European Respiratory Society, with a moderate level of evidence 12, 13
In patients with disproportionate differences between spirometric impairment and dyspnea, measurements of flow parameters may provide additional useful information, as noted by the European Respiratory Society, with a low level of evidence 14

Responses to bronchodilator therapy are often underestimated by FEV1 and FVC compared to airway resistance or flows measured during partial expiratory maneuvers, as stated by the European Respiratory Society, with a moderate level of evidence 12, 13