Establishment and application of reference equations for FEF₅₀ and FEF₇₅ in the Chinese population

Reference equations for forced expiratory flow at 50% and 75% of forced vital capacity (FVC) (FEF₅₀ and FEF₇₅) in the Chinese population are lacking. It is of great importance to establish equations covering most age groups and to study their applicability in clinical practice.

Using the lambda-mu-sigma (LMS) method, reference equations for FEF₅₀ and FEF₇₅ were constructed based on pulmonary function data from healthy subjects collected from January 2007 to June 2010 at 24 centers throughout China. Differences between the established equations and extraneous equations were compared using standardized means (Z values) and percentage errors (PE). The proportion of small airway dysfunction (SAD) defined by the present equations was calculated. The Fisher precision probability test and the Mann-Whitney test were used to analyze the magnitude of changes in small and large airway indices after bronchodilator inhalation in patients with suspected asthma and chronic obstructive pulmonary disease (COPD).

Reference equations for FEF₅₀ and FEF₇₅ were established based on data from 7,115 healthy individuals (aged 4 to 80 years, 50.9% female, height between 95 and 190 cm). The present equations (all Z values were -0.0 and PE ranged from 2.0% to 4.2%) showed advantages over the European Community for Steel and Coal (ECSC) equations in 1993 (with Z values ranging from -0.7 to -0.2 and PE ranged from -23.4% to -4.5%). A total of 4,356 patients with suspected asthma (51.1% female; a mean age of 45.4 years) and 6,558 patients with suspected COPD (10.1% female; a mean age of 65.0 years) were included. The present equations defined 95.7% and 99.9% of SAD in these patients. After bronchodilator inhalation, greater mean improvement rates in small airway indices were observed both in patients with suspected asthma [mean ± standard deviation (SD) =48%±47%] and in patients with suspected COPD (mean ± SD =20%±30%) (P<0.05).

The reference equations for FEF₅₀ and FEF₇₅ established in this study should be considered for use in China. Further studies are needed to validate their value in the diagnosis of some chronic respiratory diseases.