Chronic obstructive pulmonary disease (COPD) is a leading cause of morbidity and mortality. It is predicted to become the fourth leading cause of death worldwide by 2030 [1]. Its prevalence and mortality will increase over time and its direct and indirect socio-economic burden will be extremely high [2].

To treat COPD properly, early diagnosis is important to prevent deterioration of pulmonary function and improve treatment outcomes. Pulmonary function tests using conventional spirometry are required to diagnose airflow limitation of COPD. The ratio of forced expiratory volume in 1 second (FEV₁) and forced vital capacity (FVC) needs to be measured in order to calculate whether postbronchodilator FEV₁/FVC is equal to or less than 0.7 as an indicator of airflow limitation [3-5]. However, conventional spirometry is not easy to execute [4,5], because FVC can not be easily measured. In addition, it causes discomfort to patients. Moreover, the result is difficult to reproduce. Thus, COPD is well known to be underdiagnosed in primary care setting [4].

Forced expiratory volume in 6 seconds (FEV₆) has been recently suggested as an alternative to FVC to detect airflow limitation [6,7]. In many patients who have difficult with exhalation, measuring FEV₆ could shorten expiratory time and reduce patients’ discomfort during examination [8]. Several researches have suggested that cut-off values of FEV₁/FEV₆ with conventional spirometry are between 0.7 and 0.8 [9,10]. Handheld spirometry can measure both FEV₁ and FEV₆ simultaneously. Recent active case-finding studies for COPD have shown the effectiveness of handheld spirometry in detecting airflow limitation [9-11]. Therefore, the aim of this study was to find the appropriate cut-off value of FEV₁/FEV₆ to diagnose COPD using handheld spirometry.

In this study, we tried to find out the best cut-off value of FEV₁/FEV₆ for diagnosing COPD. With both results of handheld spirometry and conventional spirometry from patients having the risk of COPD, FEV₁/FEV₆ less than or equal to 0.73 was found to be the best cut-off value to detect airflow limitation. It had high sensitivity and specificity. Furthermore, correlation analysis was performed between FEV₁/FEV₆ by handheld spirometry and postbronchodilator FEV₁/FVC by conventional spirometry. The correlation coefficient was 0.901, showing strong positive correlation between the two.

COPD is a preventable and treatable disease when it is diagnosed and treated early. However, a considerable number of patients might be undiagnosed [14]. One of the reasons is that it is difficult to perform conventional spirometry as a diagnostic tool in primary clinics [4,5]. In Korea, only 19.4% of primary care physicians used spirometry with bronchodilator test to assess COPD patients in 2008 [15]. Unfortunately, this figure did not improve much so far. In recent COPD Clinical Appropriateness Report in Korea, spirometry performance rates in primary clinics were still low, ranging from 37.8% to 42.4% [5]. The main reason for not using the spirometer frequently was the difficulty in performing the test [16]. The obstacle of performing conventional spirometry came from measuring FVC. Thus, FEV₆ was suggested as an alternative to FVC. In fact, in previous studies, FEV₁/FEV₆ and FEV₁/FVC showed good agreement, proving that FEV₁/FEV₆ could be a substitute of FEV₁/FVC for detecting airflow limitation. However, these studies were conducted using conventional spirometry [6,7,16].

The most noteworthy aspect of our study was that we analyzed the best value of FEV₁/FEV₆ using handheld spirometry and that it could detect airflow limitation of COPD as a diagnostic value. There have been studies about the best FEV₁/FEV₆ value on the use of handheld spirometry to assess subjects at risk for COPD. One cross-sectional study enrolled patients aged 50 years or older who were current smoker or ex-smoker (at least 1 pack-year) and defined the cut-off value to be 0.73 [10]. Another multicenter study recruited patients with high-risk COPD and set the cut-off point for the FEV₁/FEV₆ ratio < 0.70 with COPD-6 device as prescreening modality of COPD [11]. Although both studies have been conducted to explore active case-finding for COPD with the optimal FEV₁/FEV₆ value, they used FEV₁/FEV₆ value obtained from handheld spirometry as a screening purpose [10,11]. On the other hand, our study enrolled participants aged 40 years or older with risk factors for COPD who had respiratory symptoms and more than 10 pack-years of smoking history and found a cut-off value of 0.73 for FEV₁/FEV₆ with handheld spirometry for diagnosis.

This study has some limitations. First, this study was conducted in a population of patients at risk of COPD, and the cut-off value of FEV₁/FEV₆ < 0.73 obtained in this study was intended to be a guideline for primary care physicians. In recent COPD Clinical Appropriateness Report in Korea [5], spirometry performance rates in primary care clinics just range from 37.8% to 42.4%. We suggest that the cut-off value we proposed in this study can be helpful in diagnosing and treating COPD patients through COPD-6 device, which is more easily available to primary care physicians who have difficulty in using conventional spirometry. However, even though we considered the value from the previous studies, there was no result from the random population. Therefore, it is also necessary to obtain the validity of our values by collecting nation-wide data in the future. Second, the rate of COPD patients in this study was 47.9%, which was higher than that in other studies [16]. Participants of this study were patients who visited a tertiary hospital directly or were referred from primary clinics with high risk of COPD. Thus, the COPD rate was higher than that in the general population. Third, in our study, of 139 patients diagnosed as COPD with conventional spirometry, approximately 15.1% of patients had FEV₁/FEV₆ > 0.73 and reported as normal by handheld spirometry. Patients measured with FEV₁/FEV₆ > 0.73 had higher predicted FEV₁ values in both prebronchodilator (84.4% ± 17.3%) and postbronchodilator (86.5% ± 17.5%) results. They also had higher postbronchodilator FEV₁/FVC than those with FEV₁/FEV₆ ≤ 0.73 (86.5 ± 17.5 vs. 53.8 ± 11.4, p < 0.001). Patients with FEV₁/FEV₆ > 0.73 have relatively good lung function. They may not be treated immediately. Additionally, according to updated GOLD guideline [17], the value between 0.6 and 0.8 for postbronchodilator FEV₁/FVC might change as a biological variation. Indeed, in a study of two large cohorts [18], diagnostic reversal was observed in some patients with mild to moderate airflow limitation. However, less than 0.6 of postbronchodilator FEV₁/FVC at a single measurement would rarely rise to be above 0.7 [17,19]. For these reasons, patients with FEV₁/FEV₆ > 0.73 should be routinely followed up with handheld spirometry. Fourth, we did not show the gender data. Because there was no gender difference in conducting the study, it was not necessary to distinguish it.

In conclusion, our study indicates that the best cut-off value of FEV₁/FEV₆ using handheld spirometry to detect airflow limitation would be 73% or less in subjects with COPD risk factors. Further studies are needed to validate this cut-off value.