1,650 subjects were initially recruited (978 from urban region, 672 from the rural county). We excluded 120 subjects because of missing data, and 43 subjects whose quality results of spirometry were under grade C. Information about symptoms was available from 1486 subjects. There were 845 (56.9%) never-smokers and 641 (43.1%) smokers in our study. The prevalence of COPD was different among diverse smoking status and respiratory symptoms population (Table 1). 1258 (84.7%) subjects with FEV1/FVC ≥ 70%, 229 (15.4%) were diagnosed COPD (FEV1/FVC < 70%). 105 COPD patients (45.9%) were GOLD stage I, 91(39.7%) were GOLD stage II, 28(12.2%) were GOLD stage III, and 5(6.3%) were GOLD stage IV.
Table 1
Characteristics of all participants
|
Non-COPD (N = 1258)
|
COPD (N = 229)
|
Area
|
|
|
Urban
|
945
|
121
|
Rural
|
542
|
108
|
Sex
|
|
|
men
|
877
|
188
|
women
|
610
|
41
|
Age (years)
|
|
|
40 ~ 49
|
222
|
7
|
50 ~ 59
|
641
|
73
|
60 ~ 69
|
464
|
92
|
≥70
|
160
|
57
|
BMI (kg/m2)
|
|
|
<18.5
|
116
|
32
|
18.5 ~ 23.9
|
782
|
129
|
24.0 ~ 27.9
|
460
|
57
|
≥28.0
|
129
|
11
|
Smoking status
|
|
|
Never-smoker
|
845
|
70
|
Smoker
|
641
|
159
|
Respiratory symptoms
|
|
|
With respiratory symptom
|
362
|
111
|
Without respiratory symptom
|
1124
|
117
|
Lung function
|
|
|
FEV1/FVC > 0.7
|
1258
|
-
|
GOLD stage I
|
-
|
105
|
GOLD stage II
|
-
|
91
|
GOLD stage III
|
-
|
28
|
GOLD stage IV
|
-
|
5
|
Figure 1 (A) shows the strong correlation between FEV1 measured by two machines in total population (r1 = 0.889, P < 0.001), non-COPD group (r2 = 0.869, P < 0.001) and COPD group (r3 = 0.907, P < 0.001). Significant difference was observed between non-COPD group and COPD group (z = 2.509, P = 0.012). Figure 1 (B) shows strong relationships between FEV1 measured by spirometry and COPD-6® in groups of GOLD stage I (rI=0.810, P < 0.001), stage II (rII=0.802,P < 0.001) and stage III (rIII=0.637,P < 0.001) but nonsignificant correlation was found in GOLD stage IV group (rIV=0.844,P = 0.072). No statistical significance was found among GOLD stage I, stage II and stage III (rI vs. rII: z = 0.141, P = 0.887; rI vs. rIII: z = 1.675, P = 0.094; rII vs.rIII: z = 1.558,P = 0.119). Bland-Altman graph of FEV1 measured by spirometry and COPD-6® is shown in Fig. 1 (C). The limit of Agreement (LoA) was 0.445 ~ 0.816 L, and 4.5% (67/1487) points were out of the 95% LoA.
Figure 2 (A) shows strong correlations between FVC measured by spirometry with FEV6 measured by COPD-6® in total population (r1 = 0.835, P < 0.001), non-COPD group (r2 = 0.865, P < 0.001) and COPD group (r3 = 0.807, P < 0.001). Statistical difference was detected between non-COPD group and COPD group (z = 2.668, P = 0.008). Figure 2 (B) shows strong relationships between FVC measured by spirometry and FEV6 measured by COPD-6® in groups of GOLD stage I (rI=0.737,P < 0.001), stage II (rII=0.724,P < 0.001), stage III (rIII=0.574,P = 0.0014), but no significant correlation was found in GOLD stage IV group (rIV=0.615,P = 0.269). No significant difference was found among groups (rI vs. rII: z = 0.187, P = 0.851; rI vs. rIII: z = 1.301, P = 0.193; rII vs. rIII: z = 1.161, P = 0.246). Figure 2(C) shows the Bland–Altman graph of FVC by spirometry and FEV6 by COPD-6®. LoA was 0.514 ~ 1.297L, and 5.2% (77/1487) points were out of the 95%LoA.
Figure 3 (A) shows the relationship between FEV1/FVC measured by spirometry and FEV1/FEV6 measured by COPD-6® in total group (r1 = 0.647, P < 0.001), non-COPD group (r2 = 0.343, P < 0.001) and COPD group (r3 = 0.686, P < 0.001). Figure 3 (B) presented the relationship between FEV1/FVC measured by spirometry and FEV6/FVC measured by COPD-6® in groups of GOLD stage I (rI=0.197, P < 0.044), stage II (rII=0.641, P < 0.001), stage III (rIII=0.715, P < 0.001) and stage IV (rIV=0.784, P = 0.117). Figure 3 (C) is the Bland–Altman graph of FVC by spirometry and FEV6 by COPD-6® and the 95%LoA is -20.944 to 12.822.
Table 2 presented the AUC, sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and positive likelihood ratio (LR+) at different cut-off points of FEV1/FEV6 ratios. The AUC was largest (i.e., 0.80) when the cut-off point is 0.77.
Table 2
The measures on sensitivity and specificity at different cut-off points of FEV1/FEV6.
Cut-off point
|
AUCa
|
SE%b
|
SP%c
|
PPV%d
|
NPV%e
|
LR + f
|
< 0.65
|
0.62
|
25.82
|
98.93
|
80.80
|
88.02
|
23.15
|
< 0.70
|
0.70
|
41.95
|
97.92
|
78.75
|
90.34
|
20.28
|
< 0.75
|
0.77
|
60.22
|
92.96
|
60.83
|
92.37
|
8.52
|
< 0.77
|
0.80
|
69.94
|
89.98
|
55.78
|
94.29
|
6.92
|
< 0.80
|
0.79
|
77.78
|
80.13
|
41.63
|
95.24
|
3.91
|
a AUC = area under the receiver operator characteristic curve |
b SE = sensitivity |
c SP = specificity |
d PPV = positive predictive value |
e NPV = negative predictive value |
f LR + = positive likelihood ratio |
ROC curve was used to determine the best corresponding cut-off for FEV1/FEV6 (Fig. 4A). When the FEV1/FEV6 cut-off value of was 0.77, the area under the receiver operator characteristic curve (AUC) was 0.86 (95% CI: 0.83–0.89) and the sensitivity (71.2%) and specificity (89.8%) was greatest. Table 2 shows AUC, sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and positive likelihood ratio (LR+) at different cut-off points of FEV1/FEV6 ratios.
Figure 4 (B) shows ROC curves of FEV1/FEV6 measured by COPD-6® to identify airflow obstruction in the symptoms group and asymptomatic patients. AUC were 0.87 (95%CI: 0.82–0.916) and 0.84 (95%CI: 0.802–0.884), no significant difference (z = 0.789, P = 0.425) was observed between two groups. The AUC of FEV1/FEV6 that used to identify airflow obstruction in the smoker group (including current smokers and ex-smokers) and the never-smokers were 0.86 (95%CI: 0.82 to 0.90) and 0.84 (95%CI: 0.79 to 0.89), respectively. No significant difference was observed (z = 0.651, P = 0.515) (Fig. 4C).