Between 16 July and 27 August 2019, 900 participants completed spirometry measurements. After exclusions, 622 participants (204 males and 418 females) aged 4–82 years met the selection criteria for the reference sample.

The mean (range) age of men and women was 38.34 (4–82) and 44.55 (4–80) years, respectively. In this sample, the mean height was 1.72 (0.08) m in males and 1.58 (0.08) m in females aged over 21 years. Thirty-nine (19.2%) men and 131 (31.4%) women had a BMI ≥ 30 kg/m2. The demographic and clinical characteristics of the participants are described in Table 1.

Table 1

Demographical and clinical characteristics of the reference population

Variables | Total(620) | Females(414) | Males (206) | P-Value |

Mean age(SD);year Range | 42.29(17.34) 4–82 | 45.54(15.67) 4–80 | 37.78(19.56) 4–82 | < 0.001 |

Mean weight (SD);kg Range | 70.73(18.18) 14-132.5 | 68.61(16.25) 15–120 | 74.88(20.99) 14–132 | < 0.001 |

Mean height(SD); cm Range | 160.18(14.21) 101–193 | 156.18(11.09) 101–120 | 168.22(16.27) 109–193 | < 0.001 |

Mean BMI(SD) Range | 27.17(5.65) 11.16–49.95 | 27.84(5.74) 13.63–49.95 | 25.8(5.23) 11.16–39.84 | < 0.001 |

Mean FEV1(SD Range | 2.97(0.94) (0.79–6.58) | 2.63(0.65) 0.79–5.01 | 3.65(1.04) 0.86–6.58 | < 0.001 |

Mean FVC(SD) Range | 3.61(1.12) 0.90–7.5 | 3.20(0.78) 0.9 − 0.78 | 4.42(1.24) 1.15–7.5 | < 0.001 |

Mean SVC(SD) Range | 3.35(1.17) 0.54–7.24 | 2.93(0.85) 0.34–6.9 | 4.18(1.26) 0.91–7.24 | < 0.001 |

Mean FEV1/FVC(SD) Range | 0.82(0.069) 0.5–1.49 | 0.820(0.70) 0.65–1.49 | 0.826(0.067) 0.51-1.0 | 0.878 |

Mean FEV1/VC(SD) Range | 0.920(0.358) 0.342–8.82 | 0.89(0.42) 0.413–8.82 | 0.892(0.169) 0.34–1.64 | 0.156 |

Mean FEF25 − 75%(SD) Range | 1.36(0.457) 0.46–2.22 | 3.88(1.29) 0.49–8.13 | 1.51(0.523) 0.55–2.22 | 0.181 |

SD = Standard Deviation, BMI = Body Mass Index, FEV1 = Forced Expiratory Volume in one second, FVC = Forced Expiratory Vital Capacity, SVC = Slow Expiratory Vital Capacity, VC = Vital Capacity, FEF25 − 75% = Forced expiratory flow at 25 and 75% of the pulmonary volume |

The Caucasian GLI-2012 was applied to our population. Overall, the mean Z-scores of FEV1, FVC and the FEV1/FVC ratio for males and females in various age group were higher than the Caucasian predicted values (range: 0.01 to 1.05) except for the FEV1/FVC in the age group under 21 years (range:-1.11 to -0.09).

The normal distribution curves of FEV1, FVC and FEV1/FVC based on observed GLI Z-score mean and standard deviation values in men and women are presented in Fig. 1a, b, c, d, e, and f.

The distribution of the Z-scores of FEV1, FVC, FEV1 /FVC ratio and FEF25 − 75% stratified by sex and age in the reference sample of healthy individuals is shown in Table 2.

Table 2

Mean GLI Z- scores for FEV1, FVC, the FEV1 /FVC ratio and FEF25 − 75% by age group and sex (Assumption test: Compare means with zero)

Age group(Y) | Sex | n | FEV1 | FVC | FEV1/FVC | FEF25 − 75% |

| | | mean | SD | P-Value | Mean | SD | P-Value | mean | SD | P-Value | mean | SD | P-Value |

< 10 | male | 13 | 0.52 | **0.62** | 0.010 | 0.72 | **0.63** | .001 | -1.11 | 1.22 | 0.001 | **-0.11** | **0.82** | 0.637 |

female | 20 | **0.40** | **0.90** | 0.061 | 0.62 | **0.79** | .003 | -1.2 | 1.08 | 0.003 | -0.59 | **0.62** | 0.000 |

10–21 | male | 38 | **0.27** | 1.29 | 0.206 | **0.39** | 1.3 | .065 | **-0.09** | 1.01 | 0.065 | **-0.09** | 1.02 | 0.595 |

female | 15 | **0.37** | 1.09 | 0.203 | 0.50 | **0.69** | .013 | **-0.32** | 1.22 | 0.013 | **-0.19** | 1.46 | 0.617 |

22–29 | male | 21 | **0.16** | **0.79** | 0.357 | **0.29** | **0.71** | .075 | **0.10** | **0.52** | 0.075 | **-0.44** | **0.94** | 0.047 |

female | 23 | **0.14** | **0.89** | 0.446 | **0.42** | 1.02 | .060 | **0.01** | 1.04 | 0.060 | -0.66 | 1.00 | 0.004 |

30–39 | male | 43 | **0.46** | 1.27 | 0.022 | 0.54 | 1.22 | .006 | **0.23** | **0.69** | 0.006 | **-0.11** | 1.1 | 0.532 |

female | 84 | 0.50 | 1.24 | 0.000 | 0.89 | 1.29 | .000 | **0.06** | **0.67** | 0.000 | -0.58 | 0.86 | 0.000 |

40–49 | male | 25 | 0.65 | 0.85 | 0.001 | 0.65 | **0.93** | .002 | **0.42** | **0.63** | 0.002 | **-0.07** | **0.77** | 0.646 |

female | 99 | 0.57 | 1.16 | 0.000 | 0.91 | 1.2 | .000 | **0.28** | **0.77** | 0.000 | **-0.49** | 1.08 | 0.000 |

50–59 | male | 29 | 0.59 | 1.09 | 0.006 | 0.62 | **0.97** | .002 | **0.42** | **0.88** | 0.002 | **-0.22** | **0.89** | 0.192 |

female | 106 | 0.83 | 1.11 | 0.000 | 1.05 | 1.48 | .000 | **0.45** | **0.78** | 0.000 | **-0.41** | **0.91** | 0.000 |

60–69 | male | 19 | 0.51 | 1.02 | 0.041 | **0.30** | **0.9** | .161 | 0.95 | **0.83** | 0.161 | **0.19** | 1.08 | 0.438 |

female | 63 | 0.72 | 1.15 | 0.000 | 1.02 | 1.18 | .000 | **0.32** | **0.76** | 0.000 | -0.53 | **0.86** | 0.000 |

> 70 | male | 16 | **0.36** | 2.23 | 0.531 | **0.48** | 2.02 | .361 | -0.61 | 1.65 | 0.361 | **0.0** | 1.28 | 0.989 |

Female | 8 | **0.36** | **0.82** | 0.260 | **0.06** | **0.66** | .805 | **0.07** | **0.98** | 0.805 | **0.09** | 1.08 | 0.830 |

Y = Years, SD = standard deviation, P-values by independent samples T-test, GLI = Global Lung Function Initiative, FEV1 = Forced Expiratory Volume in one second, FVC = Forced Expiratory Vital Capacity, FEF25 − 75% = Forced expiratory flow at 25 and 75% of the pulmonary volume |

The FEV1 Z-score was smaller than 0.5 in men and women aged 10–21, 22–29, and 30–39 years; however, its standard deviation was often above 1. Moreover, this value was below 0.5 in girls under 10 years old and men 40–49 years old and over 70. The FEV1 Z-score was not different from zero(by one sample t- test analysis) in the age groups 10–29 and over 70 years in both genders (P > 0.05) (Table 2).

The FVC Z-score exceeded the predicted values (0.5) across age groups < 10, 30–39, 40–49, and 50–59 years in both genders, but it was below 0.5 in individual aged 10–21 (males), 22–29 (both gender), 60–69 (males), and > 70 (both gender) years old (Table 2).

The Z-score of FEV1/FVC was below 0.5 in all age groups except for the age group under 10 (both genders) and 60–69 (males) years (Table 2).

The mean Z-score of FEF25 − 75% was between 0 and − 0.5 in males in all age groups; the same finding was found in women aged 10–21, 40–49, 50–59 and > 70 years (Table 2).

In the age group over 21 years, the Z-score of FEV1/FVC was below the LLN in seven men (3.43%) and eight women (2.01%). However, these values were significantly higher in six boys (46.2%) and eight girls (40.0%) under 21 years old.

Z-score of FEV1/FVC less than LLN was zero in women over the age of 60 and in men aged 22–39 and 60–69 years. It was also less than 5% in women aged 50–59 and men aged 30–59 years (1- 3.4%).

Totally, the FEV1/FVC Z-score was above the upper limit of normal in 20 (9.8%) men and 69 (16.5%) women (ULN > 1.64).

The mean Z-score of FEV1/FVC above the upper limit of normal (ULN, > 1.64) ranged between 0% (age group 22–29 years) and 25% (over 70 years( in men and between 0% (age group 10–21 and over 70 years) and 28.3% (age group 50–59 years) in women.

The FEV1 Z-score was within the 90% limits of normality (-1.64 to 1.64) in 81.3% of the observations (83.7% in males and 80.4% in females). The corresponding figure was 78.9% for FVC (84.5% in males and 76.1% in females) and 93.3% (90.2% in males and 94.8% in females) for the FEV1/FVC ratio in the age group over 21 years.

The Z-scores of FEV1, FVC, FEV1/FVC ratio and FEF25 − 75% were analyzed according to age, height, weight and gender using a linear regression model.

Age, weight and height (but not gender) had an impact on the FEV1/FVC Z–score in univariate regression (P < 0.05). In multiple linear regression (in the presence of height, weight and age as variables with P < 0.2 in univariate linear regression) height and age remained associated (B-coefficient = 0.012 and 0.007; P = 0.008, P = 0.001 respectively) with the FEV1/FVC Z-score (Fig. 2a). There was a significant association between age and FEV1 Z-score (B-coefficient = 0.007; P = 0.011). In the multiple linear regression (in the presence of age, gender and height), age remained statistically significant (B-coefficient = 0.008; P = 0.006) (Fig. 2b).

There was a significant association between gender and FVC Z-score (B- coefficient = 0.389; P < 0.001) in univariate linear regression; this association was maintained in the multiple regression model too (B-coefficients = 0.346; P = 0.004) (Fig. 2c).

Height and gender had an effect on the FEF25 − 75% Z-score (B-coefficients=-0.371 and 0.006; P < 0.001 and P = 0.021, respectively) in univariate linear regression. In the multiple linear regression model, gender had a significant relationship with the FEF25 − 75% Z-score (B-coefficient=-0.358; P = 0.001) (Fig. 2d).

The prevalence of COPD defined by spirometry based on the fixed ratio (FR) criterion increased with age from 0.8% in the age group 30–39 years to 16.7% in the age group > 70 years ( P for linear by linear association <0.001). The prevalence of COPD according to the LLN criterion did not follow a specific trend (P for linear by linear association =0.749). There was a 98.13% agreement between FR and LLN method (Fleiss' kappa coefficient = 0.58, P < 0.001). The prevalence of COPD based on FR and LLN according to age and sex is presented in Table 3.

Table 3

Prevalence of COPD based on FR(FEV1/FVC < 0.70) and LLN (FEV1/FVC < LLN)criteria by age and sex (n(%))

Age groups | Total(589) | Males(191) | Females(398) |

FR | LLN | FR | LLN | FR | LLN |

< 10(33) | 0 | 14(42.4) | 0 | 6(46.2) | 0 | 8(40.0) |

10–21(53) | 0 | 4(7.5) | 0 | 2(5.3%) | 0 | 2(13.3) |

22–29(44) | 0 | 2(4.5) | 0 | 0 | 0 | 2(8.7) |

30–39(127) | 1(0.8) | 1(0.8) | 0 | 0 | 1(1.2) | 1(1.2) |

40–49(124) | 1(0.8) | 1(0.8) | 0 | | 1(1.0) | 1(1.0) |

50–59(135) | 2(1.5) | 3(2.2) | 1(3.4) | 1(3.4) | 1(0.9) | 2(1.9) |

60–69(82) | 4(4.9) | 0 | 0 | 0 | 4(6.3) | 0 |

> 70(24) | 4(16.7) | 4(16.7) | 4(25.0) | 4(25.0) | 0 | 0 |

FR = fixed ratio (FEV1/FVC < 0.7); LLN = lower limit of normal (FEV1/FVC < LLN) |