Table 3

Characteristics of subjects who had follow-up spirometry

CharacteristicsAll patients with follow-up spirometryPatients with normal RV:TLCPatients with abnormal RV:TLCDifference (95% CI)P values
Demographics
 n (% of the study cohort)1706 (22.8)1124 (21.7)582 (25.3)−3.6% (−5.7 to 1.5)<0.001
 Age (years)61.3±10.460.9±10.362.1±10.6−1.2 (−2.2 to 0.2)0.023
 Sex (female), n (%)123 (7.2)109 (9.7)14 (2.4)7.3% (5.2 to 9.4)<0.001
 Height (cm)174.7±7.8175.0±7.9173.9±7.71.1 (0.3 to 1.9)0.006
 BMI (kg/m2)29.7±5.629.7±5.629.6±5.70.004 (−0.6 to 0.6)0.989
 Years of follow-up10.3±4.610.0±4.610.8±4.6−0.8 (−1.3 to 0.3)<0.001
Baseline airflow indices
 FEV1 (L)3.07±0.583.15±0.602.93±0.520.22 (0.16 to 0.28)<0.001
 FEV1 (% predicted)88±990±1084±85.7 (4.8 to 6.6)<0.001
 FVC (L)4.00±0.764.09±0.783.82±0.680.27 (0.20 to 0.34)<0.001
 FVC (% predicted)89±1091±1186±95.4 (4.4 to 6.4)<0.001
 FEV1:FVC (%)77.0±4.877.1±4.976.7±4.60.4 (−0.1 to 0.9)0.093
 FEV1:FVC (% predicted)98.3±5.898.5±5.897.9±5.70.58 (0.004 to 1.16)0.048
 FEF25–75 (L)2.85±0.942.92±0.972.73±0.850.19 (0.10 to 0.28)<0.001
 FEF25–75 (% predicted)86±2487±2582±215.4 (3.1 to 7.8)<0.001
 Reversibility in FEV1 (mL)187±207173±198216±220−43 (−64 to 22)<0.001
 Reversibility in FEV1 (%)7±96±89±11−2.5 (−3.5 to 1.6)<0.001
 Reversibility in FEV1, n (%)709 (41.6)435 (38.7)274 (47.1)−8.4% (−13.3 to 3.4)<0.001
Lung volume indices
 TLC (L)6.67±1.096.44±1.017.12±1.10−0.68 (−0.78 to 0.57)<0.001
 TLC (% predicted)100±1296±10107±13−10.5 (−11.7 to 9.4)<0.001
 RV (L)2.67±0.772.30±0.503.39±0.69−1.09 (−1.14 to 1.03)<0.001
 RV (% predicted)121±33104±20154±29−50 (−52 to 48)<0.001
 FRC (L)3.57±0.863.33±0.744.07±0.86−0.74 (−0.83 to 0.66)<0.001
 FRC (% predicted)103±2395±19118±23−22 (−24 to 20)<0.001
 IC (L)3.10±0.713.12±0.713.06±0.690.06 (−0.02 to 0.14)0.121
 IC (% predicted)74±1494±1473±141.3 (−0.2 to 2.8)0.089
 RV:TLC (%)40±836±648±5−11.7 (−12.3 to 11.2)<0.001
 RV:TLC (% predicted)119±22107±14143±14−36 (−37 to 34)<0.001
 FRC:TLC (%)53±952±857±8−5 (−6 to 4)<0.001
 FRC:TLC (% predicted)95±1592±14101±13−9 (−10 to 7)<0.001
 IC:TLC (%)47±848±843±75.4 (4.5 to 6.2)<0.001
 IC:TLC (% predicted)93±2096±1986±1910 (8 to 12)<0.001
Follow-up spirometry*
 Age at follow-up spirometry (years)65.4±10.464.7±10.366.7±10.4−2.0 (−3.0 to 1.0)<0.001
 Height at follow-up spirometry (cm)174.5±8.0174.9±8.1173.8±7.81.1 (0.3 to 1.9)0.007
 Time from index PFT to follow-up spirometry (years)4.1±3.13.8±2.94.6±3.3−0.8 (−1.1 to 0.5)<0.001
 FEV1 (L)2.69±0.652.77±0.652.53±0.620.25 (0.18 to 0.31)<0.001
 FEV1 (% predicted)79±1581±1575±146.5 (5.0 to 7.9)<0.001
 FVC (L)3.67±0.843.75±0.863.51±0.800.25 (0.16 to 0.33)<0.001
 FVC (% predicted)84±1586±1581±144.8 (3.3 to 6.3)<0.001
 FEV1:FVC (%)74±874±872±82.0 (1.2 to 2.8)<0.001
 FEV1:FVC (% predicted)95±1095±1093±102.5 (1.5 to 3.5)<0.001
 FEF25–75 (L)2.26±0.982.38±1.002.04±0.880.34 (0.24 to 0.43)<0.001
 FEF25–75 (% predicted)72±2975±2965±2510.0 (7.2 to 12.8)<0.001
  • P-values for significant comparisons are shown in bold.

  • Demographics and lung function in patients with preserved spirometry. Patients were stratified according to abnormal and normal RV:TLC by the upper limit of normal. Data are presented as mean±SD or number of patients with positive value for the variable (n) out of the total number of patients (N) and percentage of patients (%) and difference (95% CI) with p value for unadjusted comparison between normal and abnormal RV:TLC.

  • *Follow-up spirometry (last available postbronchodilator spirometry) was available for a subgroup of patients. Reference equations: Measures of pulmonary function and % predicted of normal values were calculated using Crapo predicted formulas (Crapo et al, 1981 and 1982), except for FRC:TLC, IC and IC:TLC. Quanjer predicted formulas were used for FRC:TLC and IC (Quanjer et al, 1993), and Francisco predicted formulas were used for IC:TLC (Francisco et al, 2009). Reversibility defined as ≥12% and ≥200 mL increase in FEV1 after bronchodilator administration.

  • BMI, body mass index; FEF25–75, maximum airflow at mid-lung volume; FRC, functional residual capacity; IC, inspiratory capacity; PFT, pulmonary function test; RV, residual volume; TLC, total lung capacity.