Spirometry variables below the lower limit of normal at any time
| Cohort | 82–85 cohort | 91–92 cohort | 99–00 cohort | |||||||
| Z-FEV1 below LLN at any time, n (%), p value | Term-born (ref.) | 3 | (7) | – | 3 | (9) | – | 6 | (11) | – |
| EP no BPD | 10 | (28) | 0.01 | 4 | (17) | 0.31 | 7 | (23) | 0.13 | |
| EP BPD | 7 | (54) | <0.001 | 6 | (50) | 0.002 | 8 | (24) | 0.08 | |
| Z-FVC below LLN at any time, n (%), p value | Term-born (ref.) | 3 | (7) | – | 0 | (0) | – | 4 | (7) | – |
| EP no BPD | 8 | (22) | 0.04 | 2 | (9) | 0.08 | 1 | (3) | 0.46 | |
| EP BPD | 2 | (15) | 0.31 | 1 | (8) | 0.08 | 4 | (12) | 0.41 | |
| Z-FEV1/FVC below LLN at any time, n (%), p value | Term-born (ref.) | 1 | (2) | – | 5 | (14) | – | 1 | (2) | – |
| EP no BPD | 8 | (22) | 0.004 | 3 | (13) | 0.89 | 9 | (29) | <0.001 | |
| EP BPD | 5 | (38) | <0.001 | 9 | (75) | <0.001 | 10 | (30) | <0.001 | |
| Z-FEF25-75% below LLN at any time, n (%), p value | Term-born (ref.) | 2 | (4) | – | 6 | (17) | – | 6 | (11) | – |
| EP no BPD | 11 | (31) | 0.001 | 3 | (13) | 0.67 | 9 | (29) | 0.03 | |
| EP BPD | 7 | (54) | <0.001 | 9 | (75) | <0.001 | 12 | (36) | 0.003 | |
| Any variable below LLN at any time, n (%), p value | Term-born (ref.) | 5 | (11) | – | 7 | (20) | – | 8 | (14) | – |
| EP no BPD | 17 | (47) | <0.001 | 6 | (26) | 0.59 | 12 | (39) | 0.008 | |
| EP BPD | 9 | (69) | <0.001 | 10 | (83) | <0.001 | 15 | (45) | <0.001 | |
The presented data are the number (fraction) of subjects with at least one value ≤LLN at least one visit.
P value refers to comparisons between term-born and EP-born groups using Pearson’s χ2 test.
EP, subjects born extremely preterm or with extremely low birth weight (gestational age ≤28 weeks or birth weight ≤1000 g); FEF25–75%, forced expiratory flow at 25–75% of FVC; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; LLN, Lower limit of normal (z-score ≤ −1.645); ref., reference level.