Longitudinal versus cross-sectional data
There is sufficient evidence (ref. 1) that age-related trends in cross-sectional studies of lung function differ from those found in longitudinal investigations. There are various explanations for this.
- A selection effect occurs, i.e. the weakest individuals die at an early age, so that increasingly the older subjects in the population represent a positive selection (e.g. with the most favorable lung function).
- A cohort effect may be involved, i.e. that persons born for example 50 years ago had a different lung development during growth than those born 20 years ago. Infectious diseases, nutrition, smoking during gestation, better survival after premature birth, exposure to environmental factors, etc. might all leave their marks so that the lung function of a 40 yr old person born in 1920 would be different from that in a 40 yr old person born in 1950.
- In a cross-sectional study measurements may have been influenced by seasonal and other climatological factors, whereas in longitudinal studies carried out over a sufficient time span such chance events would tend to average out.
- Technical problems cannot be excluded. Unfortunately the equipment used may still play a role: it is far from easy to properly take into account the effects of gas temperature, humidity and gas composition in calibrating equipment, and even more difficult to do this reproducibly.
Implicit in most studies is that the age-related decline in lung function in adults is attributable to the effect of aging on the lung. However, there is some evidence that it may also be related to changes in body mass; in follow-up studies in males (ref. 2) a 1 kg weight gain has been found to account for a reduction in FEV1 of between 13.9 and 23 mL.
| Ref. 1 - Cross-sectional versus longitudinal data | |
| 1 | Glindmeyer HW, Diem JE, Jones RN, Weill H. Non-comparability of longitudinally and cross-sectionally determined annual change in spirometry. Am Rev Respir Dis 1982; 125: 544-548. |
| 2 | Burrows B, Lebowitz MD, Casmilli AE, Knudson RJ. Longitudinal changes in forced expiratory volume in one second in adults. Methodologic considerations and findings in healthy nonsmokers. Am Rev Respir Dis 1986; 133: 974-980. |
| 3 | Ware JH, Dockery DW, Louis TA, Xu XP, Ferris BG Jr, Speizer FE. Longitudinal and cross-sectional estimates of pulmonary function decline in never-smoking adults. Am J Epidemiol 1990; 132: 685-700. |
| 4 | Van Pelt W, Borsboom GJJM, Rijcken B, Schouten JP, van Zomeren BC, Quanjer PhH. Discrepancies between longitudinal and cross-sectional change in ventilatory function in 12 years of follow-up. Am J Respir Crit Care Med 1994; 149: 1218-1226. |
| 5 | Xu X, Laird N, Dockery DW, Schouten JP, Rijken B, Weiss ST. Age, period, and cohort effects on pulmonary function in a 24-year longitudinal study. Am J Epidemiol 1995; 141: 554-566. |
| 6 | Kerstjens HAM, Rijcken B, Schouten JP, Postma DS. Decline of FEV1 by age and smoking status: facts, figures and fallacies. Thorax 1997; 52: 820-827. (Review article) |
| Ref. 2 - Change in body mass and in FEV1 | |
| 1 | Cotes JE, Gilson JC. Effect of inactivity, weight gain and antitubercular chemotherapy upon lung function in working coal-miners. Ann Occup Hyg 1967; 10: 327-335. |
| 2 | Bande J, Clément J, van de Woestijne KP. The influence of smoking habits and body weight on vital capacity and FEV1 in male airforce personnel: a longitudinal and cross-sectional analysis. Am Rev Respir Dis 1980; 122: 781-790. |
| 3 | Chen Y, Horne SL, Dosman JA. Body weight and weight gain related to pulmonary function decline in adults: a six year follow up study. Thorax 1993; 48: 375-380. |
| 4 | Chinn DJ, Cotes JE, Reed JW. Longitudinal effects of change in body mass on measurements of ventilatory capacity. Thorax 1996; 51: 699-704. |