BACKGROUND: Forced midexpiratory time (FET25-75%), the time required to exhale from 25 to 75% of forced vital capacity (FVC), has been advocated as a relatively volume-independent measure of obstruction. Previous estimates of normal FET25-75% have not systematically studied the effect of age, height, sex, and smoking history. METHODS: We analyzed flow-volume loops from 369 normal lifetime nonsmokers and smokers, a random sample of the population of Michigan. Linear models including age and/or height were considered, with and without logarithmic transformation. RESULTS: The best models used age, or age and height, and no transformation. Age and height contributed much less to the variability of FET25-75% than to spirometric flows previously investigated. The largest r2 was found in the male smokers, with the largest contribution from age, suggesting a smoking effect. CONCLUSIONS: As age was the major contributor to the value of FET25-75% and that contribution was small, we suggest the use of simplified age-stratified reference values derived from our data.
BACKGROUND: Forced midexpiratory time (FET25-75%), the time required to exhale from 25 to 75% of forced vital capacity (FVC), has been advocated as a relatively volume-independent measure of obstruction. Previous estimates of normal FET25-75% have not systematically studied the effect of age, height, sex, and smoking history. METHODS: We analyzed flow-volume loops from 369 normal lifetime nonsmokers and smokers, a random sample of the population of Michigan. Linear models including age and/or height were considered, with and without logarithmic transformation. RESULTS: The best models used age, or age and height, and no transformation. Age and height contributed much less to the variability of FET25-75% than to spirometric flows previously investigated. The largest r2 was found in the male smokers, with the largest contribution from age, suggesting a smoking effect. CONCLUSIONS: As age was the major contributor to the value of FET25-75% and that contribution was small, we suggest the use of simplified age-stratified reference values derived from our data.