Standardisation of lung function testing: the authors' replies to readers' comments.

Rationale The end of test (EOT) criteria are applied in order to ensure that efforts are made to achieve the best estimate of FVC. When a subject cannot meet the plateau criterion (,25 mL exhaled in the previous second of the blow) this may be for reasons other than premature volitional cessation of the blow. For example, in some young subjects or patients with a rigid chest wall, it is chest wall limitation that suddenly causes exhalation to stop [2] and it is difficult for them to achieve a volume–time plateau of .1 s. Under these circumstances, the FVC values returned will be repeatable, whereas if the cessation of flow is volitional, then the repeatability is usually poor. The length of time necessary to achieve a plateau is dependent on many factors, including, but not limited to, the subject’s age, body size and presence of lung disease. In general, children and adolescents reach their plateau more quickly than adults, as do adults with small body frames (e.g. Asian females). Patients with restrictive disease can also reach their EOT more quickly than patients with airflow obstruction. The 3 s expiratory time attributed to children was given as a guide to the fact that longer expiratory times, o6 s, can be difficult to achieve in younger subjects.