Why is spirometry the measurement of choice in COPD

The FEV1 is accurate and reproducible. The variance of repeated measurements in the same individual is low and normally less than 200 mL.

There are well-defined tables of normal values for FEV1 based on age, sex, height and ethnic origin.

FEVj is the best predictor of prognosis in COPD. Spirometry is relatively quick and easy to measure and is applicable for children over 6 years to old age. It is appropriate for all levels of severity of COPD.

The FVC, being more effort-dependent, is slightly less reproducible and more susceptible to errors such as poor effort and stopping blowing too early. In addition, some flow-dependent spirometers may underestimate FVC at the low flow rates found in severe COPD. If the FVC is underestimated, the FEVj/FVC ratio may give a falsely high result.

Peak expiratory flow (PEF) is unreliable for airflow obstruction in COPD, often seriously underestimating the degree of airflow present [2]. This is related to COPD being a disease of smaller airways that is identified by FEVj but not by PEF, which measures flow mainly in the larger airways.

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