Reference Values for Residual Volume, Functional Residual Capacity and Total Lung Capacity

Reference values play an important role in establishing whether, in an individual, measured volumes fall within a range to be expected for a healthy person of the same sex, similar stature, age, and other characteristics. Hence, reference values should be chosen that are appropriate to the person being investigated. Ideally, the reference values should be consistent; in the context of this paper, it implies that predictions for the various lung volumes have been derived from the same reference population, with the same techniques. Comparing measured with reference values is fraught with difficulties, as it may lead to disease going undetected. This is because the scatter in predicted values is sufficiently large to allow a sick person to lose, e.g. much of his total lung capacity (TLC), whilst still remaining in the range observed in comparable healthy persons. In addition, measurements in an "abnormal range" are not diagnostic of a specific disease, as many diagnostic entities may cause static lung volumes to be abnormal. Hence, in the follow-up of a patient, reliance should be placed much more on the effects of drugs or challenges with pharmacological, chemical or physical stimuli or allergens, and on the rate of change of lung volumes, than on the relationship of an isolated result to published "normative data". In fact, the best reference value is the value observed in a period when the disease is stable and minimal for that person. In studies such as the effects of drugs in patients with lung disease, or of adverse effects of environmental factors on the lungs, observed values should not be compared with reference values; instead, a comparison should be made with values obtained in an appropriately chosen control group.