ABSTRACT
The percentage predicted has long been the favoured method of expressing lung function results amongst clinicians. It has the advantages of being easy to calculate and intuitive to understand. The percentage predicted is also used to grade severity of disease by comparing test results with a table of cut-off ranges. Different lung function tests and indices have different degrees of natural variation within the population. This shortcoming has led clinical physiologists to favour the concept of the standard residual as a statistically more valid approach to identifying normal ranges. This method involves using standard deviations to identify the upper and lower limits of normality. The percentage of predicted is the most commonly used expression of normality, which is simple to calculate and intuitively understood. However, the cut-off for normality may result in under- or overdiagnosis of pathology. The use of standard residuals provides a statistically valid method to identify values that fall below the limits of normal physiological variability.
