What is Abnormal?    Page 3

   When someone of their age, sex, height and ethnicity (all factors known to influence their lung function results).

   The from their predicted . This result is called the "Standardised Residual" (SR) or the z-score and is given by:

Plot of FEV1 data showing definition of a residual

                SR = (Recorded - Predicted) / RSD

   RSD is the residual standard deviation from the prediction equation used. The term "residual" refers to the difference between a given subject's result and their predicted value. In the figure to the right the subject's data value A is 2.1 L and the subject has a predicted value B of 3.8L and so the residual is 2.1 - 3.8 = -1.7L. This residual is then standardised by the spread (standard deviation) of all the residuals found in the population used for the prediction. For these data the RSD = 0.51L so the SR value is -1.7/0.51 = -3.3

   Calculating the predicted value is usually done using a prediction equation taken from a large population of comparable subjects who have never smoked. The GLI-2012 equations are derived from over 70,000 subjects and give the most reliable predictions for European subjects based on their sex, age, height and ethnicity [1]. The GLI-2012 equations return the z-scores (SR values) given the subject's sex, age, height and ethnicity.

1. Quanjer PH, Stanojevic S, Cole TJ, Baur X, Hall GL, Culver BH, et al. Multi-ethnic reference values for spirometry for the 3-95-yr age range: the global lung function 2012 equations. Eur Resp J 2012; 40: 1324-43.