Interpreting results with GLI-2012 using Z-score and LLN

Let’s try to calculate %Predicted and Z-Score

The heights of the dogs (in mm) are: 600, 470, 170, 430 and 300

CLASSIC METHOD: %PREDICTED = MEASURED / PREDICTED x 100
Rottweiler 600/394 x 100 = 152% taller then PREDICTED
Dachshund 170/394 x 100 = 43% shorter then PREDICTED
Schnauzer 430/394 x 100 = 109% very close to PREDICTED
It is a very intuitive method and indicates what is the percentage value of the MEASURED VALUE in relation to the PREDICTED VALUE

NEW METHOD: Z–SCORE = (MEASURED – PREDICTED) / STANDARD DEVIATION
Rottweiler (600-394)/147 = +1.40= more then 1 SD taller
Dachshund (170-394)/147 = -1.52= more then 1 SD shorter
Schnauzer (430-394)/147 = +0.24= less then 1 SD taller
This method is less intuitive and indicates how often a measurement is greater (+) or less (-) of the STANDARD DEVIATION.

The benefits of Z-Score

The LLN value can also be expressed as a percentage of the PREDICTED:
1) In a healthy person, 20 years old: LLN of FEV1 = 80% of his PREDICTED
2) In a healthy person, 80 years old: LLN of FEV1 = 66% of his PREDICTED

It is clear that the true LLN, when expressed as %PREDICTED, varies considerably with the age. FEV1 value of 66% of the PREDICTED may be fine in a 80 years old person but it would be abnormal for a 20 years old person!

Therefore evaluate a respiratory disease according to the calculation method% PREDICTED is a very popular but should be replaced by the method of the Z-SCORE.

The benefit is that Z–SCORE is completely independent of age, height and sex.

For example, if the Z–SCORE for any parameter is –1.64, this signifies in males, females, children and adults that the measured value is at the 5th percentile.

In lung function testing this is regarded as the LLN.