Interpreting results with GLI-2012 using Z-score and LLN
Predicted LLN and Z-Score
Results of spirometric tests characteristically lead to values for FVC and FEV1 which are too low rather than too high in disease.
This explains why the LLN is defined as that value which identifies the lower 5th percentile of a healthy population of non-smokers.
If a patient has an FEV1 MEASURED = LLN, it means that he is at the limit of the 5th percentile.
Only 5% of the healthy population, with his own anthropometric data, has a lower value.
Therefore 5% of the healthy population represents false positives, ie healthy subjects having values below the LLN and then, at first sight, could be considered sicks!
There are two methods of comparison MEASURED vs PREDICTED.
1) CLASSIC: %PREDICTED =
MEASURED / PREDICTED x 100
2) NEW: Z–SCORE =
(MEASURED – PREDICTED) / STANDARD DEVIATION
In the dogs example, for simplifying we assume that the PREDICTED value of the height is equal to the MEAN value.
How to use the Standard Deviation
The heights of the dogs (in mm) are: 600, 470, 170, 430 and 300
MEAN = 394 mm (green line)
STANDARD DEVIATION = 147 mm (blue line)
LLN = MEAN – 1.64 x STANDARD DEVIATION = 394 – 1.64 x 147 = 153
NOTE: the STANDARD DEVIATION is calculated by taking in account the differences between
the MEASURED VALUE and the MEAN VALUE for all dogs.