Interpreting results with GLI-2012 using Z-score and LLN
Interpretation of Spirometric results
When interpreting test results there is uncertainty about the “abnormality” of a finding near the LLN than if the test result is within the normal range or in a pathological range.
Measured values of FEV1, FVC, FEV1/FVC with additional indices, such as PRE and POST bronchodilator, predicted values, LLN, % of predicted, confuses most users.
Conversely, PICTOGRAMS in which Z–SCORE are depicted relative to a normal range allow a simple interpretation of the spirometric results.
When Z-SCORE is -1.64, it means that the parameter of the spirometry is placed exactly on the LLN that represents the point below which only 5% (5th percentile) of the normal population had a value lower than predicted.
In the center of PICTOGRAM the Z-SCORE is equal to 0, it indicates that the measured value is exactly equal to the predicted one.
In this point we find 50% of the healthy population (the center of the bell).
In conclusion, 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. In short, it is still healthy but he is very close to…
…the population considered sick!