Z-score for spirometries

Z-score is a statistical measure that indicates how a single spirometry result compares to the average (predicted) value for a population, based on factors such as age, sex, height, and ethnicity. It is calculated using reference equations, like the Global Lung Initiative (GLI) 2012/Quanjer equations, which provide predicted values and the range of normal variability for lung function parameters.

Z-scores provide a standardized way to assess lung function by accounting for individual variability, making them particularly useful for diagnosing and monitoring conditions like asthma and COPD.

• Mean (M): The predicted value for a given individual based on reference equations.
• Lower Limit of Normal (LLN): The value below which lung function is considered below normal. This is typically set at -1.645 Z-scores (5th percentile).
• Upper Limit of Normal (ULN): The value above which lung function may be considered abnormally high. This is typically set at +1.645 Z-scores (95th percentile).
• Actual Result: The individual’s measured spirometry value expressed as a Z-score.

• A Z-score of 0 indicates the result matches the predicted value for the population.

• A Z-score below -1.645 indicates the result is below the lower limit of normal (LLN), suggesting potential lung impairment.

• A Z-score above +1.645 is above the upper limit of normal (ULN), though this is less commonly used for diagnostic purposes.

• Values between -1.645 and +1.645 are considered within the normal range.

Our spirometry tool includes a built-in Z-score graph for easy interpretation. Here’s how to access and use the feature:

Steps to View Z-Scores

1. Select the Reference Values:
   • Ensure the GLI/Quanjer 2012 reference values are selected in the top-right dropdown menu of the Spirometry page. Z-scores are not available if Hedenström reference values are selected.
2. Choose Spirometry Curves:
   • On the “Individual sessions” tab, you can select up to three spirometry tests to compare. Each test you select will display a curve on the main graph.
3. Access the Z-Score Graph:
   • For each selected spirometry test, the Z-score graph appears in the patient details section. This graph visualizes:
     • The Mean (M) value (predicted).
     • The Lower Limit of Normal (LLN) and Upper Limit of Normal (ULN) lines.
     • A circle representing the Actual Result.
4. Hover for Details:
   • Hover over any element on the Z-score graph to see detailed information, including the parameter name (e.g., FVC, FEV1, FEV1/FVC) and its corresponding Z-score.

Let’s say you’ve selected a spirometry test, and the Z-score graph for FEV1 shows:

• The circle (actual result) is at -2.0.
• The LLN is at -1.645.

This indicates that the patient’s FEV1 is below the normal range and suggests a possible airflow limitation or restrictive lung disease. Further evaluation may be required to confirm a diagnosis.