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 a Spirometry or Bronchodilator test 
   • Select a session from the list on the Spirometry page
2. Select the Reference Values:
   • Ensure the GLI/Quanjer 2012 reference values are selected for the test. Z-scores are not available if Hedenström or Solymar reference values are selected.
3. Scroll down to session details:
   • Expand the session details by pressing “Show all values”
4. View the Z-Score Graph:
   • For each trial, the Z-score graph appears in the table. The graph visualizes:
     • The Mean (M) value (predicted).
     • The Lower Limit of Normal (LLN) and Upper Limit of Normal (ULN) lines.
     • A dot representing the Actual Result.
5. Hover for Details:
   • Hover over any element on the Z-score graph to see detailed information and the corresponding Z-score.