What is the z score in spirometry?

Z-Score in Spirometry: Definition and Clinical Significance

The z-score in spirometry is a standardized measure that indicates how many standard deviations an individual's measured lung function value deviates from the predicted mean value for someone of similar age, height, sex, and ethnicity. 1

Definition and Calculation

The z-score is calculated using the following formula:

z-score = (observed value - predicted mean value) / RSD

Where RSD is the residual standard deviation of the reference population 1.

This statistical approach provides several advantages over traditional percent predicted values:

• Z-scores account for the normal distribution of lung function values in a healthy population
• They have a mean of 0 and a standard deviation of 1 in healthy individuals
• They provide probability information about how likely a result is to occur within a normal population
• They indicate how far a result deviates from the predicted mean

Clinical Application and Interpretation

Advantages Over Percent Predicted

The American Thoracic Society and European Respiratory Society recommend using z-scores rather than percent predicted values for several important reasons:

• Z-scores account for the interindividual variability of the normal distribution
• They provide more information than simply categorizing results as "normal" or "abnormal"
• They avoid the false positives that can occur with conventional fixed cutoffs (like 80% of predicted) 1, 2

Normal Range and Clinical Thresholds

• The lower limit of normal (LLN) is typically set at the 5th percentile, equivalent to a z-score of -1.645
• This means 95% of healthy individuals fall within the normal range 1
• Z-scores can be interpreted in probability terms when data are normally distributed

Practical Uses

Z-scores are particularly valuable for:

1. Tracking changes in lung function over time
2. Comparing various lung function parameters from different techniques
3. Evaluating lung function in children and adolescents where normal values change with growth 1
4. Providing more accurate assessment across different ages, heights, and ethnicities

Reference Equations

The Global Lung Function Initiative (GLI) 2012 provides multi-ethnic spirometric reference equations that allow calculation of z-scores across a wide age range (3-95 years) 3. These equations:

• Use modern statistical techniques to establish continuous equations for all ages
• Include data from multiple ethnic groups
• Allow for more accurate interpretation of spirometry results

When selecting reference equations, it's important to consider:

• The population from which the equations were derived
• Whether height or age is the main predictor (height is generally preferred)
• The applicability to the specific patient population being tested

Limitations and Considerations

While z-scores provide significant advantages, there are important considerations:

• Some studies suggest that z-score classification may underestimate airflow limitation severity in patients older than 60 years with severe functional impairment 4
• Reference equations may not be equally applicable across all populations, particularly in regions like Sub-Saharan Africa that are underrepresented in reference datasets 5
• Software for calculating z-scores should be provided with commercial equipment to ensure accurate interpretation 1

Clinical Implementation

When implementing z-scores in clinical practice:

• Use appropriate reference equations for your patient population
• Consider validating reference equations by testing healthy local subjects
• Report both z-scores and percent predicted values during transition periods
• Ensure that interpretation software is updated with the most current reference equations

By using z-scores, clinicians can more accurately assess lung function, track changes over time, and make more informed diagnostic and treatment decisions for patients with respiratory conditions.