The Z-score: A new tool in the interpretation of spirometric data

Spirometry is an important tool in the diagnosis and management of patients with respiratory pathology. An appropriate interpretation of the spirometric data requires the use of a population-specific reference equation. However, the most widely used equations were established in European populations with limited age groups. The extrapolation of these equations, based on a specific population, and their uses for a different population led to measurement and interpretation biases. In 2012, an international working group conducted a multicenter study and published new reference equations called The Global Lung Initiative (GLI). These enabled the modeling of spirometric parameters from a very large sample collected in several ethnic groups using modern statistical techniques to establish continuous equations for all ages and in many countries. The GLI also recommends the use of a new statistical tool for the expression of results: The Z-score. This tool allows to express, in a simple way: how many standard deviations a subject is deviated from its reference value. The Z-score is calculated by the ratio of the difference between the measured value and that predicted with the residual standard deviation. This simple approach has reduced the false positive results found by the use of the conventional limits of 80% compared to a predicted value or 0.70 in absolute value for the definition of bronchial obstruction that remain still used.