An system for monitoring alcohol in the breath of a test client confirms the identity of the test client based on spirometric data. The system includes a sample chamber receiving a breath sample, an alcohol sensor measuring the alcohol content of the breath sample, and a spirometric sensor generating spirometric data from the breath sample over the test clients entire exhalatory phase. A processor analyzes the spirometric data with stored client characterization data for a known client to confirm the identity of the test client. The client characterization data can be a probability density in a phase space in which at least two spirometric variables (e.g., flow and volume time-series data) are correlated.
