A method of avoiding a contaminant which would skew an analyte result in a breath analysis method and of calibrating subject of the breath analysis includes, immediately before the breath analysis method or the collection of breath for the breath analysis method, administering to the subject a predetermined gas composition. A system for analyzing an analyte in breath of a subject while avoiding a local contaminant which would skew an analyte result and calibrating the subject of so that the result of the analyte analysis will be the same regardless of where the test is performed geographically, includes a source of a predetermined gas composition immediately before the breath analysis method or the collection of gas for the breath analysis method, administering to the subject a predetermined gas mixture.
