Apparatuses are described to accurately determine a gas concentration of a sample of a patient s breath. The apparatuses may include a sample compartment, a breath speed analyzer, a gas analyzer, and a processor. The sample compartment includes an inlet that receives the breath. The breath speed analyzer determines the speed of a portion of the breath. The gas analyzer determines a gas concentration. The processor includes an algorithm that determines a degree of non-homogeneity of the sample based on the speed, and a corrected gas concentration based on the degree of non-homogeneity. In some variations, the gas correction is determined independently of patient cooperation. Apparatuses may be tuned based on the intended population s expected breathing pattern ranges such that the sample compartment is filled with a homogenous end-tidal gas sample regardless of an individuals breathing pattern. These apparatuses are useful, for example, for end-tidal CO analysis. Methods are also described.
