Abstract: Asthma creates inflation of the epithelial cells in the airway of a patient. Inflammation causes epithelial cells to increase the production of nitric oxide far above the normally low levels. Therefore, clinicians can detect biomarkers of Asthma and other maladies by measuring the concentration of exhaled nitric oxide (“eNO”) for fractional exhaled nitric oxide (“FeNO”). However, current systems require the patient to exhale at a constant rate to estimate the concentration eNO. This rough approximation may under or overestimate the FeNO, which can cause misdiagnosis. Accordingly, disclosed are systems and methods to determine the amount of nitric oxide exhaled that compensate for a variable flow rate of exhaling, and do not assume a constant flow rate.