COVID-19 disease occurs primarily in the alveoli, where the virus attacks type II pneumocytes, which can lead to an abnormal alveolocapillary membrane and the so-called post-COVID syndrome. A diffusion assay is the primary measurement technique used to detect this pathophysiology. Patients suffering from post-COVID syndrome complain of symptoms such as shortness of breath and fatigue, but usually have a normal spirometry reading. In these cases, diffusion testing is the method of choice to confirm the diagnosis of post-COVID syndrome, which is psychologically important for patients and a significant measure for monitoring COVID-19 rehabilitation.