Functional residual capacity is best measured by?

Pulmonary plethysmography is commonly used to measure FRC and TLC The difference between full and empty lungs can be used to assess diseases and airway passage restrictions. An Obstructive disease will show increased FRC because some airways do not empty normally, while a Restrictive disease will show decreased FRC. Body plethysmography is paicularly appropriate for patients who have air spaces which do not communicate with the bronchial tree; in such patients, helium dilution would give an incorrectly low reading. Although Helium dillution and Nitrogen washout methods are also used to measure TLC but overall best is pulmonary plethysmography.