Dynamic lung volume tests such as spirometry are limited to measurement of volumes of gas which may be inspired or expired from the lungs. However, a residual volume (RV) of gas remains within the lungs, even at full expiration. Measurement of this volume provides additional information to supplement the spirometric values. Lung volumes which cannot be measured by spirometry alone are termed static lung volumes. Measurement of static lung volumes may be helpful to evaluate the cause of a reduced forced vital capacity (FVC), particularly if there is suspicion of a mixed restrictive/obstructive defect. Measurement of functional residual capacity (FRC) also allows calculation of lung volume subdivisions and derived parameters. A variety of methods exist for measurement of static lung volumes, each of which requires differing levels of comprehension, cooperation, and compliance to produce accurate and meaningful results. Helium dilution is the easiest method to perform, but is not the cheapest due to the relative expense of helium gas.