The<unk>  is the way of measuring the functional residual capacity of the lungs (the volume left in the lungs after normal expiration). This technique is a closed-circuit system where a spirometer is filled with a mixture of<unk>  (He) and oxygen. The amount of He in the spirometer is known at the beginning of the test (concentration × volume = amount). The patient is then asked to breathe (normal breaths) in the mixture starting from FRC (functional residual capacity), which is the gas volume in the lung after a normal breath out. The spirometer measures<unk>  concentration. The<unk>  spreads into the lungs of the patient, and settles at a new concentration (C2). Because there is no leak of substances in the system, the amount of<unk>  remains constant during the test, and the FRC is calculated by using the following equation: V2 = total gas volume (FRC + volume of spirometer) V1 = volume of gas in spirometer C1 = initial (known)<unk>  concentration C2 = final<unk>  concentration (measured by the spirometer)
Helium dilution technique
Cloned enzyme donor immunoassay
Bolton analysis
Sereny test
Enzyme multiplied immunoassay technique
Hydrostatic weighing
Radioimmunoassay
Nephelometry 