A key component to spirometry is the conversion of gas properties from ATPS to BTPS.

The air within the lungs after-all is influenced by its surroundings, which makes the temperature 37 degrees Celsius and 100% saturated with water.

The ambient air temperature is usually much cooler and dryer, thus exhaled air contracts as it leaves the lungs into a spirometer. For spirometers which do not calculate the BTPS value for expired air for you, you may either measure the the value according to the BTPS axis on the graph paper or calculate the result manually.

ATPS: Ambient Temperature and Pressure Saturated with water vapour

BTPS: Body Temperature and Pressure Saturated with water vapour

The volume of air recorded by most spirometers is usually 6-10% less than the true volume. The BTPS correction adjusts the measured result to the actual volume contained within the lungs. In volume measuring devices, the measurement is usually as ATPS therefore a correction to BTPS is required, either by manual calculation using this table:

Many digital flow devices make this conversion during the measurement therefore no further action is required. Evidently, it’s important to note whether this calculation is performed by the device or not.

The limitation to this conversion, however, is the temperature of the ambient air. The BTPS conversion assumes that the patients’ exhaled breath cools immediately. However, evidence suggests this is not the case, particularly in cooler environments.

In these cases, the BTPS conversion factor will overestimate FEV1 and peak expiratory flow (PEF). Consider an inverse relationship between this error and air temperature; so the cooler the air the more likely FEV1 and PEF is overestimated. As FVC involves a longer expiration, this has been shown to reach equilibrium and so this temperature effect does not influence the values measured.