Standardising predicted values over many spirometers

We have a number of spirometers in our unit, using different measurement techniques: the standard bellows (Vitalograph), rotary device (Micromedical 3000)& pneumotachograph (Vitalograph Alpha) Distressingly, each appears to give different results and also have different predicted values. We've just had then all serviced (at huge expense. Makes plumbing call-outs appear cheap!) Is this an inevitable consequence of having different types of spirometer? Is there a means to standardise results obtained on different machines?

Oasys, Specialist, 2/20/2006, 2/20/2006,

This is an answer from a clinician, I hope somebody more technical will also reply. The predicted value are the easiest bit to answer. Predicted values for adults are based on age, sex, height and race. There are many different studies producing estimates, but at least in Europe there is general agreement that we should be using ECCS equations, which are an amagamation of many individual studies. You can set most electronic spirometers to use various predicted equations, or have the spirometer loaded with the predicted equations of your choice. There is less agreement about corrections for race (African origin races have smaller volumes related to standing height than white Europeans by about 15%, but probably no difference in peak expiratory flow). All measuring methods have their limitations; bellows spirometers often have a relatively high resistance, but checking for leaks and non-linearity is quite easy; vane spirometers are inherently non-linear and require sophisticated software to linearise the output, there is usually an acceleration of the vane at the beginning and some momentum at the end, end of test criteria may be a problem; pneumothachographs are less stable than the rest and require frequent calibration but are potentially more sensitive to small changes in flow. The reason for calibration checking is to make sure that any differences between spirometers, and with the same spirometer over time, are within guidelines. Measurement of peak expiratory flow is much more problematical, as there is no satisfactory definition of PEF (some pneumotachographs can detect very high flows which are exceedingly transitory). For an individual person PEF is best measured on the same device on every occasion as changes will then be due to the person rather than the measuring device.

2/20/2006