What about Accuracy & Calibration?

VacuMed Vista VO2 Measurement Systems

What do we know about accuracy? Actually, a lot, as you will find out after reading this!

A. Deceptions of the Douglas Bag Validation Method
(The old gold standard)

C. The Metabolic Calibrator
The New Gold Standard!

Accuracy, Calibration, Validation: Our CPX system is calibrated essentially the way all others are, but there is one advantage: Our flow sensor is permanently calibrated, so only the gas analyzers need to be checked every day.

We do have an automatic software procedure that simplifies the gas analyzer calibration. As is common practice, we use air as one calibration gas, and a tank containing 4% CO₂, 16% O₂ as a second calibration gas.

After this calibration procedure your CPX system, of whatever make, is presumed to be accurate.

How accurate?

Some manufacturers publish an accuracy specification, others don't. Some publish only the gas analyzer specifications, a number such as 0.1% accuracy (or perhaps better, linearity) may easily be confused with the accuracy of the VO₂ measurement. Not so, but we will address this elsewhere.

So how do you know the accuracy of your VO₂ and VCO₂ measurement?

Well here comes validation. A popular method is comparing the so-called unknown CPX system with a so-called "well-known" system. The assumption here is that if 500 people bought this well-known system, it must be accurate. Really?

The other validation method is the "Douglas Bag" method. More problems, in the next chapter we will discuss the many errors inherent in this method.

We maintain that the only way to know the accuracy of any metabolic cart (CPX system) is to present to a system under test a known metabolic gas mixture, as described in "Metabolic Calibration".