These values are based on certain parameters, calculations and/or algorithms from a specific test protocol or maximal effort. So to get a comparison, it’s best to use the same method to test, and then look at the trend over time. Knowing what impacts doing test/effort, as well as general health at the time can also affect the result, just like a metabolic cart test would also be dependent on how you perform on the day.
Some links on Intervals where different testing methods have been discussed:
Doing a ramp test, you can estimate using this site.