I split my time between living at 5,280 feet and 7,100 feet. The higher elevation is where I do most of my longer weekend rides. 5,280 is where I tend to do shorter, structured rides on weekdays, often on the trainer.
Given all that, where should I do my threshold tests? I’m assuming 2,000 feet is enough to affect the results.
2,000ft would effect your power at threshold. The amount really depends on how acclimated you are. But even acclimated it has an effect. However most likely we are talking about 1 - 3%. So when looking to ride in a certain zone remember when at 7,100ft you will most likely be at the middle to lower end of your power target at the same heart rate and perceived exertion as say 5200ft.
I would just recommend doing your power test at 5200ft as that is where you do most of your intervals. When and if you feel you need to retest I would make sure you test at the same elevation. You need to keep things as consistent in terms of testing as possible. You could also test twice in the same week or two to see how much of a difference it is for you personally. I wouldn’t do that every time, maybe just once, but you could see the effects it has on you and adjust your zones accordingly.