Training & Nutrition - FTP Testing on a Hill

Is it inaccurate to gauge FTP from a hill? I ask because one of the best places to test FTP, little traffic, no stoplights and consistent terrain, is a local mountain climb. Before it gets asked, this climb does force me to be at a lower cadence then normal, I usually spin at 90-100 and this climb forces be down to 70-80. Does this throw off what my estimated FTP would be, assuming a 20min interval?

Thanks!

Is it inaccurate to gauge FTP from a hill? I ask because one of the best places to test FTP, little traffic, no stoplights and consistent terrain, is a local mountain climb. Before it gets asked, this climb does force me to be at a lower cadence then normal, I usually spin at 90-100 and this climb forces be down to 70-80. Does this throw off what my estimated FTP would be, assuming a 20min interval?

Thanks!I don't train with power, but from long practice of watching vertical feet/hour vs. HR and cadence, I find my fastest ascent for a 20 minute interval to center around a 78 cadence and 92%-95% MHR, so that would be FTP I suppose. For longer climbs, I do better at a higher cadence/lower cog. My cadence on the flat hovers around 90, TT cadence nearer 100.

The answer is no. Quoting Andy Coggan, "Alls you can do is alls you can do." Doesn't matter if it's uphill, downhill, into the wind, or whatever. If you can hold a certain power output, that is your FTP.

Just note, however, that it may be difficult to replicate on the flat ground. Difficult, but not physiologically impossible.

I do all my ftp stuff indoors because there is no 20 min stretch of uniform road here.