Glossary: Lactate threshold vs FTP

Definitions

Lactate threshold (LT): the exercise intensity at which blood lactate accumulation begins to exceed clearance. Measured in a lab via blood draws across a ramped test.
Functional Threshold Power (FTP): the highest average power sustainable for approximately one hour. Field-tested, not lab-tested.

Relationship

For most athletes, FTP and the power at LT are very close (within 5 to 10 W of each other), but they are not the same thing:

LT is a physiological event (lactate kinetics).
FTP is a performance ceiling (sustained power).

A well-trained athlete can hold a small range above LT for short periods, so FTP can sit just above LT, not exactly at it.

Which one Grepiac uses

Grepiac anchors training zones to:

CP (Critical Power) for the dashboard bike zones (see Glossary: zones).
FTP for activity time-in-zone calculations.
Run threshold pace for run zones (analogous to FTP).
CSS for swim zones (the swim equivalent).

LT itself is not a stored Grepiac metric type. If you have a lab LT value, log it as FTP if it matches your sustained hour power, or as a CoachNote with the lab context.

How to test each

FTP test: 20 minutes all-out, take 95 percent. Or a ramp test. Or the CP test (3 + 12).
LT test: lab visit with blood draws every 3 to 4 minutes during a graded ride.

Most athletes never have a lab LT measured. The FTP / 20-min test is the practical substitute and is close enough for training prescription.

When the distinction matters

You have both LT and FTP and they disagree by more than 15 W. Then the LT data point is useful for placing zone boundaries; FTP alone may not be enough.
You are doing structured lactate-test-based training. Most age-groupers do not, and this is fine.