Compensating for changes in the threshold voltage of the drive transistor of an OLED drive circuit, the drive transistor includes a first electrode, second electrode, and gate electrode; connecting a first voltage source to the first electrode, and an OLED device to the second electrode and to a second voltage source; providing a test voltage to the gate electrode and connecting to the OLED drive circuit, a test circuit, that includes an adjustable current mirror causing voltage applied to the current mirror, to be at a first test level; providing a test voltage to the gate electrode of the drive transistor and connecting the test circuit to the OLED device producing a second test level after the drive transistor and the OLED device age; and using the first and second test levels to calculate changes in the voltage applied to the gate electrode of the drive transistor to compensate for drive transistor aging.