How many questions on the questionnaire for depression must be endorsed as 2 or 3 to require administration of the phq-8?

Phq8: usually people use the phq 9 the phq8 is the same questionnaire without the question about suicidality the measure is scored as a total score. more than 15 usually means depression sever enough to require treatment

PHQ-8 and PHQ-9: I administer the PHQ-9 (previously the PHQ-8) to all new patients and then intermittently thereafter. I do not base it' s need for administration on other screening tests.

That depends: I'm not sure which questionnaire you're referring to here (The MDQ is developed as a bipolar disorder screening tool), but the most common way to universally look (screen) for depression in adults is to begin with the PHQ-2, and if either answer is positive, it would then be routine practice to administer a PHQ-9 to consider a diagnosis of depression.