Do nitrites indicate kidney infection or bladder infection?

Could be either: Infection in the urinary tract often begins with a bladder infection; this may extend further "upstream" to involve the kidneys (but the kidneys could also be a primary source of infection). Either way, nitrates on the dipstick urinalysis usually is how's up as a "byproduct" of bacteria commonly infecting the urinary tract. A kidney infection is usually more symptomatic and more serious.