Which of the following accurately predicts problems with cortisol levels blood test, 24 hr urine or midnight saliva swab?

Depends: Blood cortisol is useful in the diagnosis of cushings (hypercortisolemia) or adrenal insufficiency. A 24 hr urine cortisol is helpful to diagnose excess cortisol. Saliva cortisol is only helpful in diagnosing excess cortisol, not adrenal insufficiency. If a blood cortisol level is low, further evaluation is required to establish if your adrenal function is truly low.