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.