Inform your doctor if you are on any medications, have any allergies or underlying medical conditions before your Metanephrines Fractionated 24 Hr Urine Lc-Ms/Ms 24 Hour Urine. Your doctor will give specific instructions depending on your condition on how to prepare for Metanephrines Fractionated 24 Hr Urine Lc-Ms/Ms 24 Hour Urine.