The metanephrine 24-hour urine test quantifies the levels of metanephrine, a metabolite of adrenaline and noradrenaline, excreted in the urine throughout a 24-hour period. Abnormal results may indicate an adrenal gland tumor or related conditions, prompting further investigation. The test also aids in diagnosing conditions like pheochromocytoma or paraganglioma, known for excessive adrenaline production.