24-Hour Urine Collection and Analysis

Excerpt

A 24-hour urine collection is a timed specimen used for evaluating urinary stone disease, proteinuria (eg, amyloidosis, lupus nephritis, nephrotic syndrome, and preeclampsia), multiple myeloma, pheochromocytoma (fractionated metanephrines and catecholamines), and renal function through urea and creatinine clearance. This test is usually conducted in an outpatient setting while the patient maintains their usual diet. Results are analyzed alongside detailed medical and dietary history, serum chemistry, and stone composition (if available) to assist in diagnosis and guide therapy or prophylactic treatment. Further, this urine study can be used in pediatrics to test for inherited conditions such as primary hyperoxaluria and cystinuria. Pheochromocytomas and paragangliomas, which are rare conditions causing catecholamine excess, are diagnosed in symptomatic individuals through elevated urine and plasma metanephrines. The collection procedure is similar to other conditions; however, the data must be interpreted cautiously due to the very low pretest probability.

Current laboratory values estimate the glomerular filtration rate (GFR) based on creatinine levels. However, these estimates can vary significantly from the true GFR if creatinine levels are unstable or for other reasons. Therefore, a 24-hour urine collection can be a valuable tool in evaluating abnormal renal function, but adherence to proper collection techniques is essential, as is educating the patient about the strict guidelines for a 24-hour collection. Shorter urine collections, such as 8- or 12-hour, can improve patient adherence and accuracy in specimen collection. However, 24-hour collections are generally preferred because they account for daily creatinine variations and solute excretion.