A precise quantitative test that measures the total amount of protein excreted in the urine over a 24-hour period. This test is considered the gold standard for assessing proteinuria. It helps in the diagnosis and monitoring of the severity of kidney diseases affecting the glomeruli (the kidney's tiny filters), such as nephrotic syndrome, diabetic nephropathy, and autoimmune kidney diseases. It provides an accurate measure of daily protein loss that cannot be obtained with the same precision from random urine samples.