Proteinuria
Proteinuria refers to the presence of protein in the urine. In healthy persons, the urine contains no protein or only a trace amount of protein. Proteins in the urine are comprised of albumin and globulin from the plasma. Because albumin is filtered more easily than globulin, it is the predominant type of protein that may appear in the urine. The term albuminuria may be used interchangeably with proteinuria.
Detection of protein in the urine, combined with a microscopic exam of urinary sediment, provides the basis for the differential diagnosis of renal disease. Normally, the glomeruli prevent passage of protein from the blood to the glomerular filtrate. A small amount of filtered plasma proteins and proteins secreted by the nephrons can be found in normal urine; however, in healthy individuals, protein excretion does not exceed 150 mg in 24 hours or 10 mg per 100 ml of any single specimen. Glomerular injury causes increased permeability to plasma proteins, resulting in proteinuria. A persistent finding of proteinuria is the single most important indication of renal disease. If more than a trace of protein is found on a routine UA, a quantitative 24-hour evaluation of protein excretion is done.
The dipstick method of checking for protein does not detect the presence of abnormal proteins such as globulins and Bence-Jones proteins excreted in multiple myeloma. Persons with diabetes have a higher than normal potential for renal disease, so they may have regular checks for proteinuria. If random samples are persistently positive for protein, a quantitative (24-hour) sample may be done.
Proteinuria is described based on the following scale:
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When proteinuria occurs, the primary protein detected is albumin.
Proteinuria occurs in both renal and nonrenal disease. Renal diseases that produce proteinuria include nephritis/glomerulonephritis, nephrosis, renal vein thrombosis, malignant hypertension, polycystic kidney disease, and chronic urinary tract obstruction. Proteinuria does not always accompany renal disease. Pyelonephritis, urinary tract obstructions, kidney stones (nephrolithiasis), tumors, and congenital malformations can cause severe illness without producing proteinuria. Nonrenal causes of proteinuria include: fever and acute infection, trauma, leukemia and multiple myeloma, toxemia and preeclampsia of pregnancy, diabetes mellitus, Waldenstrom's macroglobulinemia, systemic lupus erythematosis, and poisoning from heavy metals, phenol, opiates, or other drugs. Proteinuria is often associated with casts on the microscopic examination of urine sediment exam because protein is necessary for cast formation.
Protein losses of over 2000mg/24 hours in adults and over 40 mg/24 hours in children usually indicates a glomerular etiology. Preeclampsia (toxemia of pregnancy) causes massive loss of protein in the urine. In what is called the nephrotic syndrome, which may be the end result of many diseases that cause kidney dysfunction, the protein loss may exceed 4000 mg/day.
Postural proteinuria is a condition in which protein appears in the urine in otherwise healthy people who have been standing for a period of time. This type of proteinuria is intermittent and disappears when the person lies down. Postural or orthostatic proteinuria occurs in approximately 3 to 15% of healthy young adults. To diagnose this benign condition, two urine specimens are collected - one right after waking, taken while the person is lying down and the second about 2 hours after being upright. If the person has postural or orthostatic proteinuria, the first specimen is negative for protein and the second specimen is positive for protein.
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Some renal diseases do not cause proteinuria.