Protein is in our food and is also made by the liver. It circulates in the blood and its concentration can be measured by a blood test. The kidneys make urine by filtering the blood. Normally, urine consists of only waste chemicals dissolved in water. The kidneys do not normally allow protein to “leak” through its filtering system from the blood into the urine. Thus, the finding of protein in the urine by the dipstick (a screening tool) test CAN mean that there is kidney disease.
Sometimes, the dipstick can register a small amount of protein in the urine because the urine is concentrated. To evaluate this possibility, we often recommend that the urine be tested in a laboratory. “Physiologic” proteinuria that is due only to concentrated urine has no health consequence.
Normal healthy teenagers can have proteinuria that is related to posture and activity. That is, protein appears in the urine during the upright posture and normal activity, but is NOT present in the urine formed by the kidneys when the individual is lying down (during sleep). This is the purpose of the timed overnight urine collection. If there is no protein in this urine specimen, but protein is present in the urine at other times, the diagnosis is postural (or orthostatic) proteinuria. This is a benign finding in many normal healthy teenagers. The cause is unknown. However, we do know that it generally disappears with age and does not cause any harm. No additional testing is required and no treatment is necessary.
If proteinuria is present (by the dipstick) in the timed overnight urine specimen, and NOT related to body posture or position, additional diagnostic testing is required to determine its cause. This may include laboratory measurement of the exact amount of protein in the urine and blood tests to measure blood protein concentration and kidney function (the level of waste chemicals). Frequently a kidney biopsy is recommended to examine the kidney tissue and its filters under a microscope. This allows a specific diagnosis to be made as well as determining possible treatment, as many kidney conditions characterized by persistent proteinuria can progress to kidney failure.