Does foam in urine mean kidney problems? Not necessarily...

Author: Zuo Li, Chief Physician, Peking University People's Hospital

Reviewer: Mao Yonghui, Chief Physician, Beijing Hospital

In daily life, people sometimes observe foam in urine after urination and may worry about kidney health because of this.

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In fact, foam may also be produced during urination under normal circumstances, and these foams are usually large and disappear quickly after urination. In contrast, when there is a problem with the kidneys, the urine may contain protein, which causes the foam to be smaller and last longer, making it difficult to dissipate.

If you find persistent foam in your urine, a simple urine routine test can be done to determine if there is a kidney problem.

1. Urine protein is positive, where might the protein come from?

When urine routine examination shows positive urine protein, the following situations may exist:

1. Increased albumin filtration: The glomerulus is the basic functional unit of the kidney and is responsible for filtering blood. Under normal circumstances, large molecules such as albumin should not pass through this filtration system into the urine. However, when kidney function is impaired, albumin may abnormally pass through the glomerular filtration membrane into the urine.

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2. Overflow proteinuria: When there are a large number of small molecular proteins in the blood circulation that can freely pass through the glomerular filtration membrane, if it exceeds the reabsorption capacity of the renal tubules, these proteins will appear in the urine and form proteinuria. In this case, the structure and function of the kidney itself are usually normal.

3. Kidney structure damage: When kidney tissue is damaged, protein in the kidney tissue may leak into the urine and may also cause positive urine protein.

2. What diseases may cause proteinuria?

In my country, the most common cause of proteinuria is primary glomerular disease, such as nephrotic syndrome, which is characterized by heavy proteinuria, hypoproteinemia, edema, and hyperlipidemia. Problems with the renal tubules or renal interstitium may also lead to the appearance of small amounts of proteinuria.

Diabetic nephropathy has become the second leading cause of proteinuria in my country and is gradually becoming the most common cause of proteinuria. Therefore, diabetic patients should undergo regular urine routine tests to identify the occurrence of diabetic nephropathy at an early stage.

Hypertension can also cause proteinuria. In healthy individuals, renal perfusion is maintained within a specific pressure range and can self-regulate to maintain a constant blood supply. Long-term hypertension can weaken the kidney's ability to regulate blood flow, leading to increased pressure within the glomerulus, which in turn increases the filtration pressure, causing albumin to be filtered into the primary urine and form albuminuria. This condition is called hypertensive renal damage.

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Overflow proteinuria is common in patients with light chain multiple myeloma, which secretes large amounts of light chain proteins. Light chain proteins can be filtered freely through the glomeruli, and when they exceed the reabsorption capacity of the renal tubules, they appear in the urine. In addition, light chain proteins in the blood may also deposit in the glomeruli, damaging their filtration barrier and leading to albuminuria; or they may deposit in the renal tubules or be reabsorbed by the renal tubules, causing renal tubular damage.

3. What to do next if urine protein is positive?

When a routine urine test shows positive urine protein, the cause of the disease can be preliminarily determined based on the amount of urine protein and accompanying symptoms.

If the amount of urine protein is large, it usually indicates glomerular disease. Positive urine protein combined with hematuria, especially macroscopic hematuria, also often points to glomerular lesions. Glomerular diseases include a variety of primary and secondary pathological types, and the treatment options vary significantly. When the clinical manifestations are not sufficient to determine the specific type, a renal biopsy is required to confirm the pathological diagnosis.

In the case of only mild urine protein (a plus sign) and no hematuria, it is difficult to directly determine the specific source of urine protein. At this time, a 24-hour urine protein quantitative test should be further performed, and the nature of urine protein should be evaluated in combination with urine albumin excretion.

If the urine protein is approximately 0.5 g per day and the urine protein test is a plus sign, the two match, indicating that a mild glomerular disease may be present.

The urine routine protein test shows three to four plus signs, and the urine protein quantity is about 4 grams per day, the two are consistent and also indicate glomerular disease.

If the urine protein quantity is as high as 10 grams per day, but the urine routine protein test only shows a plus sign, the so-called "qualitative and quantitative separation of urine protein" phenomenon occurs, indicating that most of the urine protein does not come from albumin and may not be caused by glomerular disease.