Urine routine is one of the most basic tests in physical examinations and disease screening, and urine protein is a crucial indicator among them. It can signal potential early kidney problems. The qualitative test for urine routine is normally negative. The normal range for 24-hour urine protein quantification in healthy adults is generally 0–80 mg/24h. When the quantitative test exceeds 150 mg/24h, it is called proteinuria. Values between 80–150 mg/24h fall within the borderline range and require dynamic clinical observation. Healthy kidneys act like a precision sieve, retaining large-molecule proteins needed by the body in the blood. Only when this "sieve" is damaged do proteins leak into the urine.

What is proteinuria?

Under normal conditions, due to the filtration of the glomeruli and the reabsorption of the renal tubules, urine contains only trace amounts of small-molecule proteins. When the protein content in urine increases abnormally (>150 mg/24h), it is called proteinuria, which is one of the most common manifestations of kidney disease.

How to interpret urine protein test results?

Negative (–): Indicates that urine protein is within the normal range, and kidney filtration function is basically normal.

Positive (± ~ ++++) or elevated quantitative value: Suggests possible proteinuria and requires further evaluation. The approximate correlation between qualitative results and 24-hour urine protein quantification is as follows:

(±): Usually corresponds to microproteinuria (<0.2 g/24h); further confirmation with urine microalbumin test is needed.

(+): Approximately corresponds to 24-hour quantification of 0.2–0.5 g/24h.

(++): Approximately corresponds to 24-hour quantification of 0.5–2.0 g/24h.

(+++) ~ (++++): Approximately corresponds to 24-hour quantification of >3.0 g/24h.

The more "+" signs or the higher the quantitative value, the more severe the urine protein loss.

What problems may positive/elevated urine protein indicate?

The presence of urine protein is not necessarily indicative of kidney disease; clinical context must be considered. Common causes can be divided into two categories: pathological and physiological.

I. Pathological

1. Glomerular proteinuria
This is the most common and most concerning type, originating from damage to the glomerular filtration barrier.
Based on the degree of damage to the glomerular filtration membrane, glomerular proteinuria can be further divided into:
Selective proteinuria: The filtration membrane is mildly damaged, and the urine mainly contains medium-molecular-weight proteins (such as albumin), indicating a mild lesion and a good response to steroid therapy.
Non-selective proteinuria: The filtration membrane is severely damaged, and both large-, medium-, and small-molecule proteins (such as IgG, IgM, etc.) appear in the urine, indicating a severe lesion and a relatively poor prognosis.
By etiology, it can be divided into:
Primary glomerular diseases: such as acute glomerulonephritis, chronic glomerulonephritis, nephrotic syndrome (often presenting as massive proteinuria, >3.5 g/24h), etc.
Secondary glomerular damage: This is a clinical focus; the core mechanism is that lesions in other systems or organs lead to structural and functional damage to the glomerular filtration barrier, such as diabetic nephropathy, hypertensive renal damage, lupus nephritis, gouty nephropathy, Henoch-Schönlein purpura nephritis, etc.
Glomerular injury and inflammation caused by other factors: such as drugs/toxins, severe infections, preeclampsia, etc.

2. Tubular proteinuria
Due to impaired reabsorption function of the renal tubules, normal filtered small-molecule proteins (such as β2-microglobulin) cannot be reclaimed and are excreted. Commonly seen in interstitial nephritis, drug-induced kidney injury (such as certain analgesics, heavy metals), hereditary tubular diseases (such as Fanconi syndrome), etc.

3. Overflow proteinuria
The kidney's own filtration and reabsorption functions are normal, but due to a massive increase in certain abnormal small-molecule proteins in the blood (such as light-chain proteins produced in multiple myeloma, free hemoglobin in massive hemolysis, free myoglobin in severe rhabdomyolysis), after filtration by the glomerulus, they far exceed the reabsorption capacity of the renal tubules, thus appearing in the urine.

4. Mixed proteinuria
Kidney lesions involve both glomeruli and renal tubules simultaneously, and the urine contains both medium/large-molecule proteins (such as albumin) and small-molecule proteins (such as β2-microglobulin). Commonly seen in advanced chronic glomerulonephritis, progressive diabetic nephropathy, etc.

5. Tissue proteinuria
Caused by destruction of renal tissue (renal tubules, renal pelvis, ureteral mucosa, etc.) or abnormally increased secretion, such as increased secretion of Tamm-Horsfall protein (uromodulin). Usually small-molecule proteins in small amounts, which can be seen in urinary tract inflammation, tumors, etc.

II. Physiological/Transient proteinuria

Physiological proteinuria is usually benign and temporary, and will disappear after removing the triggering factor. Common types include:

Orthostatic proteinuria: More common in tall and thin adolescents; appears when standing upright or with lumbar lordosis, and disappears after lying down.

Functional proteinuria: Seen during fever, strenuous exercise, cold exposure, mental stress, etc., representing temporary changes.

Intake-related influence: Intake of a large amount of high-protein food in a short period.

What to do if urine protein is positive?

1. Do not self-diagnose: A single positive urine protein result (especially if trace) does not necessarily indicate disease; retesting is required for confirmation.

2. Rule out false proteinuria: Contamination of urine with blood, pus, mucus, semen, or vaginal secretions can also cause false-positive urine protein tests. The external genitalia should be cleaned before collecting the specimen, and a morning midstream urine sample should be collected to avoid contamination. If specimen contamination is highly suspected, re-collection after local cleaning is required for retesting.

3. Perform quantitative testing: After abnormal qualitative urine routine results, 24-hour urine protein quantification is the gold standard for assessing severity, or random urine protein/creatinine ratio (UPCR) can be used as an alternative.

4. Comprehensive assessment of kidney function: Usually combined with tests such as serum creatinine, blood urea nitrogen, and kidney ultrasound.

5. Identify potential causes: Tests such as blood glucose, immune indicators (such as ANA, ANCA), serum protein electrophoresis, etc., may be needed.

6. Consult a specialist: A nephrologist will integrate all information to determine the source and severity of proteinuria and formulate an appropriate treatment plan.

Dietary and Lifestyle Management Recommendations

For patients with pathological proteinuria, lifestyle management is an important part of treatment:

1. Low-salt, high-quality protein diet: Limit salt intake, and under the guidance of a doctor or nutritionist, moderately consume high-quality proteins (such as eggs, milk, lean meat, fish), avoiding high-protein diets that increase kidney burden.

2. Strictly control underlying diseases: Strict glucose control for diabetic patients and stable blood pressure for hypertensive patients are the core of delaying kidney damage.

3. Avoid nephrotoxic drugs: Use non-steroidal anti-inflammatory drugs (such as ibuprofen) and certain antibiotics with caution.

4. Regular follow-up monitoring: Patients with chronic kidney disease need regular rechecks of urine routine, urine protein quantification, and kidney function to monitor disease progression.

Summary

Urine protein is a window into kidney health, and its significance should not be underestimated. There is no need to panic excessively upon discovering abnormalities; identify the cause through scientific examinations. Early detection, standardized diagnosis and treatment, and good self-management are key to protecting kidney function and delaying disease progression.

Disclaimer: This article is for health science popularization reference only and cannot replace professional medical advice. For specific diagnosis and treatment, please consult a regular medical institution.