Urinalysis is a basic test in physical examinations and disease screening, and urinary protein is one item well worth paying attention to.

I. What Is Urinary Protein?

The basic functional units of the kidney include the glomerulus and renal tubules. When blood flows through the glomerulus, water, metabolic waste, and small molecules can enter the filtrate, while most proteins are retained in the blood. Subsequently, the renal tubules reabsorb a portion of useful substances. Healthy people also excrete trace amounts of protein in their urine, so "urinary protein being absolutely zero" is not the sole standard for judging kidney health. Clinically, the greater focus is on:

• Whether urinary protein persistently exceeds the normal reference range;
• Whether it is albumin or total protein that is elevated;
• Whether it is accompanied by hematuria, changes in kidney function, or other abnormalities;
• Whether there are risk factors such as diabetes, hypertension, immune disease, medications, or systemic disease.

Traditionally, the 24-hour total urine protein in healthy adults is typically below 150 mg. This value is the commonly used upper reference limit for total daily urinary protein excretion; it does not mean that everyone's urinary protein must fall below some precise integer, nor can it alone prove that the kidneys are "completely normal." It should be emphasized that 150 mg refers to total protein, whereas early kidney injury often first manifests as microalbuminuria (24-hour urine albumin <30 mg), at which point total protein may still be within 150 mg. Therefore, doctors now more commonly recommend using UACR (urine albumin-to-creatinine ratio) for early detection of kidney problems.

II. How to Interpret Urinary Protein Results?

1. Routine Urine Dipstick: Suitable for Initial Screening, Not Equivalent to Quantitative Results

Qualitative urinalysis typically uses a dipstick, with results reported as "negative," trace "±," or "1+ to 4+." It is mainly sensitive to albumin; a positive result merely indicates that a certain amount of protein is present in the urine and cannot be used to precisely determine how many grams are lost over 24 hours. Results can be affected by various factors:

• Overly concentrated or dilute urine;
• Fever, strenuous exercise, cold exposure, or stress;
• Hematuria, hemoglobinuria, or myoglobinuria;
• Urinary tract infection;
• Prolonged standing of the specimen or contamination by secretions;
• Urine pH, medications, and other chemical substances.

Therefore, "±, 1+, 2+, 3+, 4+" should not be mechanically converted into fixed numerical values.

2. UACR: Focus on Albuminuria

The urine albumin-to-creatinine ratio (UACR) measures the ratio of albumin to creatinine in urine. It corrects for urine concentration and is a commonly used screening indicator for people at risk of diabetes, hypertension, and chronic kidney disease. According to the KDIGO 2024 general risk stratification:

These categories are mainly used to assess kidney and cardiovascular risk. For patients with diabetes, hypertension, and chronic kidney disease, doctors usually assess risk based on both UACR and eGFR together, rather than looking at a single urine protein plus sign.

3. UPCR: Focus on Total Protein

The urine protein-to-creatinine ratio (UPCR) measures the ratio of total protein to creatinine in urine, reflecting more than just albumin. It is suitable for cases with obvious proteinuria, follow-up of glomerular disease, or when the doctor suspects light-chain protein or proteins of tubular origin. UPCR and UACR measure different targets and their numerical values should not be directly interchanged.

4. 24-Hour Urine Protein Quantification: For Total Daily Quantification in Specific Situations

The 24-hour urine protein quantification collects a full day's urine and directly calculates the total daily protein excretion. It is suitable for assessing obvious proteinuria, tracking treatment response in nephrotic syndrome, and resolving discrepancies between UPCR and clinical presentation. The premise of this method is complete collection. If one void is missed, the result may be underestimated; if several extra hours are collected, the result may be overestimated. Women should avoid the menstrual period; the specimen should be stored as required and protected from contamination. On the day of collection, maintain normal daily fluid intake and diet — neither deliberately drinking more water nor deliberately drinking less. Deliberately altering drinking habits will not make the result "better" and may instead introduce bias.

III. Does Elevated Urinary Protein Necessarily Mean Kidney Disease?

Not necessarily. A single abnormal result should first rule out transient changes, then repeated testing should be used to determine whether it is persistent proteinuria. Common transient influencing factors include:

• Strenuous exercise;
• Fever;
• Cold exposure;
• Mental stress;
• Severe heart failure;
• Upright posture or lumbar lordosis;
• Urinary tract infection;
• Menstruation or specimen contamination.

Among these, exercise-induced or functional proteinuria may resolve once the triggering factor is removed. Orthostatic or postural proteinuria is pronounced when standing and decreases or disappears when lying flat; it is most common in adolescents and young adults and is generally benign, usually disappearing on its own with age. However, a doctor must still rule out persistent proteinuria, hypertension, hematuria, abnormal kidney function, and other diseases; one should not self-label as "definitely fine." It should be emphasized that in people with normal kidney function, a single high-protein meal generally does not directly cause persistent pathological proteinuria. Occasionally consuming a high-protein meal may cause brief fluctuations in urinary protein excretion, but this is not sufficient to establish "intake-induced proteinuria" as a stable, independent disease category. However, in people who already have underlying kidney disease, a high-protein diet may exacerbate glomerular hyperfiltration, thereby worsening proteinuria and kidney burden.

IV. What Are the Common Sources of Pathological Proteinuria?

When evaluating proteinuria, doctors pay attention not only to the amount of protein but also to where the protein comes from and why it increases. Common causes include the following:

1. Glomerular Proteinuria

When the glomerular filtration barrier is damaged, relatively large proteins such as albumin more easily enter the urine. This is a clinically common and important type. Such proteinuria can be further divided into selective and non-selective forms, but this classification is mainly used to assist in assessing the degree of damage to the filtration barrier.

2. Tubular Proteinuria

When the reabsorptive capacity of the renal tubules declines, low-molecular-weight proteins that have been filtered cannot be fully recovered, resulting in tubular proteinuria. Common proteins include β₂-microglobulin, α₁-microglobulin, and lysozyme.

3. Overflow Proteinuria

In early stages of disease, the kidney's own filtration and reabsorption functions may have no obvious structural damage, but certain small-molecule proteins in the blood are abnormally increased, exceeding the reabsorptive capacity of the renal tubules and thus entering the urine.

4. Mixed Proteinuria

When both glomeruli and renal tubules are involved, the urine may contain both medium-to-large proteins such as albumin and small proteins such as β₂-microglobulin. This is commonly seen in advanced chronic glomerulonephritis and progressive diabetic nephropathy.

5. Organ/Tissue Proteinuria

The urinary tract mucosa, renal tubules, or inflamed tissue may also secrete small amounts of protein, such as Tamm-Horsfall protein. Healthy people also excrete trace amounts of such protein; its clinical significance is usually limited, and it is not suitable for inclusion as a major point for the general public alongside ordinary proteinuria etiologies.

V. What to Do After Discovering Positive Urinary Protein?

Do not self-diagnose based on a single result; consult a specialist. A nephrologist will determine the source and risk based on whether the proteinuria is persistent, the degree of elevation, changes in kidney function, urine sediment, medical history, and examination results.

VI. Lifestyle and Dietary Management

1. Control Salt Intake, but Less Is Not Always Better

The Dietary Guidelines for Chinese Residents (2022) recommends that general healthy adults consume no more than 5 g of salt per day, counting "hidden salt" in soy sauce, oyster sauce, chicken bouillon, pickled foods, and processed foods. Whether patients with chronic kidney disease, obvious edema, or hypertension require stricter salt restriction (such as 3–4 g/day) should be determined individually based on blood pressure, kidney function, electrolytes, and urine output; the general-population recommendation should not be directly applied to everyone.

2. Protein Intake Should Be Moderate, Not Generalized

Healthy people with normal kidney function do not need to blindly increase high-protein intake out of concern for urinary protein, nor do they need to completely avoid meat, eggs, or dairy. Fish, eggs, milk, soy products, and moderate lean meat are all sources of daily protein. People with positive urinary protein should not self-prescribe long-term high-protein or strict low-protein diets; it is best to have them evaluated jointly by a nephrologist and a clinical nutritionist.

3. Use Pain Relievers and Be Alert to Nephrotoxic Risks with Caution

Nonsteroidal anti-inflammatory drugs (NSAIDs) may increase the risk of kidney injury when used during dehydration, low blood pressure, or with long-term or high-dose use. Certain antibiotics, contrast agents, lithium, and some traditional Chinese herbs and health supplements may also carry risks.

VII. Summary

Urinary protein can indeed indicate that kidney filtration or reabsorption function may have changed, but kidney disease cannot be diagnosed based on a single "±" or "1+." The current overall framework for proteinuria assessment still centers on UACR screening, repeated confirmation, and integration with clinical context.

References

[1] KDIGO. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of CKD.

[2] Chinese Diabetes Society, Chinese Medical Association. Guidelines for the Prevention and Treatment of Diabetic Kidney Disease in China (2021 Edition).

[3] Beijing Municipal Health Commission. Instructions for 24-Hour Urine Protein Quantification Specimen Collection.

[4] Chinese Nutrition Society. Dietary Guidelines for Chinese Residents (2022).

Disclaimer:​ This article is for health education purposes only. Reference ranges and testing methods may vary across laboratories. For specific tests, rechecks, frequency, and treatment plans, please follow the judgment of a qualified medical institution's doctor based on your personal medical history.