Evaluation oof Proteinuria

Proteinuria is a well-established marker of kidney disease. It can manifest as transient, benign, or persistent, with clinical implications for kidney health.

Proteinuria is a common finding among pediatric and adolescent patients. In children, it is defined as urinary protein excretion at levels higher than 100-150 mg/m2/day.

Neonates, however, may have a higher normal urinary protein excretion, reaching up to 300 mg/m2/day. This variation is due to reduced reabsorption of filtered protein in neonates.

Approximately half of normal protein excretion consists of proteins secreted by tubular epithelium, with a significant portion being Tamm-Horsfall protein (uromodulin). The remaining half comprises plasma proteins, including albumin (making up 40 percent of the total urinary protein) and low molecular weight (LMW) proteins such as beta-2 microglobulin and amino acids.

The typically low rate of urinary protein excretion can be attributed to two main factors:

• Restriction of the filtration of proteins across the glomerular capillary wall.
• Reabsorption of freely filtered LMW proteins (those with a molecular weight less than 25,000 Daltons) by the proximal tubule.

Abnormal protein excretion in children is characterized by urinary protein levels exceeding 100-150 mg/m² per day or 4 mg/m² per hour. Nephrotic range proteinuria, indicating heavy proteinuria, is defined as ≥1000 mg/m² per day or 40 mg/m² per hour.

Increased protein excretion can occur through three primary mechanisms:

• Glomerular proteinuria
• Tubular proteinuria
• Overflow proteinuria

Glomerular Proteinuria

Glomerular proteinuria occurs due to the increased filtration of macromolecules, especially albumin, across the glomerular capillary wall. It can result from anatomical or functional lesions and is a common cause of proteinuria in children.

It can be associated with glomerular diseases, such as minimal change disease (MCD), or non-pathological conditions like fever, intensive exercise, and orthostatic (or postural) proteinuria, where protein excretion is increased only in the upright position.

Tubular Proteinuria

Tubular proteinuria is less frequent and results from the increased excretion of low molecular weight (LMW) proteins like beta-2-microglobulin, alpha-1-microglobulin, and retinol-binding protein.

These molecules are typically filtered across the glomerulus and then reabsorbed in the proximal tubule. Interference with proximal tubular reabsorption, often due to various tubulointerstitial diseases, can lead to increased excretion of these smaller proteins.

Tubular proteinuria is often associated with other defects in proximal tubular function, including glycosuria, proximal renal tubular acidosis with bicarbonate wasting, and phosphaturia. In Fanconi’s syndrome, all four of these proximal tubular defects may occur.

However, only albumin is detected by urine dipsticks, while tubular proteinuria is not identified in screening dipstick urinalysis.

Overflow Proteinuria

Overflow proteinuria results from increased excretion of LMW proteins due to excessive production of a specific protein, surpassing the tubular reabsorptive capacity.

This type of proteinuria is rarely observed in children and is mainly seen in adults with plasma cell dyscrasias, such as multiple myeloma, where they overproduce immunoglobulin light chains.

Similar to tubular proteinuria, overflow proteinuria with low molecular weight proteins cannot be detected through screening dipstick urinalysis.

Proteinuria can be evaluated using various methods, including:

• Urinary dipstick reagent,
• Sulfosalicylic acid (SSA) turbidity tests,
• Random or first morning urine (FMU) for spot urine protein-to-creatinine ratio (UPCR),
• 24 hours urinary protein and creatinine,
• Urine protein electrophoresis,
• Urinary microalbumin.

Urinary Dipstick Reagent Test

The urinary dipstick reagent test is one of the most routinely used methods in clinical practice for assessing proteinuria. It has the following characteristics:

• High sensitivity for measuring albumin concentration via colorimetric reactions.
• Limitation in that it cannot detect other types of proteins, such as plasma proteins, globulins, and low-molecular-weight proteins.
• The graded scale is as follows:

1. Negative (less than 10 mg/dL)
2. Trace (10-29 mg/dL)
3. 1+ (30-100 mg/dL)
4. 2+ (100-300 mg/dL)
5. 3+ (300-1,000 mg/dL)
6. 4+ (>1,000 mg/dL)

It's important to be aware of potential sources of false-positive and false-negative results. False-positive results can occur in alkaline or highly concentrated urine specimens, macroscopic hematuria, pyuria, and with certain antiseptic agents like chlorhexidine or benzalkonium chloride, as well as iodinated radiocontrast agents and detergents. On the other hand, false-negative results may be observed in patients with very dilute urine specimens or in disease states where albumin is not the predominant urinary protein.

Qualitative SSA Turbidity Test

The qualitative sulfosalicylic acid (SSA) turbidity test is not commonly employed in clinical practice to assess proteinuria. However, it has the potential to detect a broad range of urinary proteins. Key points about this test include:

• It has the capability to identify various urinary proteins.
• This turbidometric method is particularly useful in diagnosing multiple myeloma, a condition characterized by the excretion of light-chain immunoglobulins.
• During the SSA turbidity test, SSA reagent is added to a fresh urine specimen, and the resulting turbidity is correlated with the degree of proteinuria using a predetermined scale.
• One limitation of this testing method is its subjective nature, as it relies on a grading scale to interpret results.
• As a result, this technique is rarely employed for routine proteinuria assessment in clinical practice.

Spot UPCR

The spot urine protein-to-creatinine ratio (UPCR) is a widely utilized tool for quantitative measurements of pediatric proteinuria. This method is favored due to the inaccuracies associated with collecting 24-hour urine specimens from infants or young children, particularly those who are not toilet-trained. Key points about the spot UPCR include:

• It provides a practical approach for quantifying proteinuria, making it suitable for pediatric patients.
• The spot UPCR can be extrapolated to approximate the results of a 24-hour urine collection process.
• Collecting the first morning urine (FMU) is crucial because urinary protein levels exhibit significant variability throughout the day.
• The normal UPCR range for infants under 6 months of age is not precisely defined, but ratios higher than 0.8 to 1.0 are generally considered abnormal.
• For children aged 6 to 24 months, ratios lower than 0.5 are typically considered normal, while those less than 0.2 are considered normal for children older than 24 months and adults.
• In pediatrics, pathologic proteinuria is often diagnosed when the UPCR exceeds 1 to 2.

24-Hour Protein

The 24-hour urine specimen collection remains the gold standard for quantitative urinalysis. To standardize quantification and account for variations in body size, measurements are often adjusted to body surface area. Here are the results based on this adjustment:

• Normal protein excretion is defined as ≤4 mg/m²/hour.
• Proteinuria, indicating increased protein excretion, falls within the range of 4 to 40 mg/m²/hour.
• Nephrotic-range proteinuria is diagnosed when protein excretion exceeds 40 mg/m²/hour.

Microalbuminuria

Definition: Microalbuminuria is the presence of albumin in the urine at levels above normal but below the detectable range using the conventional urinary dipstick method.

Quantification: The urine albumin-to-creatinine ratio (UACR) is used to quantify microalbuminuria. It is calculated similarly to the UPCR.

Normal Range: The normal range for UACR, measured using the first morning urine (FMU), is typically less than 20-30 mg of urine albumin per gram of creatinine.

Diagnosis: Microalbuminuria is defined as the excretion of 20-200 µg/min/1.73m² or 30-300 mg of albumin per gram of creatinine per day.

Types of Proteinuria: Proteinuria can manifest in various ways, including:

• Asymptomatic
• Symptomatic (can be transient or persistent)
• Isolated or associated with other systemic symptoms

The initial step in assessing proteinuria is to distinguish between pathological and benign causes.

Transient Proteinuria

Transient proteinuria is characterized by the following features:

• Associated with stress, fever, seizure activity, cold exposure, or strenuous exercise.
• Temporary and disappears when the inciting factor is resolved.
• May not be associated with any significant renal disease.
• Proteinuria does not exceed 1+ to 2+ when assessed using the urine dipstick method.
• Caused by hemodynamic changes in glomerular blood flow.
• Although these changes result in increased protein diffusion, further evaluation or treatment for these children is unnecessary.

Orthostatic or Postural Proteinuria

Orthostatic proteinuria is characterized by the following features:

• More common in older children and adolescents.
• Usually asymptomatic and can be easily detected using urinary screening tests.
• Increased protein excretion in the upright position is the hallmark; other symptoms such as hematuria, edema, hypertension, and renal dysfunction must be absent.
• Total urinary protein excretion may be up to 1 g/day, but it rarely exceeds this level.
• The collection of first morning urine (FMU) is critical for diagnosis.
• The exact cause of orthostatic proteinuria is still unclear; multiple factors have been proposed, including renal hemodynamic changes, partial left renal vein compression, increased permeability of the capillary walls, or circulating immune complexes.
• Orthostatic (or postural) proteinuria is defined as increased protein excretion in the upright position which returns to normal when the patient is recumbent.
• Long-term studies have documented the benign nature of this condition, with recorded normal kidney function up to 50 years later.
• The diagnosis is established by a negative dipstick on the first morning voided specimen.

Persistent Asymptomatic Proteinuria

Persistent asymptomatic proteinuria is characterized by the following features:

• Proteinuria persists on FMU for more than 3 months.
• The prevalence of persistent proteinuria in children may be as high as 6%.
• This type is not associated with edema, and the average amount of proteins excreted is approximately <2 g/day.
• Causes of persistent asymptomatic proteinuria include membranous and membranoproliferative glomerulonephritis, pyelonephritis, developmental anomalies, hereditary nephritis, hepatitis B infection.

General Information:

• A positive dipstick for protein on a random urinalysis is common in children.
• General screening of normal school-age children and adolescents with a urine dipstick will be positive (defined as ≥1+) in 5 to 10 percent.
• However, only 0.1 percent of children have persistent proteinuria.
• It is this small subset of children who are at the highest risk for kidney disease.

Asymptomatic child:

• The diagnostic evaluation of the child with dipstick-positive proteinuria depends on the presence or absence of symptoms.
• In the asymptomatic child with an incidentally discovered positive dipstick for proteinuria, the first step is to repeat the test.
• The simplest approach is to measure the protein/creatinine (Pr/Cr) ratio on a first morning void obtained at home.

Symptomatic child:

• Clinical manifestations in the symptomatic child with proteinuria may be general and nonspecific (e.g. fever, malaise, weight loss), nonurinary specific (e.g. rash, purpura, arthritis), or urinary specific (e.g. edema, hypertension, kidney insufficiency).
• The underlying disorder may be primarily renal in origin or secondary to a systemic process.
• Diagnostic categories include infections, rheumatologic and immunologic disorders, and primary and secondary glomerular and interstitial diseases of the kidney.

History and physical examination:

• The evaluation begins with a thorough history and physical examination to determine the type of proteinuria.
• Findings suggesting underlying kidney disease and persistent proteinuria include a change in urine volume or color, evidence of edema or increased blood pressure, recent streptococcal infection, a positive family history for kidney disease, and hearing loss (most suggestive of Alport disease).
• An elevated Pr/Cr ratio on the first morning void and a positive dipstick on the second specimen indicate persistent proteinuria that requires further evaluation.
• Examination of the urine sediment is important, looking for other signs of glomerular and/or parenchymal disease such as hematuria, red cell casts, pyuria, and/or lipiduria.

Additional Considerations:

• Symptoms of hypertension, oliguria, polyuria, weight loss, skin lesions, joint symptoms, recent infections, previous abnormal urinalyses, and recent intake of medications (such as NSAIDs, gold, angiotensin converting enzyme inhibitors (ACEi), and penicillamine).
• Family history of hypertension, renal disease, autoimmune disease, and visual impairment or deafness should also be considered.
• Growth is an important clue for chronic diseases such as renal disease and needs to be measured.
• Blood pressure also needs to be routinely measured.
• Children with heavy proteinuria and periorbital or peripheral edema must be evaluated promptly for nephrotic syndrome.

Treatment Approach:

• The treatments should be directed at the underlying causes of the proteinuria.
• If the patient is confirmed to have transient or orthostatic proteinuria, no treatment is required.
• For patients with isolated proteinuria, treatment recommendations range from none to protein-lowering agents like angiotensin receptor blocker (ARB) or ACEi.
• ARB or ACEi medications can be used as adjunctive or primary treatments in patients with high-grade proteinuria and those with abnormal levels of microalbuminuria with type I or type II diabetes mellitus.

Prevention and Monitoring:

• Although there are no recommendations to prevent proteinuria, prompt evaluation, diagnosis, treatment, and long-term monitoring of these pediatric patients can significantly prevent potential progression of the underlying disease process.
• Dietary protein restriction is rarely recommended to avoid harmful effects on their growth and development. Hence, pediatric patients must receive the proper daily protein intake depending on their ages.
• A positive treatment response to proteinuria is not only an indicator of a good prognosis but also a predictor of high renal functional survival rate as well as subsequent changes in glomerular filtration rate.

• The first step in the evaluation of proteinuria is to differentiate between the pathologic and benign etiologies.
• Persistent proteinuria may be indicative of underlying renal pathology, and treatments should be directed at the underlying causes of the proteinuria.
• Although there are no recommendations to prevent proteinuria, prompt evaluation, diagnosis, treatment, and long-term monitoring of these pediatric patients can significantly prevent potential progression of the underlying disease process.