PATHOPHYSIOLOGY AND CLASSIFICATION
Normal protein excretion — Urinary protein excretion in the normal child is less than 100 mg/m2 per day or a total of 150 mg per day. In neonates, normal urinary protein excretion is higher, up to 300 mg/m2, because of reduced reabsorption of filtered proteins.
Approximately one-half of normal protein excretion consists of proteins secreted by tubular epithelium, mostly Tamm-Horsfall protein (uromodulin). The other half consists of plasma proteins including albumin, which accounts for approximately 40 percent of the total urinary protein, and low molecular weight (LMW) proteins, such as beta-2 microglobulin and amino acids.
The normally low rate of urinary protein excretion is due to two factors:
●Restriction of the filtration of proteins across the glomerular capillary wall
●Reabsorption of freely filtered LMW proteins (less than 25,000 Daltons) by the proximal tubule
Abnormal protein excretion — Urinary protein excretion in excess of 100 mg/m2 per day or 4 mg/m2 per hour is considered abnormal in children. Nephrotic range proteinuria (heavy proteinuria) is defined as ≥1000 mg/m2 per day or 40 mg/m2 per hour.
Glomerular proteinuria — Glomerular proteinuria is due to increased filtration of macromolecules (particularly albumin) across the glomerular capillary wall. This may arise because of anatomical or functional lesions.
Glomerular proteinurias are a common cause of proteinuria in children. They may result from glomerular disease (most often minimal change disease [MCD]) or from nonpathologic conditions such as fever, intensive exercise, and orthostatic (or postural) proteinuria, in which protein excretion is increased only in the upright position.
Tubular proteinuria — Tubular proteinuria, which is less frequent, results from increased excretion of LMW proteins such as beta-2-microglobulin, alpha-1-microglobulin, and retinol-binding protein. These molecules are normally filtered across the glomerulus and then largely reabsorbed in the proximal tubule. Interference with proximal tubular reabsorption, due to a variety of tubulointerstitial diseases, can lead to increased excretion of these smaller proteins.
Tubular proteinuria often is 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 occur.
As will be described below, only albumin is detected by the urine dipstick, while tubular proteinuria is not detected by screening dipstick urinalysis.
Overflow proteinuria — Overflow proteinuria results from increased excretion of low molecular weight proteins due to marked overproduction of a particular protein to a level that exceeds tubular reabsorptive capacity. Overflow proteinuria is not observed in children; it is primarily observed in adults with a plasma cell dyscrasia (eg, multiple myeloma) who overproduce immunoglobulin light chains.
As with tubular proteinuria, overflow proteinuria with low molecular proteins will not be detected by screening dipstick urinalysis.
MEASUREMENT OF URINARY PROTEIN — There are several tests available for the measurement of urinary protein. The most common test, the urine dipstick, and other tests such as the sulfosalicylic acid (SSA) test that detects all proteins, measure only the urine protein concentration and cannot be used to quantify protein excretion. A dilute urine, for example, will underestimate the degree of proteinuria, while a highly concentrated urine may have a protein concentration greater than 100 mg/dL, but not be indicative of increased protein excretion.
Urine dipstick — The urinary dipstick measures albumin concentration via a colorimetric reaction between albumin and tetrabromophenol blue producing different shades of green according to the concentration of albumin in the sample.
●Trace – between 15 and 30 mg/dL
●1+ – between 30 and 100 mg/dL
●2+ – between 100 and 300 mg/dL
●3+ – between 300 and 1000 mg/dL
●4+ – >1000 mg/dL
Dipstick testing will not detect low molecular weight (LMW) proteins. False-positive results may be obtained in samples that are very alkaline, or contaminated by antiseptic agents (such as chlorhexidine or benzalkonium chloride) or iodinated radiocontrast agents . Thus, the urine should not be tested for protein with the dipstick for at least 24 hours after a contrast study.
Sulfosalicylic acid test — In contrast to the urine dipstick, sulfosalicylic acid (SSA) detects all proteins in the urine including the LMW proteins that are not detected by the dipstick . The SSA test is performed by mixing one part urine supernatant (eg, 2.5 mL) with three parts 3 percent SSA, followed by assessment of the degree of turbidity. This test is infrequently necessary in children.
Quantitative assessment — Children with persistent dipstick-positive proteinuria must undergo a quantitative measurement of protein excretion. Because of the difficulty of obtaining an accurate 24 hour collection, we prefer to use the quantitative measurement of the total protein/creatinine (Pr/Cr) ratio on a spot urine sample. In particular, it is difficult to obtain accurately time urine collection in young children and infants who are not toilet trained.
Spot urine sample — The quantitative assessment is measurement of the total protein/creatinine ratio (Pr/Cr) on a spot urine sample is preferably performed on a first morning specimen The normal value for this ratio is <0.2 mg protein/mg creatinine (<20 mg protein/mmol creatinine) in children greater than two years of age and <0.5 mg protein/mg creatinine (<50 mg protein/mmol creatinine) in infants and toddlers from 6 to 24 months .
Of note, spot urine protein to creatinine ratio will overestimate the actual protein excretion in very dilute urine (ie, urine Cr <38.8 mg/dL [3.43 mmol/L]) or underestimate protein excretion for a concentrated sample (ie, urine Cr >61.5 mg/dL [5.44 mmol/L]) . Ongoing efforts are needed to enhance the accuracy of a spot urine sample to as urinary protein excretion.
Timed 24-hour urine collection — In children (except neonates), levels of urinary protein excretion higher than 100 mg/m2 per day (or 4 mg/m2 per hour) are abnormal. Proteinuria of greater than 40 mg/m2 per hour is considered heavy or in the nephrotic range.
Qualitative assessment — A qualitative analysis of urinary proteins may be necessary to differentiate glomerular from tubular proteinuria. A selective measurement, by immunonephelometry of marker proteins that include beta-2 microglobulin, alpha-1-microglobulin, lysozyme, and retinol-binding protein can distinguish glomerular from tubular proteinuria. In tubular proteinuria, these levels will be 10 to 100 times higher than normal.
APPROACH TO THE CHILD WITH PROTEINURIA — 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 renal disease.
Proteinuria in children presents in three ways: transient or intermittent; orthostatic; and persistent. Transient and orthostatic proteinuria are benign conditions that require no further evaluation:
●Transient proteinuria is the most common cause. It can be induced by a variety of factors including fever, exercise, stress, seizures, and hypovolemia.
●Orthostatic (or postural) proteinuria is defined as increased protein excretion in the upright position which returns to normal when the patient is recumbent. It is a common cause of proteinuria, especially in adolescent males . Long-term studies have documented the benign nature of this condition, with recorded normal renal function up to 50 years later . The “nutcracker syndrome” (left renal vein entrapment), demonstrated by ultrasonic imaging and Doppler flow scanning, appears to be a common cause of postural proteinuria The diagnosis is established by a negative dipstick on the first morning voided specimen.
●Persistent proteinuria should be more fully evaluated for underlying renal disease.
History and physical examination — The evaluation begins with a thorough history and physical examination to determine the type of proteinuria. A urine sample collected during a febrile illness or after intensive physical exercise or a seizure suggests the possible presence of transient proteinuria. Other findings suggest underlying renal disease and persistent proteinuria; these include a change in urine volume or color, evidence of edema or increased blood pressure, recent streptococcal infection, a positive family history for renal disease, and hearing loss, which is most suggestive of Alport disease.
Asymptomatic child — The diagnostic evaluation of the child with dipstick-positive proteinuria depends in part upon the presence or absence of symptoms. The following approach is consistent with the recommendations of the 2000 Pediatric Nephrology panel established at the National Kidney Foundation conference on Proteinuria, Albuminuria, Risk, Assessment, Detection, and Elimination (PARADE)
In the asymptomatic child with an incidentally discovered positive dipstick for proteinuria, the first step is to repeat the test, since, as mentioned above, the great majority of such episodes are transient and do not reflect any renal disease . The simplest approach is to measure the protein/creatinine (Pr/Cr) ratio on a first morning void obtained at home, and to send for urinalysis a second specimen obtained in the office. The parents should be instructed to have the child void before going to bed and to remain recumbent until the first morning sample is obtained.
The following findings may be obtained:
●A normal Pr/Cr ratio on the first morning void and a normal urinalysis indicate transient proteinuria. These patients should have a repeat urinalysis on a first morning void in one year to ensure that proteinuria does not recur.
●A normal Pr/Cr ratio on the first morning void and dipstick-positive proteinuria on the second upright specimen indicate orthostatic proteinuria (see These patients should have a repeat urinalysis on a first morning void in one year to ensure that nonorthostatic proteinuria does not occur.
●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, beginning with examination of the urine sediment, looking for other signs of glomerular and/or parenchymal disease such as hematuria, red cell casts , pyuria, and/or lipiduria. Red cell casts are pathognomonic for the presence of glomerulonephritis. Based upon the particular findings, certain patterns on the urinalysis are suggestive of glomerulonephritis, nephrotic syndrome, or other renal parenchymal disorders.
The urinalysis may also be suggestive of a urinary tract infection (UTI) with pyuria, bacteriuria, and positive nitrites or leukocyte esterase along with mild proteinuria. Proteinuria in such children typically resolves with successful treatment of the infection. If the proteinuria persists after eradication of the infection, further work-up is indicated.
Among children with persistent proteinuria, a complete history and physical examination should be reviewed, including measurement of the blood pressure, as elevated hypertension may be indicative of underlying renal disease.
Initial laboratory evaluation includes renal function tests (blood urea nitrogen and creatinine), serum electrolytes, cholesterol, albumin, and total protein.
Other tests should be considered based on the clinical setting:
●Suspected poststreptococcal glomerulonephritis following an antecedent streptococcal infection and evidence of glomerular disease (eg, hypertension, hematuria, and in some cases proteinuria)
•Serum levels of C3 are low
•Streptozyme testing is positive
●Suspected lupus nephritis with evidence of extrarenal findings (eg rash, hematologic abnormalities, joint symptoms)
•Serum levels of C3 and C4 may be low
•Antinuclear antibody (ANA)
●Patients with evidence of liver disease – Hepatitis B and C serology.
●History of exposure to human immunodeficiency virus (HIV) – HIV testing
●Renal ultrasound should be considered if there is evidence of renal parenchymal damage (eg, scarring) or congenital abnormality (eg, hypertension, impaired renal function, history of chronic UTI. A voiding cystourethrogram (VCUG) should be considered if there is an abnormal ultrasound with scarring or a history of febrile UTIs.
If this initial evaluation is normal, the urine dipstick should be repeated on at least two additional specimens. If these subsequent tests are negative for protein, the diagnosis is transient proteinuria.
If the proteinuria persists or if any of the studies are abnormal, the patient should be referred to a pediatric nephrologist. At this point, urinary protein excretion should be quantified by a timed collection, if obtainable. A detailed family history should be obtained and the parents should be screened by urinary dipstick to search for hereditary glomerular disorders.
Indications for renal biopsy — The role of renal biopsy in a child with isolated asymptomatic persistent proteinuria is controversial . Many nephrologists recommend close monitoring for those children with urinary protein excretion below 500 mg/m2 per day before considering a biopsy. Monitoring should include assessment of blood pressure, protein excretion, and renal function. If any of these parameters shows evidence of progressive disease, a renal biopsy should be performed to establish a diagnosis.
There are limited data on the results of renal biopsy in such children.
In a retrospective review of 53 Japanese children with persistent isolated proteinuria who underwent renal biopsy from 1976 to 1989, a significant glomerular disease was present in almost half of the patients (n = 25). 15 had focal segmental glomerulosclerosis (FSGS); four had IgA nephropathy, and three each had membranous nephropathy and diffuse mesangial proliferative glomerulonephritis without immunoglobulin A (IgA) deposition .
In a subsequent case series from the same institution, 52 children with asymptomatic persistent proteinuria underwent renal biopsy from 2000 to 2010 . The 29 patients with a urine Pr/Cr ≥0.5 mg/mg were more likely to have significant glomerular findings that included five with FSGS, and seven with IgA nephropathy. The remaining 17 patients had minor glomerular changes. In contrast, only one of the 15 patients with a urine Pr/Cr <0.5 mg/mg had FSGS, and the rest had minor glomerular changes. The authors of the study suggest that a renal biopsy should be considered in asymptomatic children with constant isolated proteinuria with a urine Pr/Cr ≥0.5 mg/mg.
In a report of 461 Korean children with an abnormal urinalysis detected by school screening, only nine patients had isolated persistent proteinuria with protein excretion ≥2 g in a 24-hour collection . Renal biopsy demonstrated changes consistent with minimal change disease (MCD) in seven patients, and one case each of mesangial proliferative glomerulonephritis and membranous glomerulopathy.
Symptomatic child — Clinical manifestations in the symptomatic child with proteinuria may be general and nonspecific (eg, fever, malaise, weight loss), nonurinary specific (eg, rash, purpura, arthritis), or urinary specific (eg, edema, hypertension, renal 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.
The diagnosis may be evident and straightforward from the history and physical examination, including blood pressure measurement, or it may be more complicated, with a need for early referral to a nephrologist. Initial laboratory evaluation includes renal function tests (blood urea nitrogen and creatinine), serum electrolytes, cholesterol, albumin, and total protein. The practitioner must ensure that children with significant disease are clearly identified.
●Children with heavy proteinuria and periorbital or peripheral edema must be evaluated promptly for nephrotic syndrome. The major manifestations of nephrotic syndrome are heavy proteinuria (protein excretion >1000 mg/m2 per day or spot urine Pr/Cr ratio >1), edema, serum albumin <2.5 g/dL, and hypercholesterolemia. Almost all such children have idiopathic nephrotic syndrome, and management decisions should be made in consultation with a pediatric nephrologist.
There are three main mechanisms of increased protein excretion: glomerular, tubular, and overflow proteinuria.Non-nephrotic children with persistent proteinuria who present with hypertension, an abnormal urinalysis, or an elevated plasma creatinine concentration should be referred to a pediatric nephrologist for further evaluation and possible renal biopsy.
●In patients with a history of febrile UTIs, renal ultrasound should be obtained to determine if there is evidence of renal scarring. A VCUG should be considered if there is concern for vesicoureteral reflux.