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- Protein may be found in the urine of healthy children.
- The term proteinuria is used to indicate urinary protein excretion beyond the upper limit of normal (100 mg/m2/day or 4 mg/m2/h in children and 150 mg/day in adults).
- Nephrotic range proteinuria >1,000 mg/m2/day or 40 mg/m2/h
- Nephrotic syndrome: nephrotic-range proteinuria with edema and hypoalbuminemia (<2.5 g/dL)
- Microalbuminuria: elevated urinary excretion of albumin (30–300 mg/g albumin/creatinine ratio or 30–300 mg/day). Currently, it is only used to indicate kidney disease in those with diabetes mellitus.
- Persistent or fixed proteinuria
- Urinary dipstick ≥1+ in the first morning urine specimen on ≥3 samples >1 week apart
- Requires prompt referral to nephrology
- Transient proteinuria
- Proteinuria absent on subsequent urine examinations
- It is not usually associated with clinically significant underlying renal disease.
- Often associated with high fever, cold stress, dehydration, and exercise
- Orthostatic or postural proteinuria
- Elevated protein excretion when the patient is upright that normalizes when patient is supine
- The most common cause of fixed or transient proteinuria in childhood and adolescence
- Proteinuria rarely exceeds 1 g/m2/day.
- Benign condition
- Normally, ∼50% urinary proteins are derived from tissue proteins and proteins from cells lining the urinary tract (i.e., Tamm-Horsfall protein).
- Proteinuria may be the result of glomerular proteinuria or tubular proteinuria.
- Glomerular proteinuria
- An increased permeability of the glomeruli to the passage of plasma proteins
- Normally may range from <1 to >30 mg/day
- Large amounts of glomerular proteinuria may be found in the context of edema and hypoalbuminemia (nephrotic syndrome).
- If there is hypertension, abnormal glomerular filtration rate, and hematuria, there may be nephritis as well.
- Tubular proteinuria
- Decreased reabsorption of low-molecular-weight proteins by the proximal renal tubules
- Rarely >1 g/day and is not associated with edema.
- The major marker is urinary beta-2-microglobulin.
- It may be associated with other defects of proximal tubular function (e.g., renal tubular acidosis [RTA], glucosuria, phosphaturia, aminoaciduria) and tubular interstitial processes.
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