- 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/d or 4 mg/m2/hr in children and 150 mg/d in adults).
- Proteinuria >40 mg/m2/hr is considered as nephrotic range.
- Transient proteinuria: Often associated with high fever, cold stress, dehydration, and exercise. It is not associated with underlying renal disease and by definition is absent on subsequent urine examinations.
- Orthostatic or postural proteinuria: Elevated protein excretion when the subject is upright and deambulating, but normal during recumbent position. It most commonly occurs in school-age children and adolescents and rarely exceeds 1 g/m2.
- Persistent or fixed proteinuria: Urinary dipstick ≥1 in the 1st morning urine specimen on multiple occasions for a period >3 months. Requires prompt referral to Nephrology.
- Glomerular proteinuria: The amount of proteinuria may range from <1 to >30 mg/d. It is usually found in the context of edema, hypertension, abnormal glomerular filtration rate, and hematuria. The major marker is albumin.
- Tubular proteinuria: Rarely >1 g/d and is not associated with edema. It may be associated with other defects of proximal tubular function (e.g., glucosuria, phosphaturia, aminoaciduria) and tubular interstitial processes. The major marker is โα2-microglobulin.
- ~50% of the normally excreted protein consists of Tamm-Horsfall protein, a glycoprotein secreted by the ascending loop of Henle.
- Proteinuria may be the result of an increased permeability of the glomeruli to the passage of serum proteins (glomerular proteinuria) or decreased reabsorption of low molecular weight proteins by the renal tubules (tubular proteinuria).