Proteinuria

Proteinuria is a topic covered in the 5-Minute Pediatric Consult.

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Basics

Description

  • 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/m2/24 h or 4 mg/m2/h in children and 150 mg/24 h in adults).
    • Nephrotic-range proteinuria >1,000 mg/m2/24 h 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 to 300 mg/g albumin/creatinine ratio or 30 to 300 mg/24 h); generally used to indicate kidney disease in those with diabetes mellitus
  • Classification
    • Persistent or fixed proteinuria
      • Urinary dipstick ≥1+ on 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.
      • May be associated with high fever, cold stress, dehydration, and exercise
    • Orthostatic or postural proteinuria
      • Elevated protein excretion when the patient is upright but normalizes when patient is supine
      • The most common cause of fixed or transient proteinuria in childhood and adolescence
      • Rarely exceeds 1 g/m2/24 h
      • Benign condition and not associated with edema

Epidemiology

Prevalence of asymptomatic proteinuria in school-aged children is 5.4–10.7% on single urine sample and <1% on ≥3 urine samples.

Pathophysiology

  • 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
    • 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/24 h and is not associated with edema
    • The major marker is urinary β-2-microglobulin.
    • 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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