Proteinuria

Descriptive text is not available for this imageBASICS

DESCRIPTION

  • 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 or spot urine protein/creatinine ratio >2 mg/mg
    • Nephrotic syndrome: nephrotic-range proteinuria with edema and hypoalbuminemia (<2.5 g/dL)
    • Moderately increased albuminuria (formerly called microalbuminuria): elevated urinary excretion of albumin (30 to 300 mg/g albumin/creatinine ratio or 30 to 300 mg/24 h)
  • Classification
    • Persistent or fixed proteinuria
      • Urinary dipstick ≥1+; or urine protein/creatinine ratio >0.2 mg/mg 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 fever, 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 proteinuria on non-first morning urine samples in childhood and adolescence
      • Proteinuria rarely exceeds 1 g/m2/24 h.
      • Benign condition and not associated with renal disease

EPIDEMIOLOGY

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

PATHOPHYSIOLOGY

  • Normally, ~50% of 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 a distinct or overlapping pathologic process (glomerular and/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, this may indicate glomerulonephritis (GN).
  • 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
  • Overflow proteinuria
    • Ultrafiltrate protein load above the normal tubular resorptive capabilities such as in multiple myeloma

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