Proteinuria: Causes of Protein in Urine
Definitions of Proteinuria
Proteinuria is the presence of excess serum proteins in the urine.
Normal Protein Excretion:
Protein in the urine should be in the range of 50–150 mg in a 24-hour urine collection.
Proteinuria up to 500 mg/24 h without the presence of a kidney disease is called functional proteinuria.
Microalbuminuria is the presence of more than 30 mg and up 300 mg albumin in a 24-h urine collection.
Selective Glomerular Proteinuria:
Selective glomerular proteinuria is the increased excretion of more than 300 mg medium-sized negatively charged proteins such as albumin in a 24-h urine collection.
Non-Selective Glomerular Proteinuria:
Non-selective glomerular proteinuria is the increased excretion of more than 3000 mg proteins of any size in a 24-h urine collection.
Tubular proteinuria is the presence of more than 150 mg small proteins (such as alpha 2-microglobulin) in a 24-h urine collection, while the serum proteins have normal concentrations.
The increased concentration of small proteins in the plasma, which can be filtered in the glomerulus, leads to an increased protein concentration in the primary urine and failure of the complete reabsorption by the tubular cells.
Proteinuria due to bleeding or infection of the kidneys, ureter, bladder or urethra is called postrenal proteinuria.
Differential Diagnosis of Proteinuria
- Physical activity
- Heat or cold
- Heart failure
Microalbuminuria indicates an abnormal leakage of albumin in the renal glomerulus and is a sign of kidney damage:
- Diabetic nephropathy (stage III)
- Hypertensive Nephropathy
- Also possible in fever, after physical activity or due to infections.
Selective glomerular proteinuria:
Selective glomerular proteinuria indicates moderate glomerular damage:
- Glomerulonephritis (e.g. minimal-change GN)
- Lupus nephritis with low activity
Non-selective glomerular proteinuria:
Non-selective glomerular proteinuria indicates severe glomerular damage:
- Lupus nephritis
Small proteins are in part filtrated in the renal glomerulum and reabsorbed by tubular cells. Tubular proteinuria indicates tubular cell damage:
- Interstitial nephritis
- Analgesics nephropathy
- Chronic pyelonephritis in vesicoureteral reflux
- Acute renal failure
- Tubular toxins like aminoglycosides, contrast media or ciclosporin.
Diagnostic Work-Up of Proteinuria
- Urine sediment and urine culture
- Protein concentration in 24-hour urine collection
- Protein electrophoresis of urine and serum
- Sonography of the urinary organs
- Urography or computed tomography of urinary organs, if postrenal proteinuria is suspected.
- Immun-electrophoresis, if Bence Jones proteinuria is suspected
- Renal biopsy, if severe proteinuria or signs of renal dysfunction are present.
Index: 1–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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