A normal person’s urine contains only a trace amount of proteinuria, about 20-80mg of urine protein excreted daily. If the urine protein qualitative test is positive or the urine quantitative test is over 150mg/24h, it is called proteinuria.
I. Why proteinuria occurs
1, Glomerular filtration membrane damage or increased permeability
2.Decrease in renal tubular reabsorption
3.Increased protein overflow
4.Increased excretion of renal tubules and urothelial cells
5.Destruction of renal tissue
6.The effect of body position and exercise
Two, the common causes of proteinuria
1.Transient: seen in exercise, fever or dehydration.
2.Postural proteinuria: divided into temporary or persistent.
3.Persistent benign proteinuria
4.Primary and secondary glomerulonephritis:such as acute glomerulonephritis, chronic glomerulonephritis, hereditary nephritis, nephrotic syndrome, IgA nephropathy, lupus nephritis, purpura nephritis, etc.
5.Primary tubulointerstitial disease:such as reflux nephropathy, renal dysplasia, acute interstitial nephritis, etc.
Three, pseudoproteinuria
1, mixed with semen or prostate fluid, blood, pus, inflammatory or tumor secretions, as well as menstrual blood, leukorrhea, etc., routine urine protein qualitative examination can be a positive reaction, urine sediment can be seen in a large number of red, white blood cells and flat epithelial cells without tubular type, the urine centrifugal precipitation or filtration, protein qualitative examination will be significantly reduced or even turned negative.
2, urine for a long time or after cooling, can precipitate salt crystals, so that the urine is white cloudy, easy to be mistaken for protein urine, but heating or adding a little acetic acid can make cloudy urine to clear, to help distinguish.
3.Some drugs such as rifampin and Sandoval can make the urine cloudy and similar to proteinuria when they are excreted from the urine, but the qualitative reaction of proteinuria is negative.
4.Lymphatic urine: when lymphatic urine contains less protein, it is not necessarily chylomicronous.
Four, physiological proteinuria
1.Functional proteinuria
It is generally temporary, the amount of urine protein is usually less than <1g/24h, proteinuria is mainly medium molecular albumin, it occurs after strenuous exercise or during fever, excessive cold, high temperature work, mental tension and other states of high sympathetic excitement, etc. After the cause is removed, proteinuria can disappear.
2.Postural proteinuria
Generally less than 1g/24h, we can compare the urine before waking up in the morning after lying in bed at night and the urine after standing for 4-6h for qualitative protein examination, and measure it for 3 consecutive days. It is caused by the left superior mesenteric artery and the angle of the abdominal aorta compressing the left renal vein in the upright position, which increases the pressure of the left renal vein.
3.Post-exercise proteinuria
Proteinuria can occur in normal people after exercise. The intensity of exercise is the main factor in determining proteinuria, and the highest peak of urinary protein amount usually occurs within 0.5h after the cessation of exercise.
V. Pathological proteinuria
It refers to proteinuria caused by various primary or secondary kidney diseases and hereditary diseases, characterized by persistent non-resolution of urinary protein, or accompanied by hematuria, swelling, hypertension and other manifestations; it can also present simple proteinuria without any clinical symptoms or signs.
1.Glomerular proteinuria
Glomerular proteinuria is caused by the increased permeability of the glomerular filtration membrane to plasma protein.
It is the most common type of glomerulonephritis in the clinical setting and is seen in a variety of primary or secondary glomerulonephritis.
It is caused by ischemia, toxicity, immunopathological damage that disrupts the integrity of the filtration membrane, or by a weakened charge barrier of the filtration membrane.
The characteristics of this type of proteinuria are firstly, the amount of protein is often large, with an excretion range of 1-30g/d; secondly, its composition is mainly albumin, or albumin and proteins with greater molecular weight than it.
2. Renal tubular proteinuria
It is due to the impaired reabsorption of filtered protein by the renal tubules.
It is seen in tubulointerstitial lesions, pyelonephritis, congenital tubulopathy, and hypokalemic nephropathy.
This type of proteinuria is characterized by a small amount of total urinary protein, usually <1g/d; and contains only a small amount of albumin, and is dominated by low molecular weight lysozyme, β2-microglobulin, light chain protein, vitamin A binding protein, etc.
3.Overflow proteinuria
This is due to an abnormal increase of some lower molecular weight (<60,000 to 70,000) proteins in the blood circulation, which are filtered out by the glomerulus and exceed the reabsorption capacity of the renal tubules, resulting in proteinuria.
It is seen in patients with multiple myeloma (with native-periodic protein in the urine), myoglobinuria in severe crush injury, and lysozyme urine in myeloma and monocytic leukemia.
4.Secretory proteinuria
It refers to proteinuria caused by protein or other proteins secreted from the renal tubules and lower urinary tract.
For example, renal tubules secrete IgA or large-molecule Tamn-Horsfall protein after inflammation or drug stimulation, which is the substrate for the formation of tubular urine and may also be related to the immune response, but the amount is not large.
Mucin is due to pus, blood and secretions produced by urinary tract infections and prostatic fluid, semen or mixed with vaginal secretions, and the amount is usually not much.
5.Tissue proteinuria
There are some soluble tissue decomposition metabolites in normal urine, the content is small, but when poisoning, ischemia, inflammation or tumor lead to its tissue necrosis, the content can increase.
For example, glomerular basement membrane antigen is excluded from the urine in glomerulonephritis, and specific antigenic substances related to mid-stream can be detected in the urine in tumor.
Six, how to determine the degree of proteinuria
1.Mild proteinuria: 24h urine protein quantification is less than 1g
2.Moderate proteinuria: 24h urine protein quantification in 1.0-3.5g
3.Severe proteinuria: 24h urine protein quantification is more than 3.5g