Protein total (urine)
Why this test?
- For the diagnosis of lipoid nephrosis, idiopathic membranous glomerulonephritis, focal segmental-glomerular sclerosis and other primary glomerulopathies.
- For the diagnosis of kidney damage in diabetes, systemic connective tissue diseases (systemic lupus erythematosus), amyloidosis and other diseases with possible involvement of the kidneys.
- For the diagnosis of kidney damage in patients with an increased risk of chronic renal failure.
- To assess the risk of developing chronic renal failure and coronary heart disease in patients with kidney disease.
- To assess kidney function during treatment with nephrotoxic drugs: aminoglycosides (gentamicin), amphotericin B, cisplatin, cyclosporine, nonsteroidal anti-inflammatory drugs (aspirin, diclofenac), ACE inhibitors (enalapril, ramipril), sulfonamides, penicillin, thiazides, furosemide, and some others.
In what cases is it prescribed?
- With symptoms of nephropathy: swelling of the lower extremities and periorbital area, ascites, weight gain, arterial hypertension, micro- and macrohematuria, oliguria, increased fatigue.
- In case of diabetes, systemic connective tissue diseases, amyloidosis and other diseases with possible kidney involvement.
- If there are risk factors for chronic kidney failure: arterial hypertension, smoking, heredity, age over 50 years, obesity.
- When assessing the risk of developing chronic renal failure and coronary heart disease in patients with kidney disease.
- When prescribing nephrotoxic drugs: aminoglycosides, amphotericin B, cisplatin, cyclosporine, nonsteroidal anti-inflammatory drugs, ACE inhibitors, sulfonamides, penicillins, thiazide diuretics, furosemide and some others.
Protein in the urine is an early and sensitive sign of primary kidney diseases and secondary nephropathies in systemic diseases. Normally, only a small amount of protein is lost in the urine thanks to the filtration mechanism of the renal glomerulus - a filter that prevents the penetration of large charged proteins into the primary filtrate. While low-molecular-weight proteins (less than 20,000 daltons) freely overcome the glomerular filter, the entry of high-molecular-weight albumin (65,000 daltons) is limited. Most of the protein is reabsorbed into the bloodstream in the proximal tubules of the kidney, with the result that only a small amount is excreted in the urine. About 20% of the normally secreted protein consists of low-molecular-weight immunoglobulins, and 40% is accounted for by albumin and mucoproteins, secreted in distal renal tubules. The normal loss of protein is 40-80 mg per day, the release of more than 150 mg per day is called proteinuria. At the same time, the main amount of protein is albumin.
It should be noted that in most cases proteinuria is not a pathological sign. Protein in the urine is determined in 17% of the population, and only 2% of them are the cause of a serious disease. In other cases, proteinuria is considered functional (or benign); it is observed in many conditions, such as fever, increased physical activity, stress, acute infectious disease, dehydration. Such proteinuria is not associated with kidney disease, and protein loss with it is insignificant (less than 2 g/day). One of the varieties of functional proteinuria is orthostatic (postural) proteinuria, when protein in the urine is detected only after prolonged standing or walking and is absent in a horizontal position. Therefore, with orthostatic proteinuria, the analysis of total protein in the morning portion of urine will be negative, and the analysis of daily urine will reveal the presence of protein. Orthostatic proteinuria occurs in 3-5% of people under 30 years of age.
Protein in the urine also appears as a result of its excessive formation in the body and increased filtration in the kidneys. At the same time, the amount of protein that entered the filtrate exceeds the reabsorption capabilities of the renal tubules and is eventually excreted in the urine. Such «overflow» proteinuria is also not associated with kidney disease. It can accompany hemoglobinuria with intravascular hemolysis, myoglobinuria with damage to muscle tissue, multiple myeloma and other plasma cell diseases. With this variant of proteinuria, urine does not contain albumin, but any specific protein (hemoglobin in hemolysis, Bence-Jones protein in myeloma). In order to detect a specific protein in the urine, daily urine analysis is used.
For many kidney diseases, proteinuria is a characteristic and constant sign. According to the mechanism of occurrence, renal proteinuria is divided into glomerular and tubular. Proteinuria, in which protein appears in the urine as a result of damage to the basement membrane, is called glomerular. The basal membrane of the glomeruli is the main anatomical and functional barrier for large and charged molecules, so when it is damaged, proteins freely enter the primary filtrate and are excreted in the urine. Damage to the basement membrane can occur first (in case of idiopathic membranous glomerulonephritis) or secondarily, as a complication of some disease (in case of diabetic nephropathy against the background of diabetes mellitus). The most common glomerular proteinuria.
Diseases accompanied by damage to the basement membrane and glomerular proteinuria include lipoid nephrosis, idiopathic membranous glomerulonephritis, focal segmental glomerular sclerosis and other primary glomerulopathies, as well as diabetes, connective tissue diseases, poststreptococcal glomerulonephritis and other secondary glomerulopathies. Glomerular proteinuria is also characteristic of kidney damage associated with taking certain drugs (non-steroidal anti-inflammatory drugs, penicillamine, lithium, opiates). The most common cause of glomerular proteinuria is diabetes and its complication - diabetic nephropathy. The early stage of diabetic nephropathy is characterized by the secretion of a small amount of protein (30-300 mg/day), the so-called microalbuminuria. As diabetic nephropathy progresses, protein loss increases (macroalbuminemia). The degree of glomerular proteinuria is different, it often exceeds 2 g per day and can reach more than 5 g of protein per day.
When the function of protein reabsorption in the renal tubules is impaired, tubular proteinuria occurs. As a rule, protein loss in this variant does not reach such high values as in glomerular proteinuria, and is up to 2 g per day. Violation of protein reabsorption and tubular proteinuria are accompanied by hypertensive nephroangiosclerosis, urate nephropathies, intoxication with lead and mercury salts, Fanconi syndrome, as well as medicinal nephropathy when using nonsteroidal anti-inflammatory drugs and some antibiotics. The most common cause of tubular proteinuria is hypertension and its complication - hypertensive nephroangiosclerosis.
An increase in protein in the urine is observed in infectious diseases of the urinary system (cystitis, urethritis), as well as in renal cell cancer and bladder cancer.
The loss of a significant amount of protein in the urine (more than 3-3.5 g / l) leads to hypoalbuminemia, a decrease in oncotic blood pressure and both external and internal edema (edema of the lower extremities, ascites). Significant proteinuria makes it possible to give an unfavorable prognosis of chronic renal failure. Persistent loss of a small amount of albumin does not manifest itself with any symptoms. The danger of microalbuminuria lies in the increased risk of ischemic heart disease (especially myocardial infarction).
Quite often, as a result of a variety of reasons, the analysis of morning urine for total protein is incorrect positive. Therefore, proteinuria is diagnosed only after repeated analysis. If two or more tests of the morning portion of urine for total protein are positive, proteinuria is considered persistent, and the examination is supplemented with an analysis of daily urine for total protein.
Examination of the morning portion of urine for total protein is a screening method for detecting proteinuria. It does not allow to assess the degree of proteinuria. In addition, the method is sensitive to albumin, but does not detect low molecular weight proteins (for example, Bence-Jones protein in myeloma). In order to determine the degree of proteinuria in a patient with a positive result of the analysis of the morning portion of urine for total protein, daily urine is also examined for total protein. If multiple myeloma is suspected, daily urine is also analyzed, and it is necessary to carry out an additional study for specific proteins - electrophoresis. It should be noted that the analysis of daily urine for total protein does not differentiate the variants of proteinuria and does not reveal the exact cause of the disease, so it must be supplemented with some other laboratory and instrumental methods.