Proteinuria

Proteinuria often causes no symptoms and it may only be discovered incidentally.[1]

Foamy urine is considered a cardinal sign of proteinuria, but only a third of people with foamy urine have proteinuria as the underlying cause.[2] It may also be caused by bilirubin in the urine (bilirubinuria),[3] retrograde ejaculation, pneumaturia (air bubbles in the urine) due to a fistula,[4] or drugs such as pyridium.[3]

There are three main mechanisms to cause proteinuria:[5]

• Due to disease in the glomerulus
• Because of increased quantity of proteins in serum (overflow proteinuria)
• Due to low reabsorption at proximal tubule (Fanconi syndrome)

Proteinuria can also be caused by certain biological agents, such as bevacizumab (Avastin) used in cancer treatment. Excessive fluid intake (drinking in excess of 4 litres of water per day) is another cause.[6][7]

Proteinuria may be a sign of renal (kidney) damage. Since serum proteins are readily reabsorbed from urine, the presence of excess protein indicates either an insufficiency of absorption or impaired filtration. People with diabetes may have damaged nephrons and develop proteinuria. The most common cause of proteinuria is diabetes, and in any person with proteinuria and diabetes, the cause of the underlying proteinuria should be separated into two categories: diabetic proteinuria versus the field.

With severe proteinuria, general hypoproteinemia can develop which results in diminished oncotic pressure. Symptoms of diminished oncotic pressure may include ascites, edema and hydrothorax.

Protein is the building block of all living organisms.[14] When kidneys are functioning properly by filtering the blood, they distinguish the proteins from the wastes which were previously present together in the blood.[14] Thereafter, kidneys retain or reabsorb the filtered proteins and return them to the circulating blood while removing wastes by excreting them in the urine.[14] Whenever the kidney is compromised, their ability to filter the blood by differentiating protein from the waste, or retaining the filtered protein then returning which back to the body, is damaged.[14] As a result, there is a significant amount of protein to be discharged along with waste in the urine that makes the concentration of proteins in urine high enough to be detected by medical machine.[14]

Medical testing equipment has improved over time, and as a result tests are better able to detect smaller quantities of protein.[14] Protein in urine is considered normal as long as the value remains within the normal reference range.[14] Variation exists between healthy patients, and it is generally considered harmless for the kidney to fail to retain a few proteins in the blood, letting those protein discharge from the body through urine.[14]

Conventionally, proteinuria is diagnosed by a simple dipstick test, although it is possible for the test to give a false negative reading,[18] even with nephrotic range proteinuria if the urine is dilute.[19] False negatives may also occur if the protein in the urine is composed mainly of globulins or Bence Jones proteins because the reagent on the test strips, bromophenol blue, is highly specific for albumin.[8][20] Traditionally, dipstick protein tests would be quantified by measuring the total quantity of protein in a 24-hour urine collection test, and abnormal globulins by specific requests for protein electrophoresis.[3][21] Trace results may be produced in response to excretion of Tamm–Horsfall mucoprotein.

More recently developed technology detects human serum albumin (HSA) through the use of liquid crystals (LCs). The presence of HSA molecules disrupts the LCs supported on the AHSA-decorated slides thereby producing bright optical signals which are easily distinguishable. Using this assay, concentrations of HSA as low as 15 µg/mL can be detected.[22]

Alternatively, the concentration of protein in the urine may be compared to the creatinine level in a spot urine sample. This is termed the protein/creatinine ratio. The 2005 UK Chronic Kidney Disease guidelines state that protein/creatinine ratio is a better test than 24-hour urinary protein measurement. Proteinuria is defined as a protein/creatinine ratio greater than 45 mg/mmol (which is equivalent to albumin/creatinine ratio of greater than 30 mg/mmol or approximately 300 mg/g) with very high levels of proteinuria having a ratio greater than 100 mg/mmol.[23]

Protein dipstick measurements should not be confused with the amount of protein detected on a test for microalbuminuria which denotes values for protein for urine in mg/day versus urine protein dipstick values which denote values for protein in mg/dL. That is, there is a basal level of proteinuria that can occur below 30 mg/day which is considered non-pathology. Values between 30 and 300 mg/day are termed microalbuminuria which is considered pathologic.[24] Urine protein lab values for microalbumin of >30 mg/day correspond to a detection level within the "trace" to "1+" range of a urine dipstick protein assay. Therefore, positive indication of any protein detected on a urine dipstick assay obviates any need to perform a urine microalbumin test as the upper limit for microalbuminuria has already been exceeded.[25]

Treating proteinuria mainly needs proper diagnosis of the cause. The most common cause is diabetic nephropathy; in this case, proper glycemic control may slow the progression. Medical management consists of angiotensin converting enzyme (ACE) inhibitors, which are typically first-line therapy for proteinuria. In patients whose proteinuria is not controlled with ACE inhibitors, the addition of an aldosterone antagonist (i.e., spironolactone)[27] or angiotensin receptor blocker (ARB)[28] may further reduce protein loss. Caution must be used if these agents are added to ACE inhibitor therapy due to the risk of hyperkalemia. Proteinuria secondary to autoimmune disease should be treated with steroids or steroid-sparing agent plus the use of ACE inhibitors.

• Albuminuria
• Microalbuminuria
• List of terms associated with diabetes
• Protein toxicity
• Major urinary proteins
• UPCR