What is it?
Proteinuria is a condition characterised by the presence of greater than normal amounts of protein in the urine. It is associated with a variety of different diseases and is sometimes seen in those who are apparently healthy. Mild or transient proteinuria may become more severe over time.
Plasma, the liquid portion of blood, contains many different proteins. One of the many functions of the kidneys is to retain plasma protein so that it is not excreted along with waste products when urine is produced. There are two mechanisms that normally prevent protein from passing into urine: (1) the glomeruli in the kidney provide a barrier that keeps most larger plasma proteins inside the blood vessels but allows water and small proteins to be filtered out and (2) the small proteins that do get through are almost entirely reabsorbed by the tubule.
Proteinuria most often occurs when either the glomeruli or tubules in the kidney are damaged. Inflammation and/or scarring of the glomeruli can allow increasing amounts of protein and sometimes red blood cells (RBCs) to leak into the urine. Damage to the tubules can prevent protein from being reabsorbed, meaning proteinuria may also develop when the tubules cannot reabsorb all of a small protein which has been filtered from the blood.
A variety of diseases and conditions can cause kidney damage which can result in proteinuria. Two of the most common causes are:
- diabetes
- hypertension
Others causes include:
- immune disorders (e.g. systemic lupus erythematosus (SLE), IgA nephropathy, Goodpasture’s syndrome)
- infections
- exposure to toxins
- trauma
- kidney cancer
- pre-eclampsia
- multiple myeloma
Pregnant women are frequently screened for proteinuria because its development is associated with pre-eclampsia (also know as toxaemia), a hypertensive disorder that can cause oedema, nausea, and headaches during pregnancy. Pre-eclampsia can be dangerous for both the mother and her baby.
Proteinuria due to the presence of excess small proteins in blood may be seen in multiple myeloma (free immunoglobulin light chains) and in conditions that damage red blood cells in the circulation and release of haemoglobin which appears with its red colour in the urine.
Healthy people can have short term or long term proteinuria. It is associated with stress, exercise, fever, aspirin therapy, and exposure to cold. Some people excrete more protein into the urine when they are standing up than when they are lying down (orthostatic proteinuria).
Signs and symptoms
Frequently, no symptoms are seen with proteinuria, especially in mild cases. Large amounts of protein may cause the urine to appear foamy. Loss of large amounts of protein from the blood can affect the body’s ability to regulate fluids, which can lead to swelling in the hands, feet, abdomen and face. When symptoms are present, they are usually associated with the condition or disease causing proteinuria.
The reasons for looking for protein in the urine include screening those at risk, looking for a cause for proteinuria, evaluating the type and quantity of protein being released, and assessing kidney function. When proteinuria is detected, patients are monitored at intervals to see if it gets better or becomes worse. Both urine and blood tests may be requested to evaluate proteinuria.
Laboratory Tests
Several tests may be requested on either 24-hour or random urine samples:
- Urinalysis– evaluation of a urine sample by visual inspection, chemical testing with a simple dip stick and, in some cases, microscopic examination.
- Microalbumin, 24-hour urine – a more sensitive test for proteinuria that is used to detect and monitor kidney function in diabetic patients.
- Urine protein, 24-hour urine – measures the amount of protein passed in the urine over a 24-hour period; this is a more accurate assessment of the degree of proteinuria than a random urine measurement.
- Microalbumin/creatinine ratio (ACR), random urine - measures albumin in a random sample and corrects for the volume of urine passed by measuring the amount of creatinine in urine (a substance released by the body at a steady rate); this is more convenient than having to collect a 24-hour sample.
- Urine protein/creatinine ratio (UPCR), random urine - measures protein in a random sample and corrects for the volume of urine passed by measuring the amount of creatinine in urine; this is more convenient than having to collect a 24-hour sample.
The following tests may also be requested and are measured primarily on blood alone:
- eGFR (estimated Glomerular Filtration Rate) - uses a blood creatinine level to calculate the estimated rate of urine filtration; rate decreases with progressive kidney damage.
- Creatinine - a blood test used to evaluate kidney function; less frequently, it may be done on urine. (Note: Although creatinine may be measured in urine samples, it is usually measured to be included as part of a ratio or calculation.)
- Total Protein - a blood test that measures all of the protein in the serum.
- Albumin - a blood test that measures the concentration of albumin in blood.
A kidney biopsy may also be requested. This is a procedure that is sometimes done to look at a small sample of kidney tissue under the microscope to look for evidence of kidney disease or damage.
Non-Laboratory Tests
- Imaging scans of the kidney to detect the presence and determine the severity of kidney disease or damage.
- Blood pressure; may be measured as part of investigation of cause of proteinuria; frequently monitored in those who have hypertension or are at risk of developing it.
The goals of treatment are to control the underlying conditions causing proteinuria and to minimise its progression. Treatment for each condition is likely to be different. For example, controlling blood sugar levels in patients with diabetes will help to preserve kidney function. In patients with hypertension, it is important to control blood pressure to prevent progressive kidney damage. Pre-eclampsia-related proteinuria usually resolves once the baby is born.
In some patients with persistent proteinuria, the doctor may recommend dietary changes such as altering the amount of protein consumed. No treatment may be necessary for mild or transient proteinuria.