The goals of testing for proteinuria include screening individuals who may be at risk, detecting the condition, determining its underlying cause, evaluating the type and quantity of protein being released, and evaluating kidney function. If proteinuria is detected, the person will be monitored at intervals to see if it resolves or becomes worse. Both urine and blood tests will be ordered to evaluate proteinuria.
Screening for protein in the urine may be performed as part of a general health exam or as part of a check-up for an individual who is known to have a condition that may cause proteinuria. Some screening tests include:
- Urine protein – detects the presence of any type of protein that may be in the urine. It can be performed alone on a random urine sample or as part of a urinalysis.
- Urinalysis – an evaluation of a urine sample for several different substances that may be in the urine, including protein. This test may be used as part of a general health exam.
- Urine albumin (microalbumin) – a sensitive test that is used to monitor people with diabetes for small amounts of albumin, the main blood protein, in the urine. Over time, diabetes can begin to affect kidney function and this test is an early indicator that diabetes has caused some kidney damage. The American Diabetes Association recommends that people diagnosed with type 2 diabetes be screened annually for low levels of albumin in the urine (microalbuminuria) and that type 1 diabetics be tested 5 years after diagnosis and annually thereafter.
A positive result on a screening test may be followed by further urine tests to determine how much protein and what type of protein is being lost in the urine:
- Urine albumin, 24 hour urine – measures the amount of albumin that is escaping into the urine within that timeframe. This test may give the healthcare practitioner a better assessment as to the degree of kidney damage.
- Urine albumin/creatinine ratio – as an alternative to collecting urine for 24 hours, a random urine sample may be used. In this case, creatinine is also measured. Creatinine is a substance that is released into the urine at a steady rate. When both protein and creatinine are measured in a random sample, a urine albumin/creatinine ratio can be calculated. This calculation corrects for the amount of creatinine in the random sample, more accurately reflecting how much albumin has been lost in the urine.
- Urine protein, 24-hour urine – measures the amount of protein released in the urine in a 24-hour period; this is a more accurate assessment of the degree of proteinuria than a random urine test.
- Urine protein/creatinine ratio (UPCR) – measures protein and creatinine in a random sample and corrects it for the amount of creatinine, similar to the urine albumin/creatinine ratio
- Urine protein electrophoresis – a test used to determine the different types and relative concentrations of protein present in the urine. A urine test specifically for detecting the presence of Bence-Jones protein is sometimes used when multiple myeloma is suspected.
- When the protein electrophoresis shows an abnormality, an immunofixation test may be performed to quantify the abnormal protein, in addition to an immunoassay evaluation for free light chains.
In addition to testing urine, there are several other tests that may be used to evaluate kidney function and/or assess the nature of the protein present in the urine. These tests may be done at the same time as urine protein screening or in follow-up. The tests include:
- BUN (Blood Urea Nitrogen) and Creatinine – blood tests used to evaluate kidney function; urea and creatinine are nitrogen-containing waste products that healthy kidneys move from the blood to the urine. If the kidneys are not functioning properly, urea and creatinine will remain in the blood and the levels will increase. (Note: Although creatinine may be measured in urine samples, a blood sample is also measured to be included as part of the evaluation.)
- eGFR (estimated Glomerular Filtration Rate) – uses a blood creatinine level along with age and values assigned for sex and race to calculate the estimated rate of urine filtration; the eGFR rate decreases with progressive kidney damage.
- Creatinine clearance – measures creatinine in a 24-hour urine sample and a blood sample to calculate the amount of creatinine that has been cleared from the blood and passed into the urine; this calculation allows for a general evaluation of kidney function based on the rate of creatinine excretion from the body.
- Total Protein (TP) – a blood test that measures all of the protein in the serum
- Albumin – a blood test that measures the concentration of albumin (the most prevalent protein in blood serum)
- Serum protein electrophoresis – determines the types and relative amounts of protein in blood serum and is compared to the urine electrophoresis pattern to determine if blood is the source of the protein seen in the urine
- Serum Free Light Chains (SFLC) – a blood test used to help diagnose and monitor conditions associated with an increased production of free light chains such as multiple myeloma
A healthcare practitioner may also order a kidney biopsy. In a kidney biopsy, a small sample of the kidney tissue is examined with a microscope by a pathologist and may be used to look for evidence of kidney disease or damage. (For more, read the article on Anatomic Pathology.) Additional studies using electron microscopy, immunohistochemistry, and immunofluorescence may also be performed to further evaluate a kidney biopsy.
- Imaging scans of the kidney may be performed to detect the presence and determine the severity of kidney disease or damage (For more on these, visit RadiologyInfo.org.)
- Blood pressure may be measured as part of an investigation of the cause of proteinuria. It is frequently monitored in people who have hypertension or are at risk of developing it.