A positive (+) result of protein in urine (proteinuria) may be a sign of renal disease, although it may be also due to a transient condition, such as strenuous exercise or dehydration.
Normal urine has very little protein present. The proteins founds consist primarily of low-molecular weight serum proteins that have been filtered by the kidney and proteins produced in the genitourinary tract.
The most common type of protein present in the urine is albumin due to its low molecular weight. However, its presence is usually low because the majority of albumin that reach the kidneys is not filtered and much of the filtered albumin is usually reabsorbed by the renal tubules.
Other proteins that may be present in the urine are:
The protein urine test is a screening procedure usually performed to detect kidney disorders. The presence of protein in the urine is often associated with early renal disease.
However, protein may be present in the urine when there is no renal disease or damage. For example, it may be due to a transient condition such as strenuous exercise or dehydration.
There are also other diseases not related to kidney problems that may produce protein in urine.
This test is performed along with other urine tests. The patient must collect a sample of the urine in a specific container using a special kit. This sample will be sent to the lab for the analysis.
Protein in urine is usually tested either with a dipstick or with a tablet that change of color in the presence of a high amount of protein in the urine.
In laboratories, there are automated machines that perform the analysis, but it is also possible to buy strips that let you analyze a urine sample at home. They comprise up to 10 or more different reagents or chemical pads to perform different urine tests at the same time.
Most dipsticks (reagent strips) detect only the presence of albumin. For most screening purposes, the dipstick method is adequate. However, this method does not detect the presence of abnormal proteins such as the globulins and the Bence Jones protein of myelomas. For this matter, it is necessary to use other testing methods to detect their presence, such as the sulfosalicylic acid precipitation test (SSA test) or the 24-hour urine collection.
The protein urine test must be negative to be considered within normality. For this matter, the level of protein in urine should be below 30 mg/dL (300 mg/L)
When protein in urine is above 30 mg/dL the test is considered positive.
Some dipsticks or labs report the trace readings in terms of negative, 1+, 2+, 3+, and 4+; or the semiquantitative values of 30, 100, 300, or 2000 mg/dL corresponding to each color change. The interpretation of trace readings can be difficult sometimes. To sum it up:
A positive result for protein in a routine analysis does not always indicate kidney damage. The positive require additional testing to determine whether the protein represents a normal or a pathologic condition. Benign proteinuria is usually transient and can be produced by conditions such as:
When the positive in the protein urine test is due to a pathologic condition, the main causes are:
The protein urine test may cause false-positive readings in case of:
The protein urine test may cause false-negative readings in case of a high presence of proteins different of albumin, as previously mentioned, or in case of microalbuminuria.