Proteinuria is the presence of excess proteins in the urine.
In healthy persons, urine contains very little protein; an excess is suggestive of illness.
Excess protein in the urine often causes the urine to become foamy,
although foamy urine may also be caused by bilirubin in the urine (bilirubinuria),
retrograde ejaculation, pneumaturia (air bubbles in the urine) due to a fistula.
There are three main mechanisms to cause proteinuria:
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.
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.
Conventionally, proteinuria is diagnosed by a simple dipstick test,
although it is possible for the test to give a false negative reading,
even with nephrotic range proteinuria if the urine is dilute.
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.
Trace results may be produced in response to excretion of Tamm–Horsfall mucoprotein.
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 states 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.
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 30–300 mg/day are termed microalbuminuria which is considered pathologic.
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.
The most common cause is diabetic nephropathy; in this case, proper glycemic control may slow the progression.