When it comes to monitoring proteinuria, some nephrologists prefer following urine albumin levels, whereas others prefer monitoring urine protein levels. While there is nothing inherently wrong with either, it is helpful to keep the following reference values in mind, particularly when a single patient has both proteinuria and albuminuria values in his/her medical record.
First, however, one must consider the definition of “proteinuria.” Everybody has some degree of proteins in their urine: what defines an “abnormal” degree of proteinuria has been the result of countless epidemiological studies and is still being refined today. It has been argued that our threshold of what constitutes “abnormal” should probably be lowered. In a normal individual, the standard breakdown of normal urine protein is about
- 40% albumin
- 40% Tamm-Horsfall protein produced by tubular cells
- 20% low molecular weight immunoglobulins.
Dipstick analysis is usually the first (and cheapest) test which reveals the existence of proteinuria. In the absence of protein, the dipstick panel is yellow; proteins in solution interfere with the tetrabromophenol indicator, resulting in the panel turning various shades of green. Of note, this technique does not detect urine immunoglobulins, so this may miss patients with paraproteinemias.
The limit of detection of the urine dipstick (converting to the familiar “mg/day”) for urine protein is about 150mg/day, or about 300mg/day of urine albumin.
However, it’s well-known that microalbuminuria–the existence of abnormally high amounts of albumin in the urine despite having a negative urine dipstick.