Urine Protein
The protein pad on the multireagent dipstick (Multistix®,
illustrated above) is based on the "protein error of pH indicator dyes".
Basically, the test is dependent on the ability of amino groups in proteins
to bind to and alter the color of acid-base indicators, even though
the pH is unchanged. The reaction is extremely sensitive to albumin
(as it contains the most amino groups), but is much less sensitive to
globulins. It is insensitive to Bence-Jones proteins. Generally this
differential sensitivity is not a significant problem (nearly all cases
of significant proteinuria involve albuminuria).
The table below illustrates how results from the dipstick (Multistix)
correlates to those from the Criterion, the machine used to "read"
the dipstick at Cornell University.
|
Multistix
|
Criterion
|
|
Neg
|
Neg
|
|
Trace
|
15 mg/dL
|
|
1+ (30 mg/dL)
|
30 mg/dL
|
|
2+ (100 mg/dL)
|
100 mg/dL
|
|
3+ (300 mg/dL)
|
≥500 mg/dL
|
|
4+ (>2000 mg/dL)
|
≥500 mg/dL
|
Interpretation
The urine protein results should always be interpreted in context with the urine specific gravity and pH. Normal urine contains little protein; negative to trace reactions are usual in concentrated urine. A trace to 1+ reaction in a very dilute urine is suggestive of significant proteinuria. A dipstick protein reaction > 2+ in concentrated or dilute urine indicates significant proteinuria. There are numerous causes of proteinuria, the most common of which are urinary tract inflammation,
hematuria, and glomerular disease.
False positive results
- Alkaline urine: False positives occur rarely in highly buffered
or alkaline urine samples as the citrate buffer is overcome, resulting
in a shift in pH. Titration of the sample to a more neutral pH and
retesting could be done to overcome this problem, but is not routinely
performed. The sulfosalicylic acid precipitation test (see "protein
SSA ") used to be performed as an alternative, however several
studies have shown that the urine dipstick meaurement of protein is
reasonably accurate in most urines, even those that are alkaline,
and the SSA reaction is no longer a necessary part of routine urinalysis.
- Contact time: Leaching of the citrate buffer occurs if the urine remains in contact with the pad for a long time.
- Detergents: Quaternary ammonium compounds and chlorhexidine can result in false positives.
False negative results
- Bence-Jones proteinuria: A positive SSA protein reaction,
with a negative or weak dipstick protein reaction, in a dog or cat
with a high index of suspicion for multiple myeloma, is suggestive
of the presence of free light chains in the urine.
© Cornell University
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