Proteinuria
The significance of proteinuria should always be evaluated in light of the
method of collection, urine specific gravity, and measurement technique. The
presence of protein in a urine sample collected by catheterization or manual
expression may contain blood which by definition contains RBC and blood
proteins; the proteinuria in that case is iatrogenic. The amount of protein
assessed by qualitative methods such as colormetric dip sticks becomes more
significant as the urine decreases in urine specific gravity. A +1 protein in a
urine sample with a 1.003 specific gravity is much more significant than a +1
protein reading in a urine sample with a 1.030 specific gravity.
Protein can be measured by semiquantitative methods
including colormetric dip sticks, turbidometric methods or by quantitative
methods. The semiquantitative methods of protein measurement may result in
erroneous results including both false positive and false negative results.
Quantitative methods are less likely to return false positive or false negative
results.
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Colormetric dip stick protein readings
The changes on the color pads range from negative to +4 with the following
approximate concentration correlations:
| dipstick reading |
concentration |
| trace |
20 mg/dl |
| +1 |
30 mg/dl |
| +2 |
100 mg/dl |
| +3 |
300 mg/dl |
| +4 |
2000 mg/dl and higher |
The chemicals on the color pads are more sensitive to the detection of
albumin than globulins, Bence Jones proteins (the light chains of immunoglobulin
molecules) or other proteins. These other proteins can be detected by the
dipstick but only when present in large quantities. The lower limit of
sensitivity is ~ 20 mg/dl protein.
The following readings are considered normal
| urine specific gravity |
urine protein |
| <1.020 |
negative |
| 1.020-1.035 |
+1 |
| >1.035 |
+2 |
The dipstick may register a false negative if the
urine is very dilute urine. A false negative means that a significant amount of
protein is actually present but the large urine volume dilutes the protein so
its concentration is less than 20 mg/dl which is the lowest concentration
recorded by the dipstick. The presence of Bence Jones proteins may also result
in a false negative (may not be measured as protein). False positive color
changes may be recorded if the urine sample is contaminated with the antiseptic,
chlorhexidine (Novalsan) or if the urine sample is markedly alkaline. The H2
blocker, ranitidine, may also cause false positive readings. The range of pH of
dog and cat urine in both health and disease rarely results in a pH high enough
to result in a false positive reading.
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Turbidimetric methods for protein measurement
Turbidimetric
tests are based upon the precipitation of protein by the addition of
sulfosalicylic acid (SSA) resulting in relative degrees of turbidity of
the urine sample. The sample on the far left is strongly positive (+4)
for protein and the sample on the near left is negative. This test has a
lower limit of sensitivity of ~ 5 mg/dl protein.
False positive readings may occur if the urine contains radiographic
contrast agents or large amounts of antibiotics including penicillins,
cephalosporins or sulfonamides. In treatment of higher bacterial infections,
actinomyces and nocardia, the doses of penicillins (actinomyces) or
sulfonamides (nocardia) used in treatment, often exceed the doses of the
same drugs used to treat other diseases.
Most laboratories routinely perform SSA testing on all urine samples or at
least those that are positive by dip stick.
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A new protein detection test the
E.R.D.
screening test was introduced by the
Heska
Corporation in 2002. The test is an in-office
immunoassay that detects low levels of albumin in the urine of dogs. It
is specific for albumin, not other types of protein. This test detects
microalbuminuria which is defined as a concentration of albumin greater
than 1 mg/dl but less than 30 mg/dl which is approximately the lower
limit of protein detection using a dipstick.
Microalbuminuria has been reported to be an early indicator of
progressive renal disease in humans with hypertension and diabetic
nephropathy. Studies are underway in dogs to determine if the ERD test
can be used to detect dogs with early renal dysfunction. Some
preliminary reports, published as ACVIM abstracts are posted at:
http://www.heska.com/erd/clinical_data.asp
In addition to this test detecting lower concentrations of albumin
compared with dipsticks, studies performed by the manufacturer state
that some dogs with trace positive protein using a dipstick are false
positives in 66% (33 of 50 urine samples) as defined by a negative ERD
test.
Persistent microalbuminuria indicates ongoing glomerular damage which
may be due to primary renal disease or secondary to a variety of
systemic diseases. This does not mean that all persistently
microalbuminuric dogs will progress to end stage renal disease although
the manufacturer states that persistently microalbuminuric dogs are “at
risk” for developing end stage renal disease.
Blood contamination will interfere with the ERD test when the urine sample
is visibly discolored. The affect of lower urinary tract inflammation on
ERD results is variable. In one study about half of dogs with lower
urinary tract inflammation were negative for microalbuminuria. Therefore
a positive ERD test in the presence of lower urinary tract inflammation
does not diagnose nor preclude the existence of underlying glomerular
disease. Re-testing after successful treatment of the lower urinary
tract disease is recommended.
Preliminary results suggest that corticosteroids may increase the severity
of microalbuminuria measured by the ERD test.
"A screening test is now also available for cats. 1243 cats visiting
59 clinics were tested for microalbuminuria in their urine. 24.5% of
cats tested positive for microalbuminuria with the E.R.D.-HealthScreen™
Feline Urine Test. A statistically significant correlation (P <0.0001)
was found between increasing age and a microalbuminuria positive test
result. Observed prevalence of microalbuminuria in apparently healthy
cats and increased prevalence of microalbuminuria in cats with medical
conditions support routine testing to assess the current health status
of the patient and to alert the veterinarian to identify and treat
potential causes of renal damage."
http://www.heska.com/erd/erd_datacat.asp
Quantitative tests
The dipstick readings are "semi quantitative" with the degree of positivity
providing some estimate of the amount of protein contained in the urine sample.
Quantitative tests (e.g. biurete method) use chemical methods to provide an
actual measure of protein in the urine. Urine protein should be quantitated when
glomerular proteinuria is suspected in order to provide an objective baseline
against which to compare the effects of treatment and to follow over time in
order to determine progression or regression of disease. Quantitative tests
should also be performed if the results of semi quantitative methods are in
question (e.g. you are concerned that dipstick or turbidometric methods are
yielding false positive or false negative results). Quantitative tests require a
timed collection of urine or can be accurately estimated by a protein/creatinine
ratio.
Timed urine collection
To perform a timed urine collection, collect all the urine produced over a
period of at least 24 hours. The longer the collection period, the less
significant are errors in which part of the urine produced is lost to
collection. See the section on creatinine clearance for details of timed urine
collections. Measure the volume of urine produced and determine what the 24 hour
urine production would be. Send a sample of the collected urine to a laboratory
for a chemical determination of protein concentration. This will be reported in
units of mg/dl or mg/ml. Multiply the 24 hour urine volume by the protein
concentration to determine total quantity of protein lost in the urine over 24
hours.
urine protein (mg/dl) X urine volume/24hours (dl) = mg of protein/24hours
one deciliter (dl) = 100 ml
Normal ~ < 22mg/kg/day
Example:
50 kg dog
24 hour urine production = 1,500 ml
protein concentration of 10 mg/dl
15 dl x 10 mg/dl = 150 mg
150 mg/ 50 kg = 3 mg/kg/24 hours
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Protein/creatinine ratio (UPC ratio)
The protein/creatinine ratio (UPC ratio) is not affected by urine
concentration or volume and correlates well with 24 hour urine protein
excretion. Because the correlation of UPC ratios to timed 24 hour protein losses
is good, there is little reason to perform timed urine collection to determine
magnitude of proteinuria. UPC ratios are determined from a single urine sample.
The urine can be collected by any collection method although potentially
traumatic methods such as catheterization and manual expression can create
iatrogenic hematuria and the blood proteins will result in a high upc ratio.
There is little information be gained from performing a UPC ratio on grossly
bloody urine. Remember that blood proteins are measured in units of g/dl whereas
urine protein is usually measured in mg/dl.
UPC results
- < 0.5 normal
- 0.5 - 1.0 questionable significance
- > 1.0 abnormal
Formulas and graphs are published in text books that will allow you to
convert UPC to a daily urine protein loss in units of mg/day. These formulas are
specific to the technique used to measure urine protein. UPC ratio's do not
identify the underlying disease process or location of lesion (pre- or
postglomerular, glomerular or tubular).
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Sources of urine protein
The blood entering the afferent arteriole into the capillary loop of the
glomerulus contains plasma protein at an approximate concentration of 6 g/dl and
an albumin concentration of approximately 3 g/dl. Albumin is small enough that
some of it will leave the capillary lumen and enter glomerular filtrate. Because
a large amount of glomerular filtrate is made (as much as 100 liters in large
dogs), even though the concentration of albumin in glomerular filtrate is low
(~2 mg/dl) the actual amount of albumin in glomerular filtrate is large.
The epithelial cells lining the renal tubules will reabsorb most of the
albumin from glomerular filtrate before it is excreted as urine. Additionally
the tubular epithelial cells secrete Tamm Horsfall mucoproteins into tubular
fluid.
As the urine is eliminated through the urethra, the genital tract adds
proteins to the urine. Immunoglobins (IgA) which are locally produced in the
urinary tract are also added to the urine. Therefore what is measured in the
urine as protein may originate from all of the above sources.
Types of proteinuria
- functional (transient)
- over flow (preglomerular)
- glomerular
- tubular
- secretory (post glomerular)
A transient increase in glomerular permeability can
occur as a consequence of heavy physical activity (exercise, seizures), fever,
or congestive heart failure resulting in proteinuria in the absence of renal
disease. The proteinuria is reversible if the under lying disease/circumstance
which caused the increased permeability is reversible. If one of the previous
circumstances/diseases exists, urine protein should be re evaluated several
times to determine if it is transient or permanent. This type of proteinuria is
also called "functional" proteinuria.
Over flow (preglomerular) proteinuria is the
presence of protein in the urine that originated from a small protein which is
increased in blood. Substances which may be increased in the blood and which are
small enough proteins to pass the glomerular filtration barrier and enter the
urine include hemoglobin, myoglobin, or Bence Jones proteins. Bence Jones
proteins are the light chains of immunoglobin molecules which are produced by
abnormal plasma cells in patients with multiple myeloma (plasma cell tumor).
Bence Jones proteins will not be measured by the semiquantitative methods of
protein determination but will be measured by quantitative methods. To determine
if proteinuria is overflow, look at the patient's blood. Is the plasma red
indicating the presence of free hemoglobin from intravascular hemolysis or free
myoglobin released from muscle damage? Is the plasma globulin markedly increased
as it will be in patients with multiple myeloma? If the protein in the urine is
hemoglobin or myoglobin the urine sample will be red colored.
A small amount of albumin will normally
pass the glomerular filtration barrier
and enter tubular fluid. In health, tubular epithelial cells will reabsorb most
of the filtered albumin and return it to the blood. If the tubules are diseased
they may not reclaim the filtered albumin resulting in its loss in urine. Other
evidence of tubular malfunction includes failure to reabsorb filtered glucose
leading to glucosuria, isosthenuric urine specific gravity indicating inability
to remove water from tubular filtrate and the presence of
casts.
Glomerular proteinuria is generally caused by either immune complex
glomerulopathy or amyloidosis.
Secretory (post glomerular) proteinuria originates
from hemorrhage, inflammation, or infection of the kidneys, lower urinary tract
or genital tract. When ever RBC or WBC are observed in the urine sediment,
protein will also be present as blood contains both RBC and blood proteins and
WBC are usually accompanied by proteins (immunoglobulins).
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Assessing the
significance of proteinuria
To summarize, when a urine sample contains protein you should evaluate the
significance of that protein as follows:
- it may be normal, if a small amount is present in a concentrated urine
sample
- rule out false positives (dipstick
or turbidometric)
- determine if persistent
- look at blood for possible causes of protein
overflow and evaluate urine color (hemoglobin or myoglobin will color
the urine red)
- rule out secretory proteinuria by
a sediment exam
- determine if
tubular by looking for other signs of tubular dysfunction
- if everything above is not true, then the protein is glomerular in origin
Last Edited: Apr 24, 2008 1:57 PM
