Bedside Urinalysis – Blood

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All to often now doctors in well-resourced
hospitals rely on high tech, expensive and sometimes invasive tests to make a
diagnosis. One of the most basic and cheapest tools we have at our disposal is
the bedside urine ‘dipstick’ test. What can we learn from such an easy to
perform test? 
One urinalysis reagent strip or ‘dipstick’ costs about 14cents
(Pinnacle 10SG Urinalysis).
Finnian wrote two excellent blog posts on
urine specific gravity and osmolality and on urine albumin and protein measurements
using the ‘dipstick’ test. Here’s what else this bedside test can help you with.
Urine dipstick ‘Blood’ test
Heme acts as a pseudoperoxidase and when
exposed to the peroxide and a chromagen on the test pad a colour change takes
place.
The urine dipstick is a highly sensitive
test for the presence of heme in the urine and detects as few as 1 to 2 RBCs
per high power field.
The presence of urinary ascorbic acid is
one circumstance where false negative tests for hematuria occur. Some
manufactures make test strips that can oxidize ascorbic acid to reduce false
negative tests.
Semen in the urine can cause a false
positive test.
Of course the ‘blood’ panel turns
positive due to the heme in free urinary hemoglobin or myoglobin.
Thus the presence of RBCs in the urine
needs to be confirmed by microscopic analysis of the urine.
What disorders can we assess with the
‘dipstick’ test for blood/heme?
           
Rhabdomyolysis
Myoglobin is released from damaged muscle
along with CK. 
Myoglobin is not avidly bound to protein
and is thus rapidly excreted in the urine and has a half-life of about 2 hours.
It is also rapidly metabolized to bilirubin so serum levels return to normal
within 8 hours.
Thus, a diagnosis of rhabdomyolysis is not
ruled out by negative urinary myoglobin.
On must also be aware that there may also be heme or RBCs
present in the urine for other reasons during rhabdomyolysis.
Myoglobin appears in the urine when serum
levels are above 1.5mg/dl but visible changes in urine colour only occur when
serum levels are above 100mg/dl. A clear urinary supernatant usually helps to distinguish myoglobinuria from hemoglobinuria (red supernatant) after centrifugation.
Myoglobin can be detected by the urine
(orthotolidine) dipstick at concentrations of only 0.5 to 1 mg/dL.
Mean peak serum Creatinine Kinase (CK) levels in a large study of rhabdomyolysis
was 10,000 to 25,000. 46% of this cohort had AKI. In another study that included only patients with a CK over 5000 AKI was present in 51%.
A good history and physical exam helps
makes the diagnosis.
Hemolysis
Again this diagnosis is easy
with a good history and physical exam and some basic ‘hemolysis’ lab tests. What
can you do while waiting for the lab results?
The urine dipstick will be positive for
heme in the urine. In theory there will be no RBCs in the urine and the
supernatant will be red without a red cell pellet. However, there are caveats
to this, an old urine sample and rarely a very dilute urine sample (low specific
gravity – see Finnians post) may cause red cells present in the urine to
hemolyze. Urine microscopy should be done to look for presence or absence of RBCs.
Red urine supernatant but negative
dipstick ‘blood/heme’?
Rifampin, phenytoin, food dyes, beets
(beeturia), rhubarb, senna or Acute intermittent porphyria.
Renal or urological bleeding.
In either of these circumstances urine
dipstick tests will be positive for blood.

Hematuria present with proteinuria is suggestive
of a glomerular cause. Hematuria with proteinuria greater than 1+ is almost
never due to extra glomerular bleeding even when gross hematuria is present.
However, massive bleeding can cause proteinuria. Massive bleeding is more
likely to occur due to extra glomerular bleeding and the presence of clots and
very red or pink urine as opposed to Coca-Cola coloured urine indicates extra
glomerular bleeding.

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