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?
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.
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.