Why is My Urine Purple? A Review of the "Hospital Medicine Podcast" 8/30/12, by Dr Gill Porat, MD

This is a review of "The Hospital Medicine Podcast- Uninalysis Pearls for Managing Adult Patients" added on 8/30/2012 which can be found at  https://itunes.apple.com/us/podcast/hospital-medicine-podcast/id541752791 along with many other of his podcasts


    So I listened to this quick podcast on the way to work this morning and it was prety decent. It was a brief discussion on urinalysis. First, Dr. Porat spoke about nitrates. Common urinary bugs convert nitrate into nitrite in the urine, but there are circumstances when this does not happen. There are some bugs, like yeast and E. faecalis, that do not make nitrates. Also, which is particularly interesting, is that if the patient has urinary frequency, and is emptying their bladder often, then the bacteria may not have TIME to make the nitrites. Thus don't assume that a negative nitrate means that the patient doesn't have a UTI.
     Leukocyte esterase is another test that commonly comes back positive on a UA. But where does it come from. Well ,WBC's make it , and it is also released when WBC's get killed, such as when battling an infection. If they all get killed, there will be a positive LE, but you may not see them in the UA... AH HAH! Kind of like in all those action movies where the good guys come to the enemies camp and the bad guys aren't there. The water may even be boiling on the stove and the bacon may still me sizzling... Bear in mind that contamination may give a false positive and lack of pyuria my make it false negative.
    The big "blow my mind" teaser was this idea of purple urine. And no, its not from diluted hematuria. Its from gram negative bacteria. According to Dr. Porat, the GI tract breaks down tryptophan into indole.  The bacteria then converts indole into indigo. This will then "stain the polyvinyl chloride foley bag purple". So the point being is that you have to send the urine for cultures.  While looking for other resources about this reaction, I came across a reference that said that purple urine can also be found in porphyria.
    He also mentions that brown urine can be seen in "diffuse melanosis" and that green urine can be from pseudomonas (or medications). For completeness, red urine is bloody (or myoglobin), milky or cloudy urine is pusy,  orange urine can be from medications or your diet.
 The last important point that Dr. Porat is about urine pH. I'll let you check the podcast if you want to know what he says (TEASER!)





1.MacFaddin, Jean F. "Biochemical Tests for Identification of Medical Bacteria." Williams & Wilkins, 1980, pp 173 - 183

2. http://www.mayoclinic.com/health/urine-color/DS01026