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Correct Answer: exposure to light and photo oxidation
when discussing the stability of urine samples and the accuracy of urinary bilirubin measurements, one crucial factor that often comes into play is the preservation of the sample. if urine is unpreserved, it is prone to several changes that can affect the accuracy of diagnostic tests, including the bilirubin count. the correct answer to why an unpreserved urine sample may show a decreased bilirubin count is due to exposure to light and photooxidation. here is an expanded explanation of why this occurs:
bilirubin is a yellow compound that occurs in the bile and is produced by the breakdown of heme in red blood cells. it is normally processed by the liver and excreted in bile and urine. when measuring bilirubin in urine, it serves as a useful diagnostic marker for various hepatic and biliary disorders. however, the integrity of bilirubin in a urine sample can be compromised by several factors, with exposure to light being a predominant one.
exposure to light leads to the photooxidation of bilirubin. photooxidation is a chemical process where light energy catalyzes the oxidation of bilirubin, converting it into products like biliverdin and other derivatives that are not measured as bilirubin in urine tests. this means that when a urine sample containing bilirubin is left exposed to light, the bilirubin content decreases due to its conversion to other compounds. this results in a falsely low bilirubin count, which could mislead diagnostic conclusions.
the importance of preventing photooxidation is paramount in maintaining the accuracy of urine tests. clinically, this is managed by storing urine samples in amber-colored or opaque containers that block or filter out light, thereby protecting the bilirubin from degradation. additionally, immediate processing or refrigeration of urine samples can help reduce the risk of photooxidation and other metabolic activities that can alter the composition of the urine.
while other factors like bacterial growth, the breakdown of urea, and the activity of nitrate-reducing bacteria can also affect the components of urine, they do not specifically cause a decrease in bilirubin due to photooxidation. these factors might lead to other diagnostic inaccuracies by altering different substances in the urine. for instance, bacterial growth can consume glucose in the urine, and urea breakdown can increase the ammonia concentration. however, in the context of bilirubin stability, the primary concern is its vulnerability to light-induced degradation.
by understanding these mechanisms, healthcare providers can ensure more reliable urine testing and accurate interpretation of results, particularly in the assessment of liver function and the diagnosis of related disorders.
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