Oliguria is a decrease in urine output to less than 0.5 ml/kg/hr. It is often a symptom of some quite serious disorder and should be taken seriously.

More common disorders associated with oliguria are renal failure and shock which might be caused by trauma, toxins, obstruction, diabetes, infection, etc. If oliguria is accompanied by nausea or vomiting, it should be considered quite serious and consultation with a physician should become a priority.

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http://www.ilpi.com/msds/ref/oliguria.html
http://health.allrefer.com/health/urine-output-decreased-expect.html
http://www.merck.com/mrkshared/mmanual_home2/sec11/ch143/ch143b.jsp