"Uraemia" is a term that tends to be applied to describe the syndrome associated with an accumulation of all sorts of renally cleared waste products, and is not necessarily referring to the accumulation of urea alone. However, urea is the most abundant waster product among those which accumulate, and it seems to have some important toxic effects. Urea has been asked about in:
Question 20.1 from the second paper of 2020
Question 6.3 from the second paper of 2012
Question 6.2 from the first paper of 2008
Specifically, the college had asked about the meaning of an isolated raised urea in a patient with trivially elevated creatinine. This will be addressed first, so that the time-poor exam candidate is able to rapidly refer to a table of differentials and clinical features. The excellent LITFL page on this topic was a major source of inspiration for this chapter.
Increased exogenous urea
Decreased urea clearance
Increased renal reabsorption of urea
Decreased renal clearance of urea
Increased urea synthesis
Increased protein intake
Increased protein catabolism
Post-renal obstruction probably causes the most wildly raised urea that one might see in the clinical workplace.
The clinical manifeststaions of uraemia are of course the consequence of the accumulation of numerous different uraemic toxins. Urea itself has surprisingly few ill effects. The following physiological effects have been identified from experiment and observation:
However, urea is probably one of the minor players in the pathogenesis of uraemic syndrome. It just happens to be a convenient marker solute. Specifically, an increased clearance of urea does not seem to correlate with improved survival (Eknoyan et al, 2002) - which it would, if urea was really the culprit. Moreover, the addition of urea to dialysate did not make any difference in uraemic symptoms (Johnson et al, 1972). Urea has no effect on neurology or platelet function.
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