The accumulation of nitrogenous compounds in the blood constitutes the most characteristic biochemical change of renal insufficiency. The degree of retention is variously expressed in terms of the concentration of blood urea, blood urea nitrogen (BUN), or nonprotein nitrogen (NPN), and this may occasionally cause confusion. The BUN, widely used in the United States, and the blood urea level, more frequently employed in Europe, can be used interchangeably (by using a conversion factor of 2:1). The NPN however cannot be accurately converted to BUN.
Use of the NPN in clinical medicine stems from the earlier difficulties of measuring blood urea. At the present time, urea determinations are readily available and use of the NPN can no longer be recommended. However, since many laboratories continue to estimate NPN, its significance and its relation to the more commonly used BUN must be understood.
The NPN of blood includes all nitrogen that is
Dunea G, Freedman P. The Nonprotein Nitrogen Level of the Blood in Renal Disease. JAMA. 1968;203(13):1125–1126. doi:10.1001/jama.1968.03140130037009
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