Refinements and Application of Urea Modeling

  • Thomas A. Depner
Part of the Developments in Nephrology book series (DINE, volume 29)


Nonprotein nitrogenous compounds (NPN) are the major excretory constituents of urine, so serum levels are a measure of renal function or clearance. The most abundant NPN compound in both serum and urine is urea. Urea constitutes approximately 50% of serum NPN in people with normal renal function and a higher percentage in those with renal failure (1). Traditionally, clinical laboratories report the serum urea concentration as urea nitrogen in milligrams per deciliter. This unusual practice of expressing urea concentration as urea nitrogen instead of whole urea is better understood after reviewing the history behind blood testing of renal function. Specific tests for urea have replaced the older clinical tests for nonprotein nitrogen. These older assays for total nitrogen in deproteinized serum measure a variety of nitrogenous compounds, including creatine, creatinine, uric acid, and amino acids, so the sum of their concentrations had to be expressed in terms of their common element, nitrogen. Specific urea assays replaced the more tedious NPN test, but the results continued to be reported as milligrams of nitrogen per deciliter, a more familiar term to the clinicians of old. The two nitrogen atoms in each urea molecule make up nearly half (28/60) of its molecular weight, so urea nitrogen concentrations are slightly less than half the value of total urea concentrations. Today, the assays for blood urea are more specific than they were in the past, but the results are often expressed in the same units as the older tests for nitrogen.


Urea Concentration Dialysate Flow Urea Transport Plasma Water Ultrafiltration Rate 
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Copyright information

© Kluwer Academic Publishers 1991

Authors and Affiliations

  • Thomas A. Depner
    • 1
  1. 1.University of CaliforniaDavisUSA

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