UG3, Semester 5
Clinical Biochemistry
Lecture 3
Non-Protein Nitrogen(NPN)
Compounds
Non-protein Nitrogen Compounds
• The determination of nonprotein
nitrogenous substances in the blood has
traditionally been used to monitor renal
function.
• Nitrogen containing compounds that are
not proteins or polypeptides
• Useful clinical information is obtained from
individual components of NPN fraction
Clinically Significant NPN
• The NPN fraction comprises about 15 compounds
• Majority of these compounds arise from
catabolism of proteins and nucleic acids
Urea Nitrogen (Blood) BUN
• Highest concentration of NPN in blood
• Major excretory product of protein
metabolism
• These processes release
nitrogen, which is converted to
ammonia
• Synthesized in the liver from CO2
and Ammonia that arises from
deamination of amino acids
Urea Nitrogen (Blood) BUN
• Assays for urea were based on measurement
of nitrogen, the term blood urea nitrogen
(BUN) has been used to refer to urea
determination.
• Excreted by the kidneys – 40% reabsorbed
• <10% of the total are excreted through the
gastrointestinal tract and skin.
• Concentration is determined by:
– Renal function
– Dietary intake
– Protein catabolism rate
Clinical Application
• Measurement of urea is used to:
– evaluate renal function,
– to assess hydration status,
– to determine nitrogen balance,
– to aid in the diagnosis of renal disease,
– and to verify adequacy of dialysis.
Disease Correlations
• Azotemia: elevated conc. of urea in blood
• Very high plasma urea concentration
accompanied by renal failure is called
uremia, or the uremic syndrome
• Causes of urea plasma elevations are:
– Prerenal
– Renal
– and postrenal
Pre-Renal Azotemia
• Reduced renal blood flow Less blood is
delivered to the kidney less urea filtered
– Anything that produces a decrease in functional
blood volume, include:
•
•
•
•
Congestive heart failure,
shock,
hemorrhage,
dehydration
• High protein diet or increased catabolism
(Fever, major illness, stress)
Renal Azotemia
• Decreased renal function causes
increased blood urea due to poor
excretion
– Acute & Chronic renal failure
– Glomerular nephritis
– Tubular necrosis
– & other Intrinsic renal disease
Post-Renal Azotemia
• Obstruction of urine flow
–Renal calculi
• Tumors of bladder or prostate
• Severe infections
Decreased Urea Nitrogen
•
•
•
•
Low protein dietary intake
Liver disease (lack of synthesis)
Severe vomiting and/or diarrhea (loss)
Increase protein synthesis
Analytical methods
• Assays for urea were based on measuring
the amount of nitrogen in the sample (BUN)
• Current analytic methods have retained this
custom and urea often is reported in terms
of nitrogen concentration rather than urea
concentration (urea nitrogen).
• Urea nitrogen concentration can be
converted to urea concentration by
multiplying by 2.14
Analytical methods
• Urease → hydrolysis of urea to ammonium ion ,
then detect ammonium ion (NH4+)
• Enzymatic
– The most common method couples the urease
reaction with glutamate dehydrogenase
Analytical methods
 Indicator dye
NH4+ + pH indicator → color change
 Conductimetric
◦ Conversion of unionized urea to NH4+ and
CO32- results in increased conductivity
Reference range of Urea N:
Serum or plasma: 6-20 mg/dl
24 hours Urine: 12-20 g/day