Urea
45-50%
% in total NPN
· protein catabolism o Biosynthesis of urea from ammonia is exclusively carried out in the liver by hepatic enzymes
Major metabolite product of
· Measured as BUN - Blood Urea Nitrogen o Only the nitrogen part of urea is quantitated in BUN and is correlated with the actual concentration of the entire urea compound.
Measured as
through the kidneys o Remainder excreted through the GI tract and skin (<10%) o 40%-60% is reabsorbed by passive diffusion during passage of the filtrate through the renal tubules
Most of the urea formed is excreted
acute and chronic renal diseases
Persistently increased blood urea is also seen in renal condition like
o High urea N/creatinine ratio with an elevated creatinine
Postrenal ratio
o Elevated plasma urea; Normal plasma creatinine § High urea N/creatinine ratio
Prerenal ratio
· Used as an indicator of overall nitrogen balance and as a guide to total amino acid requirements for patients with parenteral nutrition · Timed urine specimen, specifically 24-hr urine collection should be used for urine urea determination · Urine samples are stable up to 4 days if stored at 2 - 8oC
Specimen Requirements - urine
§ Caused by low protein intake, acute tubular necrosis, and severe liver disease
o Conditions associated with decreased urea production - Low urea N/ creatinine ratio
§ Due to obstruction of urine flow anywhere in the urinary tract · Caused by renal calculi, tumors of the bladder or prostate, or severe infection
o Postrenal
§ Problem in systemic circulation § Result of reduced renal blood flow § Less blood is delivered to the kidney > Less urea is filtered and excreted
o Prerenal
· Congestive heart failure, shock, hemorrhage, dehydration and other factors resulting in a significant decrease in blood volume Amount of protein metabolism · High protein diet · Increased protein catabolism o Occurs in stress, burns, fever, major illness, corticosteroid therapy, and GI hemorrhage
o Prerenal causes
§ Decreased renal function causes an increase in plasma urea concentration as a result of compromised urea excretion
o Renal
· Urea N/creatinine ratio o Normally 10:1 to 20:1
o Used to differentiate the cause of abnormal urea concentration
o Major causes: Low protein intake and severe liver disease o Poor nutrition, high fluid intake, overhydration, increased protein synthesis
· Decreased plasma urea concentration
o Used to evaluate renal function o Assess hydration status o Determine nitrogen balance o To aid in the diagnosis of renal disease o Verify adequacy of dialysis
· Functions of measurement of urea
o Protein content of the diet o Rate of protein catabolism Renal function and perfusion
· The concentration of urea in the plasma is determined by
· Post renal obstruction like kidney stones and urinary tract infection may cause mild increase blood urea concentration o May also increase in some state of dehydration and muscle wasting as in starvation
· cause mild increase blood urea concentration postrenal
An elevated concentration of urea in the blood
Azotemia
o High protein diet o Increased protein catabolism § Hemorrhage into GI tract § Shock Some chronic liver diseases
Increased by numerous factors prerenal
· Also known as carbamide, an organic compound with chemical formula CO(NH2)2 o This amide has two -NH2 groups joined by a carbonyl (C=O) functional group.
Also known as
§ Acute and chronic renal failure, glomerular nephritis, tubular necrosis, and other intrinsic renal disease
Renal causes
· Serum or plasma may be used for the measurement of urea concentration o Sodium citrate and sodium fluoride should NOT be used in plasma collection (Inhibits urease) · Fasting sample is not required because the effect of a single protein-containing meal is very minimal
Specimen Requirements - serum
Urea is often reported in terms of nitrogen concentration rather than urea concentration
Urea is often reported in terms of
Renal failure accompanied by very high plasma urea concentration o Eventually fatal if not treated by dialysis or transplantation
Uremia/Uremic Syndrome
