Urea measurement BIOCHEMISTRY AND PHYSIOLOGY Urea CONH 22

Urea measurement

BIOCHEMISTRY AND PHYSIOLOGY • Urea (CO[NH 2]2) – 75% of the nonprotein nitrogen eventually Excreted. • Protein catabolism – Amino acid nitrogen is converted to urea – in the liver by urea cycle • Urea is excreted through – The kidneys, More than 90% • Kidney disease, accumulation of urea in blood. – Gastrointestinal tract and skin

BIOCHEMISTRY AND PHYSIOLOGY • Uremic (azotemic) state. – Increase in plasma urea

The urea cycle pathway • • CPS I, Carbamyl phosphate synthetase I; *N-acetylglutamate as positive allosteric effector; OTC, ornithine transcarbamylase; MS, argininosuccinate synthetase; AI. , argininosuccinate lyase; AR, arginase; ADP, adenosine diphosphate, PI , inorganic phosphate.

CLINICAL SIGNIFICANCE • Pre-renal conditions – High-protein diet – Increased protein catabolism – Reabsorption of blood proteins after gastrointestinal hemorrhage – Treatment with cortisol – Dehydration – Decreased perfusion of the kidneys • Heart failure

CLINICAL SIGNIFICANCE • Pre-renal conditions – Creatinine concentration may be normal. • Postrenal conditions – Malignancy, nephrolithiasis, and prostatism – both plasma creatinine and urea concentrations will be increased • Discriminator – Plasma urea, plasma creatinine, urea nitrogen/creatinine ratio • reference interval (normal diet) – 12 and 20 mg urea/mg creatinine (49 and 81 mol urea/mol creatinine).

CLINICAL SIGNIFICANCE • Lower ratios – Acute tubular necrosis, low protein intake, starvation, or severe liver disease (decreased urea synthesis). • Urea clearance is a poor indicator of GFR – Production rate is dependent on several nonrenal factors • Including diet and the activity of the urea cycle enzymes. – Variable amount of back diffusion

• Blood urea nitrogen (BUN) – Converting urea mass units to urea nitrogen • 0. 467 – Converting urea nitrogen mass units to urea • 2. 14 – Urea nitrogen in mg/d. L to urea in mmol/L • 0. 357.

ANALYTICAL METHODOLOGY • Chemical • Enzymatic

Chemical Methods • Based on Fearon reaction • 540 nm.

Enzymatic Methods • Urease – Urea to ammonium ion • Ammonium quantitation – Berthelot reaction – Enzymatic assay

assay with Glutamate dehydrogenase • Decrease in absorbance at 340 nm

Interference • Endogenous ammonium – Aged samples in some urines – Particular metabolic disorders

Reference Intervals • Plasma Urea nitrogen in healthy adults – 6 to 20 mg/d. L • More than 60 years of age – 8 to 23 mg/d. L • Slightly lower in children and in pregnancy • Slightly higher in males than in females