Non-Protein Nitrogenous Compounds

By-product of protein and nucleic acid. However Creatinine is a by product of phosphocreatine utilization or muscle metabolism.

NPN are water soluble and removed from the body in urine.

Major source of ammonia is intestinal absorption from GI contents converted by action of bacteria.

It is removed from portal circulation by liver (1st pass) and converted to urea through the Urea cycle.ammonia is toxic to CNS and hyperammonia can cause encephalopathy.ammonia is higher in infants due to immature circulation.

In ISE, ammonia diffuses across "selective" membrane into a buffer of NH4Cl causing a pH change. PH change is measured potentiometrically.However, Enzymatic methods are most common, which uses spectrophotometric/ colorimetric end-point.

It is formed in liver (urea cycle) from two ammonia and CO2. it is the NPN compound in the highest concentration in plasma. it is the end-product of protein catabolism.

In kidneys, urea is "passively" reabsorbed throughout nephrons and re-enters plasma. Renal excretion depends on renal blood flow, glomerular filtration rate (GFR) and urine flow.high urine flow = 40% absorbed.low urine flow = 60% absorbed.

Uremia (uremic syndrome): increased BUN with renal failure. Pre-renalazotemia is caused by reduced Renal Blood flow. meaning that less blood is filtered by kidneys, which leads to less urea excreted.

There is a direct releationship btwn perfusion and renal function. also the energy for ultra-filtration of plasma in glomeruli of kidney is driven by the blood pressure. low blood pressure = low renal function.

Renal azotemia is intrinsic renal disease such as acute or chronic renal failure, glomerular nephritis, and tubular necrosis.post-renal azotemia is obstruction of renal flow such as kidney stones and tumors.

Liver disease, starvation, repeated hemodialysis, and inborn errors of metabolism.

Urea concentration varies with age, but NOT gender. In serum/ plasma concentrations; it is expressed as 5-17 mg/dL of urea nitrogen, and 10.7-36 mg/ dL of urea, or mmol/L urea.* anticoagulants must not contain ammonium or fluoride.

Indirect methods~ we are not directly measuring urea, we are measuring ammonia. Hydrolysis of urea by urease to form ammonia. it can calibrate using urea or ammonia standards. when testing Urine, specimen blank is required to account for endogenous ammonia.Direct methods~ Diacetyl Monoxime method.in this method, urea reacts to form a chromophore. then the amount of chromophore is quantified. this is very specific for urea because no NH3 interference.

The basis of this ratio is that Renal clearance of creatinine approximates GFR. also Renal clearance of urea is lower than creatinine and more variable due to reabsorption by distal tubules.BUN/ Creatinine ratio has limited usefulness because urea levels vary with production and excretion.Normal ratio is 12:1 to 20:1pre-renal azotemia = increased ratio with normal creatinine.Renal disease = Normal ratio despite abnormal BUN and creatinine.Post-renal obstruction = increased ratio with increased creatinine.

It is only clinically significant when one or both analytes are increased. it is used in ER and post-surgery.after surgery if the ratio decreases, it indicates renal damage. if stable it indicates stable renal function.