1CLINICAL CHEMISTRY (MLT 301) NONPROTEIN NITROGEN (NPN) LECTURE ONE Dr. Essam H. Jiffri
2Nonprotein NitrogenThe determination of nonprotein nitrogenous substances in the blood has traditionally been used to monitor renal function.- The term nonprotein nitrogen (NPN) originated in the early days of clinical chemistry when analytical methodology required that protein be removed from the sample before analysis.
3Nonprotein Nitrogen- The NPN fraction is made up of about 15 compounds of clinical interest, the majority of these compounds rise from the catabolism of proteins and nucleic acids.
4Components of the Nonprotein- Nitrogen Fraction Approximate Plasma ConcentrationCompound (% of Total NPN)UreaAminoacidsUric AcidCreatinine 5CreatineAmmonia
5UREA- Urea constitutes nearly half the nonprotein nitrogen substances in the blood.- It is synthesized in the liver from Co2 and the ammonia arising from the deamination of amino acids by means of the ornithine.
7UREAUrea constitutes the major excretory product of protein metabolism, following synthesis in the liver, urea is transported by the plasma to kidney, where it is readily filtered from the plasma by the glomerulus.- Most of the urea in the glomerular filtrate is excreted in the urine, and up to % is reabsorbed by passive diffusion during passage of the filtrate through the renal tubules.
8UREA- An elevated level of urea in the blood is called azotemia.- Very high levels of plasma urea accompanied by renal failure is called uremia or the urermic syndrome this is eventually fatal if not treated by dialysis.- The major causes of decreased plasma urea levels include decreased protein intake and severe liver disease.
9Analytical Methods- Because measurements of urea were initially done on a protein-free filtrate of whole blood, and early analytical methods were based on measuring the amount of nitrogen, this assay is commonly called a BUN or blood urea nitrogen determination.
10Analytical MethodsTwo analytical approaches have been used to assay for urea:The oldest and most often used involves the hydrolysis of urea by the enzyme urease and quantitation of the NH+4.
11Analytical MethodsEarly colorimetric methods were based on using Nesslers reagent or the Berthelot reaction to detect the NH+4 produced.
13Specimen Requirements and Interfering Substances Fluoride or citrate will inhibit the urease utilized in the coupled enzymatic methods.- When collecting plasma, NH+4 ions and high concentrations of sodium fluoride or sodium çitrate must be avoided.Urea concentration may be readily measured in serum, plasma, or urine.
14Specimen Requirements and Interfering Substances - Fasting sample is usually not required.A non-hemolyzed sample is recommended.Urea is quite susceptible to bacterial decomposition, so samples (especially urine) that cannot be analyzed within a few hours should be refrigerated.
15Reference IntervalUrea nitrogen 7 to 18 mg/dL ( mmol/L urea)
16CREATININE- Creatine is synthesized mainly in the liver from arginine, glycine, and methionine.- It is then transported to other tissues, such as muscle, where it is converted to: phosphocreatine, which serves as a high energy source.
17CREATININE- Either creatine phosphate or creatine, under physiologic conditions, spontaneously loses phosphoric acid or water, respectively, to form its anhydride, creatinine which excreted into the plasma.
19Analytical Methods for Creatinine - The methods most frequendy used to measure creatinine are based on the Jaffe reaction first described in 1886, in this reaction, creatinine reacts with picric acid in alkaline solution to form a red-orange chromogen.
20Specimen Requirements and Interfering Substances Creatinine can be measured in serum, plasma, or urine.- Hemolysis should be avoided, since there are a significant number of nonspecific chromogens in the red cells.
21Specimen Requirements and Interfering Substances Fasting is not require.A heavy protein ingestion may elevate levels of creatinine.- Urine should be refrigerated after collection or frozen if storage longer than four days is required.
22Reference IntervalReference intervals vary with age and sex and may decrease in the elderly.Cord blood mg/dl ( mmol/L)Children mg/dl ( mmol/L)AdultMale mg/dl ( mmol/L)Female mg/dl ( mmol/L)