Serum Albumin Clinical Significance From the 20 th

Serum Albumin • Clinical Significance: • From the 20 th weeke of gestation and continuing through life, albumin is most abundant protein in human plasma, representing 40 -60% of total protein. It is synthesized in the liver at a rate that is dependent on protein intake but subject to feedback regulation by the plasma albumin level.

• Hyperalbuminemia is of little diagnostic significance except in dehydration. • Meanwile, Hypoalbuminaemia is common in many disease states and results in most instances from one or more of the following factors:

• Impaired synthesis, either primary or secondary to diminish protein intake or chronic liver disease. • Increased catabolism as a result of tissue damage, inflammation or malignancy. Reduced absorption of amino acids e. g. , malabsorption syndrome.

• Protein loss in urine (e. g. , nephritic syndrome, chronic glomerulonephritis, diabetes mellitus, SLE), in faeces( protein- losing eneropathy due to inflammation or malignancy. ), or from skin ( burns). • Altered distribution(e. g. , in ascites

Specimen: . Serum is the specimen of choice, but heparinized plasma can be used if precautions are taken to prevent heparin interference. . Heparin has a positive interference with bromocresol purple and bromocresol green methods. . A fasting specimen is not required unless there is severe lipaemia.

• . Venous stasis should be avoided since haemoconcentration increases the concentration of albumin and other plasma proteins. • Cereprospinal fluid(CSF) and urine albumin can also be assyed by some of the following methods.

Analytical Methods: 1 - Electrophoresis: . The major classes of the serum proteins are separated by a serum protein electrophoresis method (cellulose acetate or agarose ). . The separated fractions are stained, and the percentage of each fraction present in the sample is determined by densitometic analysis.

The concentration of albumin is calculated by multiplication of the concentration of total protein in the sample by the percentage of albumin This method is inaccurate and is discourged because:

1 -Several problems arise from the lateral diffusion of relatevely small proteins as albumin in the supporting media. 2 -In addition to nonlinearity secondary to lack of saturation of albumin binding sites

2 -Immunological Methods: A- Radial immunodiffusion(RID) and electroimmunodiffusio(EID) Albumin either passively diffuse(RID) or is electrophoresed(EID) into a stationary phase such as agarose that contains antibodies to albumin. The preciptin lines formed by the reaction between albumin and the antibody can be fixid and stained.

. In (RID), the diameter of the preciptin ring formed is proportional to the albumin concentration. . For (EID), the hight of the rocket- shaped precipitin line is relaed to the albumin concentration. (RID)b can be used for CSF samples.

b- Turbidimetry and nephelometry: The antibody- albumin complexes that form increase the absorption (tubidity)or the scattring(nephelometry) of the incidint light which can be related to the albumin concentration. These methods are highly accurete and specific and much more sensitive than other methods. Can be used for CSF and serum samples

C- Radioimmunoassay(RIA) RIA , radiolabeled albumin competes with test albumin for limited amount of antibody. , while In EIA albumin in the sample is sandwiched between antibody bound to the surface and the enzyme labeled antibody

3 -Dye –Binding methods • These are the most widely used methods. Albumin has the ability to bind a wide variety of organic anions, including complex dye molecules. The dye binding techniques are based on a shift in the absorption maximum of the dye when bound to albumin

a-Methyl orange: • It is non specific for albumin

b- Bromocresol green(BCG) The assay is usually performed at PH 4. 2 and is monitored at 620 -630 nm at the chosen PH , albumin acts as a cation to bind the anionic dye. The specificity for albumin is an issue to debate

Hyperbilirubinaemia and hemolysis do not interfere with BCG methods. However, iterference has been shown to be by alpha- and beta globulins, as well as fibrinogen. It has been suggested that measuring the absorbance at 628 nm shortly after mixing (within 30 sec improve the specificity of the assay.

• C Bromocresol purple: • Yellow BCP dye , buffered at PH 5. 2 with acetate, turns green when complexed with albumin. Absorbance of the green complex is measured at 630 nm. • BCP methods show an exellent specificity for assay of human albumin when human albumin calibrators are used.

Urine Strips: These are used for semiquantitative measurement of urine albumin. The method is based on the change in colour of the dye bromocresol plue in the presence of a binding protein as albumin. Bromocresol plue , buffered to PH 3 with citrate, is present predominantly in the protonated form (Yellow form)

When protein is added , the affinity of the anionic form of the idicator dye for protein causes a shift of the equilibrium between anionic and protonated forms of the indicator toward formation of the blue anionic species. The intensity of the blue colour is proportional to the concentration of protein in the specimen

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