Some people coats microplates  with whole sera for certain particular applications. But I think in your case this is not a good idea at all. When you coat with a complex mixture, all the proteins are competing for the available surface. Serum contains many other proteins besides IgG, some of them in huge amounts like albumin. So you will not detect not total IgG from the sample, but the fraction of IgG that was able to bind to the plastic in strong competition with the other proteins until the surface was saturated. 

You definitely need a different assay to quantitate IgG content. The ideal situation would be to buy a second non-conjugated anti-gG antibody to coat the plates. You could try to determine IgG content by western blot using your conjugated antibody, but this would require a careful work to optimize WB conditions and to detemine the intensity of the specific signals (image processing). WB is also not well suited for multiple samples as ELISA is. I would suggest to look for the second anti-IgG antibody and set up a sandwich ELISA.

Some people coats microplates  with whole sera for certain particular applications. But I think in your case this is not a good idea at all. When you coat with a complex mixture, all the proteins are competing for the available surface. Serum contains many other proteins besides IgG, some of them in huge amounts like albumin. So you will not detect not total IgG from the sample, but the fraction of IgG that was able to bind to the plastic in strong competition with the other proteins until the surface was saturated. 

You definitely need a different assay to quantitate IgG content. The ideal situation would be to buy a second non-conjugated anti-gG antibody to coat the plates. You could try to determine IgG content by western blot using your conjugated antibody, but this would require a careful work to optimize WB conditions and to detemine the intensity of the specific signals (image processing). WB is also not well suited for multiple samples as ELISA is. I would suggest to look for the second anti-IgG antibody and set up a sandwich ELISA.

I agree with Gertrudis that a sandwich ELISA would be preferable to a simple ELISA. However, I'm not convinced that the simple ELISA would not work because of the high protein content, mainly albumin, of serum. IgG is a pretty major component of serum, so some of it should be able to bind to the plate even in the presence of a lot of albumin, enough to get a signal.

A different concern is that the concentration of total protein may vary significantly between specimens, which might introduce a variability in sensitivity of the IgG measurement between specimens. It would be a good idea to also measure the total protein concentration of each sample using a commercial protein assay kit such as the BCA or Bradford assay, and express the ELISA result as a ratio to the total protein.

Thank you so much

Gertrudis Rojas and Adam B Shapiro, these answers are very elaborative and helpful for me. 

Namdev I agree with the above.  However the following also works.  I actually do it as a

(1) Competitive ELISA and (2) Direct ELISA practical and for my undergraduates.

(1) Competitive ELISA

Isolate a small amount of IgG from serum using DEAE chromatography, or if this is not available by ammonium sulphate precipitation.  Determine the protein level using Bradford assay.  You then coat the ELISA plates with a 10 ug/ml (100 ul/well) isolated IgG protein solution in PBS overnight. Block the next day for 1 hour with 10 % skim milk in PBS.  Add various concentrations of isolated IgG or sample to wells (50 ul/well).  Immediately add 50 ul/well of anti-IgG HRP conjugate.  Final volume in wells are now 100 ul (50 sample + 50 conjugate).  Incubate for 2 hours at room temperature. Wash and add substrate.

(2) Direct ELISA

Transfer 50 ul patient blood per Epi.  Add 50 ul saturated ammonium sulphate.  Mix. Incubate overnight at 4 C.  Centrifuge on a microtube centrifuge at maximum speed for 5 minutes.  Decant supernatant.  Suspend the pellet in 500 ul PBS.  For coating of plates dilute the suspended pellet a further 1/1000 in PBS (final dilution is 1/10000). Coat plates with 50 ul/well of this overnight at 4 C.  Wash. Block with 10 % skim milk in PBS.  Add conjugate and incubate for 1 hour at room temperature. Wash and add substrate.

We actually get very good correlations between the two systems.

Thank you Sir