I've used a few Cygnus kits but not the Staph aureus one.  They have worked well for me. I have a couple of ideas.  One is to assay the samples you are making using a Biorad or similar protein assay, just to get a sense if your samples have a very low concentration.  Then I would dilute some of the standard into you samples, you buffer and PBS.  Compare the expected concentration with the measured concentration.  Something in your sample or buffer may be interfering with the readout.  

Thanks for your suggestions, Paul. I did quantify the total protein level in my sample with Bandford and Lowery method. It has about 200ug/ml of proteins.

The kit doesn't have dilution buffer in it, so I've just used my buffer (PBS) to dilute the sample. I made the sample with PBS.

We are contacting the Tech of Cygnus and still waiting for their response. I will do validation as you suggested if they don't response. The kit is expensive.

I have used and it was good.  You go a head assay protein cons.  and use PBS buffer .  Compare the expected concentration with the measured concentration.  Something in your sample or buffer may be responsible for  the readout.  

 https://www.researchgate.net/post/Anyone_has_ever_used_S_aureus_host_cell_protein_ELISA_kit_Cygnus_F320 [accessed May 22, 2017].

@Yehia A.-G. Mahmoud 

Thanks for your information. Could you tell me what sample were you testing?

@Paul Casaz

I did a validation test as you suggested. Neither PBS or PBS + my sample interfer the readout of standard sample (100ng/ml).