Methods are disclosed for detecting an antigen in a biological sample. The methods involve providing in combination a solid support, which is substantially free of specific binding proteins, and a medium comprising an antigen from the sample and an antibody for the antigen. The combination is incubated under conditions sufficient for the antibody when bound to the antigen to bind to the support. The presence or amount of antibody on the support or in the medium is detemined and is related to the presence of antigen in the sample. The methods have particular application to the detection of gram-negative bacteria.