Immunologist agreed i have malar rash. how is lupus dx'd since we can't rely on antibody or inflammatory tests w my immune deficiency and steroid tx?

Other symptoms?: If you're taking steroids, it makes me think you're having additional symptoms you didn't mention, or else your rash must be pretty severe to require that. The blood tests are not worthless in your situation, and could be checked if not already. If those are negative, a biopsy of the rash site might help, although if it shows lupus, it would not prove you have more than skin involvement.

Management/workup: The initial workup will include renal function testing and tests for circulating antibodies. Your steroid treatment may affect the tests for inflammation, moreso than autoantibodies which are still detectable even while being treated with steroids. Avoiding and treating infections, including CMV will be part of the management. I hope that this helps.