Is there a sero negative form of lupus?

Yes. A small numbr of patients with lupus do not ana, i.e. Seronegative.
Rare. Almost all patients will have labs that are suggestive of lupus if it is being considered as a diagnosis. I have patients that have positive skin biopsies consistent with lupus an dother findings but blood work is normal. I m ay treat the patient for lupus but possibly may not call the patient a definite lupus patient unless the acr criteria for the disease are met.