Does a positive ASCA mean I have Crohn disease?

No; it can increase the likelihood, especially at high titers, but it is not a diagnosis and does not replace endoscopy or biopsy.

Can ASCA be negative in Crohn disease?

Yes; not everyone with Crohn disease is ASCA-positive, per sourced material.

Each has Negative, Equivocal, and Positive cutoffs, listed above from the Quest range.

No; ASCA is one marker. The IBD Differentiation Panel (Test 16) includes additional markers.

Autoimmune and Inflammatory

A blood test for ASCA (IgG and IgA) antibodies, a marker more often associated with Crohn disease.