How do you determine if ibs is due to scleroderma when ana positive but no other symptoms?

You cannot!: ANA is not a test specific for scleroderma, and there are false positives. There are several subtypes of scleroderma too, such as localized, systemic sclerosis, crest, syndromes, and single internal organ involvement. Several major and minor criteria must be met to make a diagnosis of systemic sclerosis. A skin biopsy is essential, if clinical exam is suspicious for cutaneous scleroderma.

Not IBS: IBS is not part of scleroderma. A standard bowel work-up will usually differentiate scleroderma bowel involvement versus IBS. The ANA is not a reliable test for scleroderma. Skin biopsy of abnormal skin (if there is abnormal skin) is a reliable test.