I just got this answer from Dr. Robert Frascino at thebody.com, if anyone else would be interested about this.
"Sjgren's syndrome is an autoimmune disorder in which immune cells attack and destroy exocrine glands that produce tears and saliva. Having this condition would not alter the duration of the window period or affect the ability of the body to make anti-HIV antibodies. Therefore it would not affect ELISA test results."
I think this answers my question and I can trust my test results without doubt. Thank you for your comments!