Negative Biopsy with positive serology

My son is 10 and we were told lasat week his serology test came back consistent with crohn's disease. Had a colonoscopy and endoscopy earlier this week and just got the biopsy results that were negative. I am greatful that the biopsies were negative, but I am confused about the positive serology. What could this mean? He had one small area in the small intestine that was apparently a "healing" sore that had bled due to so much diarrhea. He also had blood in his stool. I questioned the doctor about this sore and she said it was not an ulcer nor inflammation, just probably a sore. If anyone can offer any advice, that would be great. Thanks!