How do I determine to get an abortion or not when my child has tested positive for a disability?

Difficult to answer. Dr amster's advice is good. My suggestion is to discuss the problem with a geneticicist plus a medical pediatric or pediatric surgical sub-specialist taking are of the type of disability your child would have. Then it is up to you and child's father to make a decision which of course needs should take into account child's probable quality of life as well as impact on your entire family. Not easy.
No good answer. This is one of those awful decisions nobody wants to make, and there are a lot of personal beliefs involved. A couple things to think about before you decide. 1. The tests aren't always right. Sometimes ones for down's syndrome or spinal cord defects aren't accurate, or the ultrasound showed something that wasn't there. 2. The range of the disability is vast, could be life threatening, maybe not.
Is is that your. Developing fetus or already living child is disabled? The former would perhaps be described as possesing a genetic defect. In addition to your beliefs, you need to inquire what the child's life would be like, and would you be able to love and care for the child with those specific problems. No one can probe your heart and soul for these answers better thn you and your mate, or those that will help.