Blood contains various diseases and fusing bloods that aren't the same type can cause various problems. if testing with blood it is unsure to know how much disease is in the blood.

True, unless you have a test that gives you the correct answer, you will not know how much disease is inside the blood. You shouldn't give diseased blood to a person, as it is harmful and contagious, and is a way to spread the disease. Also, if you do not test the blood, you will not know what blood type it is, which can be harmful to a person that does not have the same blood type. As the wrong blood type is injected, not only does it not help the person carry the oxygen and carbon dioxide in and out of the body because of the wrong 'type', but the immune system will attack it too

hope this helps