why does HORSE BLOOD produce an erratic result on blood agar?

Answer 1

Horse blood is the blood usually used in blood agar so I don't know why you would get erratic results...

Sheep blood is sometimes used for the purpose of providing different nutrients.

Lysed horse or sheep blood used in blood agar would produce erratic results in blood agar because that would make it chocolate agar and some of the nutrients certain bacteria need to survive would have been destroyed in the cooking of the blood to lyse it.

Another reason for strange results on blood agar may be due to the blood simply being too old. It would have a higher % of lysed cells and there would be very little glucose (the red cells would have used it up) and high potassium (due to cell lysis) which may upset whatever you're trying to grow.

hope this helps :)