Assuming there are enough blood drops for it to work on. Many deer will only leave a drop here and there. It would take gallons of the stuff sprayed all over the ground to find them. Any kind of spray is most useful for identifying SUSPECTED drops of blood, not for finding them.
I shot an elk last week with a lung shot a bit high. It didn't go anywhere but it didn't bleed out of either hole a bit. It walked about 5 yards in snow and went down leaving not a drop in the snow. If it had run a ways, there would be no blood trail to follow.
