We have had the same problem at our hospital with the positive control being very weak. I called Immucor and they sent another Rapid Screen same lot number to see if the kit was better. It was no better than the other one. I had one tech to tell me, if he let it sits for 10 min instead of 5 min, the positive control worked better. I don't know if this is allowed, since the instructions say to let it incubate for 5 min at room temp and wash four times.