The exact value for the frequency of contamination is a very difficult number to pin down, for a variety of reasons. However, two facts make it more likely that it has occurred with respect to the knife than with a typical object. One is that the sample is in the low template range of analysis but no special handling precautions (a separate building with positive flow air hoods, etc.) were taken. The mere existence of so many special handling precautions in true Low Copy Number (LCN) analysis suggest that contamination is more likely when working in this very low concentration range. Two is that there was no blood on the knife. Two experts have publicly stated that the chances of cleaning a bloody knife to the point at which blood is no longer detected but DNA is detected, are small.
The exact value for the frequency of contamination is a very difficult number to pin down, for a variety of reasons. However, two facts make it more likely that it has occurred with respect to the knife than with a typical object. One is that the sample is in the low template range of analysis but no special handling precautions (a separate building with positive flow air hoods, etc.) were taken. The mere existence of so many special handling precautions in true Low Copy Number (LCN) analysis suggest that contamination is more likely when working in this very low concentration range. Two is that there was no blood on the knife. Two experts have publicly stated that the chances of cleaning a bloody knife to the point at which blood is no longer detected but DNA is detected, are small.