Hygiene When Taking Blood

I've had a number of blood tests at my local health center recently and the nurses have never sterilised the area or their hands with alcohol first, despite then touching the area with their bare hands first.  I've always asked them to and always get the same response:  we don't do that because it corrupts the results of the blood test.

Is this correct of just an excuse?  If it is correct then why do they always sterilise the area and their hands with alcohol in hospitals with performing a blood test?