I found the following for your information. Hopefully it will help you. It is not my answer but I think it's a decent one.
Hello, I am a Registered Nurse and we use syringes to give precise doses of medications to our patients. The manufacturers of syringes have taken into account the amount in the needle as part of the measurement in small syringes (like the tuberculin you are using.) If you have a removable needle that did NOT come with the syringe, then it is usually removed prior to dosing a patient as we use blunt needles to draw from a vial and measure, then put on the appropriate needle if we are giving a shot or just into the IV as a needleless system.
So, if the needle + syringe is one unit, it measures what is in the needle. If you add a needle, the amount is not measured.
I hope this helps.