The determination of narcotic addiction and the subsequent civil commitment under this chapter shall not be deemed a criminal conviction. The results of any tests or procedures conducted by the Commissioner of Health or the supervisory aftercare authority to determine narcotic addiction may only be used in a further proceeding under this subchapter. They shall not be used against the examined individual in any criminal proceeding except that the fact that he is a narcotic addict may be elicited on his cross-examination as bearing on his credibility as a witness.