Mathew, Alice, et al. “Central catheter blood sampling: the impact of changing the needleless caps prior to collection.” Journal of infusion nursing 32.4 (2009):212-218.
This study of 91 patients compared 3 methods of blood draws (old cap (existing cap), new cap (replacing old cap with a sterile one), and peripheral methods).  Nine false positive cases of bacteremia were identified using the old cap. Researchers recommending changing the cap before drawing culture samples.