manner as the first form of retrograde flow preventer 110 negative pressure suck back upon disconnection of the external male luer 100 is automatically prevented by use of either the first or the second form of retrograde flow preventers 110 120 described above even if the male and female luer parts have inadvertently not been rotated to the closed position prior to disconnection a further optional feature of the invention comprises the provision of a cam surface on the end of the female luer part 30 such that rotation of the luer parts 23 30 to the closed position urges the seal 40 more tightly against its seat in the male luer part 20 one suitable form of achieving this result comprises a dimple 130 integrally formed on the end of the insert 90 in the position shown in fig1 15 this feature is beneficial in squeezing the seal 40 into the clearance space 118 ro minimize the amount of fluid which would otherwise remain therein prior to the use of needleless medical connectors a typical clinical protocol would require the clinician to add a heparin lock to the iv system for u201c locking in u201d heparin at the end of.