X Factor is used for the presumptive identification of Haemophilus species on the basis of their requirements for X . X-factor discs are the sterile filter paper discs impregnated with growth factor X which are used for differentiating Haemophilus species in conjuction of V factor & X+V factor discs. Bordetella and Haemophilus species can also be identified on the basis of the requirement of X and V growth factors in the basal medium.Bordetella pertussis ATCC 8467 :Positive with X factor ; Haemophilus influenzae ATCC 35056: Negative