Carrascosa and colleagues recently reported a case of hemorrhagic proctosigmoiditis in the setting of documented Blastocystis hominis infection [1]. They suggest that their case provides further evidence to support the pathogenicity of B. hominis and the idea that B. hominis should be considered a possible cause of hemorrhagic colitis in both immunocompetent and immunocompromised patients. Although I agree with their cautious conclusion, several points about the case merit comment.

First, the authors mention that the patient's stool was not examined for Shiga-like toxins. I assume that they are alluding to the fact that enterohemorrhagic infection with Escherichia coli (E. coli O157:H7) was not excluded. Because E. coli O157:H7 is a common cause of hemorrhagic colitis [2], the authors' failure to exclude infection with this organism weakens their conclusion.

Second, the authors state that the results of mesenteric and celiac arteriography were normal. I assume that the authors believe that these results rule out the possibility of a diagnosis of ischemia-related disease; unfortunately, this is not the case. Although many conditions have been associated with colonic ischemia, no precipitating factor is identified in most patients [3]. The episodes are considered to be spontaneous and occur without major vessel occlusion (that is, they are events of "nonocclusive ischemia") [4]. Therefore, normal results of mesenteric arteriography do not exclude the possibility that the patient's hemorrhagic colitis has an ischemic cause. Although rectal involvement (as in Carrascosa and colleagues' patient) is unusual in ischemic colitis, information on the proximal extent of disease (shown by colonoscopy) is not available. If the patient's colitis was segmental, as the result of barium enema suggests, an ischemic cause would still deserve consideration. The above tenets should temper the authors' conclusion about the possible role of B. hominis in hemorrhagic colitis.