(LEFT) Blastocystis in simple smear. Researchers have cited the non-descript appearance of Blastocystis as one reason for the low sensitivity of clinical diagnostic methods, along with the possibility that not all stages have been documented . (RIGHT) The classic diagnostic form of Blastocystis found in the stool of patients varies in size from 6 to 40 μm. The parasite is characterized by a central body (blue) that morphologically resembles a vacuole. The central body pushes the nuclei to the periphery of the cell. The central body is a reservoir for proteases  and may serve other functions as well. Photos courtesy of Dr. Hanaa Moussa, Cairo University.