Hematodinium infections can be diagnosed several different ways. Staining by neutral red is probably the easiest way for those who don't work with the parasite. However, the parasites may not take up the stain when cultured in long-term cultures.
Ameboid trophonts (left) and host granulocytes (right) stained with neutral red (0.05% in saline buffer). Photographs are at different magnifications. Hemocytes often take up very little stain, but Hematodinium parasites rapidly take it up.
Other methods include the obvious discoloration of the carapace (see the snow crab presentation), visualization of infections through the pleopods, blood smears, and more refined techniques such as ELISA and PCR (see Stentiford and Shields 2005 for a review).