Gayle C. wrote (regarding chromic acid): >>This actually removed hematoxylin by
overoxidation while oxidizing the Histoplasma capsule, and the methenamine
silver steps followed.  So if one ever needs to do a GMS stain on an H&E
slide, it is possible.  I am not sure periodic acid would do this.<<

In my experience, it does remove the hematoxylin.  The silver technique, unless a coated slide of some sort was used for the H&E, would almost always cause quite a bit of lifting of the tissue section, though.  I would usually ask the Pathologist whether a PAS could be used instead.

The Samurai's point about old fungus staining lightly is also excellent.  I've seen it time and again where huge fungus balls (for lack of a better term) would have weak staining in the middle, and the hearty critters on the outside would generally stain well, regardless of chromic acid or periodic acid oxidation.

I don't know whether the technique of using ammoniacal silver instead of methenamine silver makes a difference in the display of the Histoplasma using the periodic acid, but my experience with this technique showed good staining consistently.  

The takehome message here is clear...it is NOT good laboratory practice to make willy-nilly substitutions without doing the work to back up the result.  We did side-by-side stain comparisons for months before dropping the Grocott from our repertoire.

Ideally specificity AND sensitivity should be demonstrated in our control sections.  I also wonder how many AFB organisms have been missed due to the use of "loaded" control tissue.

And, RIGHT ON about the disposal commentary, Gayle!

Teri Johnson
Manager Histology Core Facility
Stowers Institute for Medical Research
1000 E. 50th St.
Kansas City, Missouri  64110
tjj@stowers-institute.org