Several posts have alluded to a problem I'd like to know the answer to.

The traditional GMS (Gomori or Grocott methenamine silver) stain for fungi 
(including Pneumocystis carinii, which alternates weekly between being a 
fungus and a protozoan) uses chromic acid as an oxidizing agent, to cleave 
vicinal diol linkages and thus produce aldehyde groups which then reduce an 
ammoniacal silver reagent (methenamine silver). 

Chromic acid, a very strong oxidizing agent, is used because it produces 
aldehyde groups in very resistant fungal cell walls.

Because of its environmental problems, chromic acid has become difficult to 
use. Kit manufacturers, with a wave of an MBA's hands I suppose, have 
declared that periodic acid, a much weaker oxidizing agent, can be used in 
its stead. But where is the literature to support this change?

Different fungi differ greatly in their demonstrability with this technique, 
and ideally one would have a different control for every fungus one wished to 
demonstrate. Probably the most difficult of common pathogenic fungi is 
Histoplasma capsulatum in old fibrotic or caseous lesions. 

If I saw a study that found periodic acid to be as good as chromic acid for 
this "acid test", then I might believe.

Bob Richmond
Samurai Pathologist
Knoxville TN