Jerry Fredenburg and I will have a paper on the inconsistencies of using
periodic acid in the GMS coming out in the JOH sometime this year.  We found
great inconsistencies in the demonstration of  Histoplasma capsulatum.
Sometime there was no staining and sometimes the staining was OK - that is on
different cases - must be some variability in the organisms.  One of the
problems is that Aspergillus sp. is frequently used for the control and it is
easily demonstrated with 5 minutes of periodic acid oxidation.  If we timed
the reaction for all slides using that as the control, then some histo cases
were negative.  It took 60 of oxidation on those cases to achieve the same
rate of impregnation with silver as we had on the aspergillus control.  We
also found that 5 minutes of oxidation with periodic acid did indeed increase
the background staining, especially the connective tissue fibers.  We know
that chromic acid is a much stronger oxidizer, oxidizing many tissue
components past the aldehyde demonstration stage, while periodic acid is a
much weaker oxidizer. Therefore it leave more of the groups demonstratable,
and thus more background staining.

Ideally the control should contain the same organism as is suspected, but
unfortunately we are not that rich in control tissue.  Since histoplasmosis is
endemic in some of the US - especially the south and southeast, I would
caution you about using periodic acid oxidation when this organism is
suspected. I know chromic acid is a hazard, but I am not sure that it should
be replaced in the GMS.

Freida Carson