REAGENTS

(R4) SAF (Sodium acetate-acetic acid-formalin) 

Composition

-     Anhydrous sodium acetate (NaC2H3O2)                       9 g

-     Glacial acetic acid                                                  20 ml

-     Commercially available formalin (37%-40%)             40 ml

-     Distilled water                                                     925 ml

In alternative to 9 g anhydrous sodium acetate, 15 g sodium acetate trihydrate (CH3CO2Na ∙ 3H2O) can be used. 

Procedure

-     Dissolve the salt in distilled water;

-     Add formalin and glacial acetic acid, and mix well;

-     Adjust to pH 4.15, if necessary.

This solution is stable, in a sealed glass bottle, for at least one year.

If just one fixing agent needs to be chosen from those commercially available, SAF is recommended for the following reasons:

      Does not contain mercuric compounds and therefore is safer, less corrosive and damaging to the environment;

      Maintains morphology of cysts, helminthic eggs and larvae, coccidian oocysts and microsporidia spores but, above all, is excellent for the morphological differentiation of the nucleus of protozoan trophozoites;

      Enables the use of concentration procedures with formalin-ether or formalin-ethyl acetate and flotation with zinc sulfate;

      Allows the search for parasites coproantigens and staining with fluorochromes;

      Enables permanent staining with iron-hematoxylin (excellent results) and trichrome staining;

      Allows modified trichrome staining for microsporidia;

      Is inexpensive and can be easily prepared in the laboratory.

However, there are some disadvantages of SAF:

-     SAF-fixed specimens stain poorly with trichrome compared to specimens fixed with Schaudinn's solution or PVA solution;

-     Due to the low formalin concentration, a ratio of 1 part feces to at least 3 parts fixative must be used to avoid fecal specimen fermentation.