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.