(R6) PVA solution (polyvinyl alcohol – mercuric chloride)

Polyvinyl alcohol is a synthetic polymer of vinyl alcohol. PVA-fixed specimens smeared onto a slide, once dried, form a very adhesive transparent plastic film that withstands alcohol, ether, xylene and acetone. The true fixative is mercury chloride, an energetic oxidizing agent that precipitates proteins. This action is enhanced by acetic acid.

PVA solution 

Composition

-     Schaudinn's stock solution                                   93.5 ml

-     Glycerol                                                              1.5 ml

-     Glacial acetic acid                                                 5.0 ml

-     Polyvinyl alcohol 90-50 (low or medium viscosity)       5 g

Procedure

-     Combine the first three ingredients and heat the mixture in a water bath (75° C), making sure that the ethanol (in Schaudinn's solution) does not burn

-     Slowly add 5 g polyvinyl alcohol, stirring continuously; if possible, use a heating plate with a magnetic stirrer.

During cooling, the solution may become slightly cloudy but this can be ignored. The solution is stable for at least 1 year if stored in a sealed glass bottle.  If it becomes too viscous, whitish or opaque over time, it can no longer be used.

PVA solution is the best fixative for trichrome staining, but has some disadvantages:

-     Mercuric chloride is highly toxic to both man and the environment, and must be disposed of as toxic waste;

-     Difficult to prepare in the laboratory, also because it is not easy to find good quality polyvinyl alcohol. If possible, purchase the fixative ready-made from a commercial supplier;

-       Because of the corrosive power of mercuric chloride, smears, before staining, must not come into contact with metals, e.g. they must not be stored in aluminum foil, otherwise the specimen will flake off from the slide;

-       Can be used for concentration with formalin-ether or formalin-ethyl acetate, but the number of eggs and cysts identifiable is far lower than for specimens fixed with formalin. When the parasite load is low, the result may be false negative;

-       The addition of iodine for direct microscopic examination can sometimes cause precipitates, which make microscopic examination difficult.

Because of the toxic nature of mercuric chloride, attempts have been made to replace it with copper or zinc sulfate. Unfortunately, with this modification, the nuclear and cytoplasmic morphology is not comparable to that seen with mercuric chloride, and in some cases it is impossible to recognize the morphology of pathogenic protozoa.