Читать книгу Practical Guide to Diagnostic Parasitology - Lynne Shore Garcia - Страница 85

Although preservatives have been developed that do not contain mercury compounds, substitute compounds have not provided the same quality of preservation necessary for good protozoan morphology on the permanent stained smear. Copper sulfate has been tried but does not provide results equal to those seen with mercuric chloride. Zinc sulfate has proven to be an acceptable mercury substitute and is used with trichrome stain. Although zinc substitutes have become widely available, each manufacturer has a proprietary formula for the fixative. Compared with mercuric chloride-based fixatives, there is much less margin for error when using modified PVA fixatives. Rapid fixation, proper stool-to-fixative ratios, and adequate mixing are mandatory for good protozoan morphology, particularly on the permanent stained smear.

Note: The important question is not “How beautiful are the organisms?” but “Can you tell which organisms are present?” With some training, microscopists can identify the organisms, although the morphology is not as clear as that seen using mercury compounds. Unfortunately, parasitology microscopy is not a perfect science; we probably miss rare organisms even when using mercury-based fixatives.

Summary: Modified PVA (Mercury Substitutes)