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266 CHAPTER 12

PROCEDURE 12-3

ZINC SULFATE CENTRIFUGAL FLOTATION PROCEDURE
Clinical Rationale
When properly performed, the flotation procedure will concentrate the organisms

from a small amount of fecal material that is then cleansed of debris with water.
The density of the zinc sulfate enables the lighter elements such as larvae, protozoa,
and ova to be found at the top of the centrifuge tube. The flotation method is the most

effective way for concentrating protozoa and their cysts, along with Cryptosporidium
oocysts. Several commercial methods exist for concentrating fecal parasites and
their ova. These kits are disposable and contain all the necessary materials in a
closed system for performing the flotation procedure. The manual method is also

quite effective but requires the making of the main reagent, that of zinc sulfate. The
manual procedure perhaps exposes the laboratory worker to the specimen and the
chemicals used to a greater extent than one of the commercial kits (see Figure 12-7).

FLOTATION

Add ZnSO 4
to sediment Coverglass
Parasites

Centrifuge Mix
Mixture of ZnSO 4
preserved feces
and water or saline
Pour off
Delmar/Cengage Learning
Supernatant

Sediment Fecal debris

FIGURE 12-7 The flotation method requires several steps in mixing and decanting of supernatant

The method called fl otation uses liquids with a relatively high specific gravity,

greater than that of parasite cysts or eggs, so that they will float to the surface. This

concentrate found at the top of the tube will contain the organisms and their eggs
that can be skimmed from the top and used to prepare slides for microscopic exami-
nation. The concentrating solution has a final specific gravity of 1.18 (compared with

water which has a specific gravity of 1.000). Most often zinc sulfate is used in the