This technique is predominantly used in parasitology laboratories. By exploiting the density of the parasites, particularly eggs, it allows the parasites to float to the top of a dense solution (final specific gravity of about 1.20) and can then be skimmed from the top of the tube.
The most commonly used reagent is zinc sulphate. Operculated eggs as well as schistosoma and infertile Ascaris eggs are not easily recovered by this method. Also trophozoites are killed due to the high specific gravity and certain other fragile eggs such as Hymenolepis nana become distorted.
1. Crush 10-20g (about 1 teaspoon) of feces with applicator sticks and mix well with 10-12ml of saline. Filter the mixture through two layers of dampened surgical gauze into a 15ml conical centrifuge tube.
2. Centrifuge the suspension at 1500 rpm for five minutes. Decant the supernatant into disinfectant. Resuspend the sediment and recentrifuge in saline if there is excessive debris in the sample.
3. Resuspend and thoroughly mix the sediment in 12ml of zinc sulphate solution (specific gravity, 1.18 to 1.20, as verified with a hydrometer).
4. Centrifuge for one minute at 2500 rpm. Place tube in a rack in a vertical position and slowly add enough zinc sulphate with a dropper pipette to fill the tube so that an inverted meniscus forms.
5. Without shaking the tube, carefully place a 22 x 22μm coverslip on top of the tube so that its underside rests on the meniscus. The meniscus should not be so high that fluid runs down the side of the tube carrying parasites away from the cover glass.
6. Allow the tube to stand vertically in a rack with the coverslip suspended on top for ten minutes.
7. Carefully lift the coverslip with its hanging drop containing parasite eggs and cysts on the underside and mount on a clean slide, liquid side down. A small drop of iodine stain may be placed on the slide prior to adding the coverslip. The slide is gently rotated after adding the coverslip to ensure a uniform mixture. The slide is then thoroughly examined microscopically.