Question

What are some of the concentration techniques used in fecal examination? Explain each.

Answer 1

Ans) Concentration procedure separate parasites from fecal debris and increase the chances of detecting parasitic organisms when these are in small numbers. They are divided into flotation techniques and sedimentation techniques.

- The use of a concentration method is essential for the examination of faeces for parasitic diseases as it increases the likelihood of finding ova, cysts and larvae, particularly in those specimens where they are present in numbers too low to be seen by direct microscopy.

- Each stool specimen was examined by the following techniques.
1. Macroscopic examination:
The colour, consistency and the nature of the faeces were recorded. The stool specimens were examined for the presence of worms like Ascaris, Enterobius, proglottids of Taenia, adult Hookworm and Trichuris, either with the naked eye or with the aid of a hand lens.
2. Direct microscopic examination by using saline and iodine preparations:
On a 1mm thick microscopic slide, a small amount of stool sample was emulsified in 1-2 drops of saline or iodine solution. A cover slip was placed on it by taking care that the preparation was free of air bubbles and macroscopic debris.
3. The microscopic examination after the various concentration techniques:
a) Simple salt floatation:Briefly, about 1gm of faeces was emulsified with 3-4 ml of saturated salt solution in a 20ml conical glass test tube. It was stirred well and more salt solution was added till the container was nearly full, with the stirring being continued. Any coarse matter which floated up was removed and the tube was placed on a levelled surface with a glass slide being placed over the top of the tube, which was in contact with the fluid. It was allowed to stand for 30 minutes. The slide was removed and observed for the presence of eggs/cysts.
b) Zinc sulphate centrifugal floatation: 1g of the stool specimen was emulsified in 10 parts of tap water and it was strained through a wire gauze. The filtrate was collected in a Wassermann tube and centrifuged at 2,500 rmp. The supernatant was discarded and the sediment was re-suspended in water. This step was repeated till the supernatant became clear. To the sediment, 3-4 ml of 33% Zinc sulphate solution was added, it was mixed well and it was filled with ZnSO4 solution, about half an inch of the rim. Several loopfuls of the supernatant fluid were removed with a bacteriological loop and they were observed for parasites.
c) Formol-ether concentration: 1g of stool was emulsified in 7ml of 10% formol saline and it was kept for 10 minutes for fixation. It was then strained through a wire gauze. The filtrate was added to 3 ml of ether and it was centrifuged at 2000 rpm for 2 minutes. It was allowed to settle. The supernatant was removed and a wet mount was made of the deposit to look for parasites.
d) Formol-ether concentration which was modified by Allen and Ridely: 5 It was a modification of the formol-ether method where the centrifugation was done at 3000 rpm for 60 seconds instead of 2000 rpm for 2 min. The sediment was used for the parasitic examination.