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262 CHAPTER 12

PROCEDURE 12-2

CONCENTRATION TECHNIQUE—FORMALIN-ETHYL ACETATE
Clinical Rationale
Fecal concentrate procedures by the sedimentation method increase the probability of
finding parasites and their ova when low numbers are present in the stool specimen.

This is a routine technique and is often used in conjunction with a flotation method that
is particularly valuable in detecting ova of parasites but the ethyl acetate method is
most commonly used for concentrating eggs and cysts. A single concentrated sample
from an infected patient is sometimes adequate for detecting helminths (worms).
It is often necessary to utilize several specimens in order to detect protozoal organ-

isms due to the difficulty in finding stages that are somewhat easily identified
(Figure 12-6).

SEDIMENTATION

Add formalin and
ethyl acetate
to sediment
Ethyl acetate
Mix and Fecal debris
Centrifuge Centrifuge
Mixture of
preserved feces Formalin
and water or saline
Pour off
Supernatant Delmar/Cengage Learning

Sediment containing
Sediment
parasites
FIGURE 12-6 The sedimentation concentration method concentrates the parasites and their ova as a button in the tip
of the centrifuge tube

Safety Alert:
Standard precautions must be employed when handling fecal samples and other
tissues and body fluids being examined for parasites. Both biological hazards and

chemical hazards may be present. Specimens may still be potentially infectious
after the addition of fixative, because helminth eggs and cysts of protozoa are

resistant to PVA.