Practical Parasitology

Stool Collection and Fixation


Overview

In view of recent literature on stool collection (see references 1, 10, 11, 12, 13, 14, 17,18) it becomes even more necessary that the laboratory receives the best possible stool specimen.

Stool container with approximately 15 ml. of SAFThe photo to the left illustrates the stool collection container with attached spoon to make collection more convenient. In this case the stool fixative is SAF. This fixative contains sodium acetate, acetic acid and formalin (SAF). Although there are other fixatives SAF is one of the best all around fixatives and unlike PVA and Schaudinn's fixative contains no mercury compounds.

What is The Best Stool Specimen?

The patient should be instructed by the physician or nurse to read the enclosed instructions carefully. The administration of drugs or anti-diarrheal substances such as mineral oil, barium, bismuth, magnesium and antibiotics should be terminated at least one week before stool collection. These substances may interfere with the ova and parasite examination by reducing the visibility of organisms.
A good stool specimen is one that has been sampled from areas containing blood, mucous and areas of loose or watery consistency. A solid stool should be sampled from both ends and the middle. The stool sample amount should be about the size of one cherry for the smaller container and two cherries for the larger stool specimen containers.
One of the most important steps is the mixing of the stool with fixative. The purpose of fixative is to preserve the morphology of the parasites. Fixative therefore, must come into contact with the organism and proper mixing will achieve this. The trophozoite stage is a very fragile stage and chances for recovery are increased when the stool is fixed completely and as quickly as possible.

New Stool Collection Guidelines

In the past a series of (3) specimens collected on alternate days was recommended as the standard for the ova and parasite examination. Recent literature (see the link above) has suggested that (1) specimen properly collected, fixed and examined for the proper length of time will yield 90% of the intestinal parasites present. A second and third specimen will yield 8% and 2% respectively.(14)
It is recommended the second and third specimen be held back until the results of the first specimen are known. If the result of the first examination is, "No Parasites Seen" and the patient is not symptomatic then the necessity for a second and third specimen examination is eliminated.
If, however the result of the first specimen examination is, "No Parasite Seen" and the patient is symptomatic then perhaps further investigation is warranted and submission of additional specimens is recommended.

Preservation

If the specimen cannot be examined within 1 hour it should be placed in fixative.

To make up 100 ml of SAF

NOTE: Some helminths are not completely fixed with SAF, therefore universal precautions should be practiced.

If you would like a copy of the guideline, entitled, "Guidelines for the Collection and Testing for Ova and Parasites in Symptomatic Patients, CLP-018" you may Email the OAML