Selection of Stool Samples:
- The specimen of choice is diarrheal stool.
- Rectal swab is recommended only for infants.
- Three specimens should be considered every other day or every third day and have been considered adequate.
- Pinworms deposit their eggs on the perianal skin.
- The worms usually lay their eggs periodically at night while the patients sleep.
- The best specimen is taken in the early morning before bathing.
Collection of Stool Samples:
Principles of Proper Sample Collection
- One of the most important steps in the diagnosis of intestinal parasites is the proper collection of specimens.
- Improperly collected specimens result in inaccurate results.
- Fresh specimens should be mandatory for the recovery of motile trophozoites.
- All fecal specimens should be collected prior to the administration of antibiotics or antidiarrheal agents.
- The use of mineral oil, bismuth, and barium prior to fecal collection should be avoided, since these substances interfere with detection or identification of intestinal parasites.
- The examination of purged specimens has to be performed less frequently.
- The same time limits for fixation and/or examination of diarrheic stools should be used for purged specimens submitted for examination.
- The fecal specimen should be collected in a clean, wide-mouthed container or on newspaper and transferred to a container with a tight-fitting lid.
- Contamination with urine or toilet water has to be avoided.
- The specimen should be transported to the laboratory as soon as possible or kept refrigerated until transport.
Recommended Materials and Precautions
- Gloves should be worn when performing this procedure.
- Areas of the feces that appear bloody, purulent, or watery should be sampled for examination as direct wet smears, fecal concentration, and permanent stained smears.
- For adequate sampling of a formed fecal specimen, material from the sides, ends, and middle should be collected and examined using fecal concentration procedures and permanent stained smears.
- To ensure the recovery of parasitic organisms that are passed intermittently, a minimum of three specimens should be examined over a 7–10-day period.
- Infections with Entamoeba histolytica/E. dispar or Giardia lamblia require the examination of up to six specimens before the organism has been detected.
- Liquid specimens should be received and examined or preserved by the laboratory within 30 minutes of passage, soft or semiformed specimens within 1 hour, and formed specimens on the same day.
- All fresh specimens should be handled carefully.
- Fixatives should be properly labeled.
- Every specimen should be labelled with
- Patient’s name and identification number
- The date and time the specimen was collected
- Each specimen should be accompanied by a request form indicating which laboratory procedures have been performed.
- Information concerning the presumptive diagnosis or relevant travel history should be included whenever possible.
Preservation of Stool Samples:
Principle and Importance of Preservation
- Fecal specimens should be preserved for further analysis if they cannot be processed and inspected in a timely manner.
- Preservatives stop existing parasites from degrading.
- There are several fixatives that can be used to preserve helminths and protozoa.
- The choice of preservative has given the laboratory the ability to perform concentration techniques and prepare permanent stained smears.
Types of Preservatives and Their Uses:
Schaudinn’s Fixative
- This preservative is used with fresh stool specimens or intestinal mucosal samples.
- Permanent stained smear is prepared from fixed material.
Advantages
- Prepared slides are stored in the fixative for up to a week without distortion.
- It is easily prepared in the laboratory.
- It is available from several suppliers.
Disadvantages
- It is recommended for use in concentration techniques.
- It contains mercury compounds (mercuric chloride), causing disposal problems.
PVA (Polyvinyl Alcohol)
- PVA has is incorporated into Schaudinn’s fixative.
- The PVA powder serves as an adhesive for stool material.
- Fixation is accomplished by the Schaudinn’s fluid itself.
- PVA fixative solution is recommended for preserving cysts and trophozoites.
- PVA use is permitted specimens to be shipped worldwide for later examination.
Advantages
- Permanent stained smears are prepared, and concentration techniques have been performed.
- Protozoan trophozoites and cysts are well preserved.
- It has a long shelf life in sealed containers.
- It is commercially available and allows specimen shipment.
Disadvantages
- Some organisms (e.g., Trichuris trichiura, Giardia lamblia, Isospora belli) is not concentrated as well from PVA as from formalin-based fixatives, and morphology is distorted.
- It contains mercury compounds (Schaudinn’s fixative), creating disposal problems.
- It turns white and gelatinous when aliquoted or refrigerated.
- It is difficult to prepare in the laboratory.
MIF (Merthiolate-Iodine-Formalin)
- MIF is used to detect protozoa, eggs, and larvae without further staining.
- Many laboratories examine the material only as a wet preparation.
Advantages
- It is combined preservative and stain, useful in field surveys.
- Protozoan cysts and helminth eggs and larvae is diagnosed from temporary wet mounts.
Disadvantages
- Permanent stained smears are difficult to prepare.
- The iodine component is unstable and needs to be added before use.
- Concentration techniques give unsatisfactory results.
- Morphology of organisms distort after prolonged storage.
Diluted Formalin
- Formalin is used as an all-purpose fixative for helminth eggs, larvae, and protozoan cysts.
- Two concentrations, 5% (for protozoan cysts) and 10% (for helminths) are used.
Advantages
- It is a good preservative for protozoan cysts and helminth eggs and larvae.
- Materials are preserved for several years.
- It is used in sedimentation concentration techniques.
- It has a long shelf life and has been commercially available.
- Neutral buffered formalin maintains morphology with prolonged storage.
Disadvantage
- Permanent stained smears cannot be prepared from formalin-preserved specimens.