Intestinal parasite concentration techniques

Purpose and indications

• Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1498
• Medical Microbiology 9e, p. 784

1. A routine direct wet mount is negative but intestinal parasitic infection remains suspected.
2. Stool is formed or semi-formed, in which cysts and helminth eggs are more likely than motile trophozoites.
3. Parasite burden is expected to be low, for example early infection, light infection, travelers, or screening.
4. A preserved stool specimen is submitted for ova-and-parasite examination.
5. Helminth eggs or protozoan cysts need better recovery from debris before microscopy.
6. Strongyloides stercoralis is suspected, especially in patients with eosinophilia, relevant exposure, unexplained gastrointestinal symptoms, or before immunosuppression. Use a larval recovery method such as Baermann rather than routine concentration alone.
7. Egg quantification is required for surveillance, mapping, assessing infection intensity, or monitoring population-level deworming programs. Kato-Katz and Mini-FLOTAC are used for this purpose.

• Fresh watery stool is available and amoebic or flagellate trophozoites are suspected. Examine a fresh saline wet mount immediately.
• Small coccidian oocysts are suspected, such as Cryptosporidium, Cyclospora, or Cystoisospora. Request appropriate modified acid-fast stain, antigen assay, or molecular testing.
• Enterobius vermicularis is suspected. The perianal adhesive tape test is preferred.
• A permanent stained smear is needed for detailed identification of intestinal protozoa. Concentrated wet mounts and permanent stains are complementary.

1. Sedimentation techniques

Principle

A. Formol-ether or formalin-ethyl acetate concentration technique

Principle

• Formalin fixes and preserves parasitic morphology.
• Ether or ethyl acetate extracts fat and helps remove debris.
• After centrifugation, parasites concentrate in the bottom sediment.

• Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1498
• Tietz Textbook of Laboratory Medicine, 7th Edition, p. 1440

Reagents and equipment

• Stool specimen, fresh or fixed in 10% formalin
• 10% formalin or saline
• Gauze or wire mesh strainer
• Centrifuge tube and centrifuge
• Ether or ethyl acetate
• Applicator stick
• Pipette
• Microscope slides, coverslips, saline and iodine

Procedure

1. Mix the stool thoroughly.
2. Emulsify approximately 1 to 2 g of stool in 7 to 10 mL of 10% formalin or saline.
3. Filter the suspension through gauze or a mesh strainer into a conical centrifuge tube to remove large particulate matter.
4. Add formalin or saline to a total volume of approximately 10 to 15 mL.
5. Centrifuge and discard the supernatant, retaining the sediment.
6. Resuspend the sediment in about 10 mL of 10% formalin.
7. Add approximately 3 to 4 mL ether or ethyl acetate. Stopper the tube securely.
8. Shake vigorously for about 30 seconds.
9. Centrifuge again. Four layers form:

1. Loosen the debris plug from the side of the tube and decant the upper three layers.
2. Resuspend the final sediment. Prepare saline and/or iodine wet mounts and examine systematically at low power followed by high power.

Advantages

• Broad recovery of protozoal cysts, helminth eggs, and many larvae.
• Useful with fresh or formalin-preserved stool.
• Good recovery of heavy eggs, operculated eggs, and many trematode/cestode eggs that may fail to float.
• Less cyst distortion than zinc sulfate flotation.
• Ethyl acetate avoids ether's major flammability hazard.
• Widely available, familiar, and suitable for routine laboratories.

Limitations

• Sediment remains relatively dirty, so scanning takes longer.
• Trophozoite motility is lost and trophozoites are poorly detected.
• Formalin and ethyl acetate require safe handling and ventilation. Ether is particularly hazardous because it is volatile and highly flammable.
• It is not the best standalone method for Dientamoeba fragilis, coccidia, or microsporidia.
• Centrifugation and careful technique are required.
• Small numbers of Strongyloides larvae may still be missed.

B. Baermann technique

Principle

Procedure

1. Set up a glass or plastic funnel in a stand, attached to rubber tubing closed with a clamp.
2. Place a wire or nylon sieve over the funnel and cover it with several layers of gauze.
3. Put a portion of fresh stool onto the gauze.
4. Fill the funnel with lukewarm water until the feces just contact the water.
5. Leave undisturbed, commonly for at least 2 hours. Motile larvae migrate through the gauze into the water and settle at the funnel tip.
6. Release a small volume of fluid from the tubing into a centrifuge tube.
7. Centrifuge if needed and examine the sediment microscopically for rhabditiform or filariform larvae.

• Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1499
• Tietz Textbook of Laboratory Medicine, 7th Edition, p. 1441

Advantages

• More sensitive than routine O&P microscopy for recovery of viable Strongyloides larvae.
• Can recover other motile nematode larvae.
• Simple equipment and relatively inexpensive.

Limitations

• Primarily detects living larvae, not cysts or eggs.
• Needs a fresh specimen and viable larvae.
• Slow and labor-intensive.
• Potential biohazard: filariform Strongyloides and hookworm larvae may be infective. Use standard precautions.
• Sensitivity still varies with intermittent larval shedding. Repeated stool examinations may be necessary.
• Agar plate culture may be more sensitive for Strongyloides in laboratories equipped to perform it.

2. Flotation techniques

Principle

• Zinc sulfate, specific gravity approximately 1.18 to 1.20
• Saturated sodium chloride, specific gravity approximately 1.20

A. Zinc sulfate flotation

Procedure

1. Mix fresh stool with saline or water and strain through gauze into a centrifuge tube.
2. Centrifuge, discard supernatant, and retain sediment.
3. Resuspend the sediment in zinc sulfate solution, specific gravity about 1.18 to 1.20.
4. Centrifuge gently or allow standing, according to the laboratory protocol.
5. Fill the tube to create a convex meniscus.
6. Place a coverslip on the meniscus for about 5 to 10 minutes.
7. Lift the coverslip vertically, place it on a slide, and examine promptly.

Advantages

• Produces a cleaner background than sedimentation.
• Good for many protozoan cysts and relatively light nematode eggs.
• Particularly useful for fertilized Ascaris, Trichuris, hookworm, Hymenolepis, and Enterobius eggs.
• Faster microscopic scanning because there is less debris.

Limitations

• Hypertonic solution can collapse or distort protozoan cysts and some eggs, especially if exposure is prolonged.
• Poor recovery of heavy eggs, including many trematode eggs, schistosome eggs, large cestode eggs, and infertile Ascaris eggs.
• Poor method for nematode larvae, including Strongyloides.
• High fat content in stool may reduce recovery.
• Microscopy should be performed promptly because morphology deteriorates over time.

B. Sodium chloride flotation

Principle and procedure

1. Mix the sample with saturated sodium chloride solution.
2. Fill the tube or flotation chamber to a convex meniscus.
3. Apply a coverslip.
4. Allow eggs to rise, usually 10 to 15 minutes.
5. Transfer the coverslip to a slide and examine.

Advantages

• Cheap and simple.
• Requires minimal equipment.
• Useful in field laboratories for many lighter nematode eggs.
• Can be used where centrifugation is unavailable.

Limitations

• Salt crystals may form and obscure the field.
• Osmotic damage may distort delicate eggs and protozoan cysts.
• Not suitable for heavy eggs and most larvae.
• Less useful for accurate protozoal cyst morphology than zinc sulfate or sedimentation.
• Not appropriate for reliable detection of Strongyloides, schistosomes, and many cestode/trematode eggs.

C. Mini-FLOTAC

Principle

Simplified procedure

1. Collect approximately 2 g of stool in a standardized collection device.
2. Fresh stool may be examined directly, or the specimen may be fixed, commonly using 5% formalin.
3. Homogenize the stool in the prescribed volume of diluent.
4. Pass the mixture through the device's filter.
5. Add the selected flotation solution.
6. Load the suspension into both Mini-FLOTAC chambers.
7. Allow eggs to float to the reading surface.
8. Rotate the reading disk and count eggs in the gridded chambers.
9. Convert the egg count to EPG using the device-specific multiplication factor.

Advantages

• Quantitative EPG result.
• Larger stool amount than a single Kato-Katz smear, which may improve recovery.
• Can use fresh or fixed stool.
• Fixed specimens can be stored and transported, allowing later examination.
• Closed device reduces some handling and contamination.
• Useful for surveys and monitoring control programs.

Limitations

• Requires proprietary devices and standardized consumables.
• Requires correct flotation solution and careful homogenization.
• Like other microscopy methods, sensitivity decreases in very light-intensity infection.
• Flotation-related distortion can occur.
• More expensive and less widely available than Kato-Katz in many endemic settings.
• Not intended for detecting trophozoites and is not the preferred test for Strongyloides larvae.

3. Filtration and Kato-Katz thick smear

Important clarification

Principle

Procedure

1. Place a small amount of fresh stool on paper or a tile.
2. Press it through a nylon or metal mesh to remove coarse fibers and debris.
3. Place a standard template with a central hole on a slide.
4. Fill the template hole completely with sieved stool.
5. Remove the template, leaving a standardized stool plug.
6. Cover the specimen with glycerol-soaked cellophane, usually containing malachite green or methylene blue.
7. Invert the slide and press gently on a smooth surface to spread the sample evenly into a thick smear.
8. Allow clearing.
9. Examine microscopically and count the eggs of Ascaris, Trichuris, hookworm, and, where relevant, Schistosoma.
10. Calculate EPG.

Advantages

• Cheap, simple, standardized, and suitable for field settings.
• Quantifies infection intensity.
• Established method for mapping and monitoring soil-transmitted helminth control programs.
• Does not require centrifugation or flotation reagents.
• Easy to train for high-volume surveys.

Limitations

• Low sensitivity in light infections, particularly when only one smear or one stool specimen is examined.
• Not suitable for protozoal cysts or trophozoites.
• Fresh stool is needed. It is not ideal for delayed processing.
• Hookworm eggs clear rapidly, so slides must be read promptly. Delayed reading can cause false-negative hookworm results.
• Stool quantity examined is small, only 41.7 mg per standard template.
• Egg counts vary between days and between samples.
• Not a reliable standalone method for Strongyloides because larvae, rather than eggs, are usually passed in stool.
• Multiple slides and/or multiple specimens improve sensitivity.

Comparison at a glance

Practical selection

• Routine broad stool O&P examination: Formalin-ethyl acetate sedimentation plus permanent stain when protozoa are a concern.
• Suspected Strongyloides: Baermann technique, agar plate culture, and/or molecular testing where available.
• Field survey for soil-transmitted helminths: Kato-Katz or Mini-FLOTAC.
• Cleaner preparation for cysts and lighter eggs: Zinc sulfate flotation.
• Heavy eggs, operculated eggs, trematode eggs, or broad screening: Prefer sedimentation over flotation.