Entero Test (String test): Principle, Procedure, Uses

Entero-Test, also known as string test or duodenal parasites test is a simple and convenient method of sampling duodenal contents/gastrointestinal fluid. The sample is examined to detect the presence of gastrointestinal parasites or any other enteric pathogens. A commercially available device that consists of a gelatin capsule containing either 90 cm (pediatric version) or 140 cm (adult version) of a highly absorbent nylon string is used for this test.

Principle

A string test involves swallowing a string with a weighted gelatin capsule, to obtain a sample from the upper part of the small intestine. The capsule is swallowed and one end of the string is taped to the side of the patient’s face. The capsule dissolves in the stomach and the string, which is weighted at its distal end, passes into the duodenum by peristalsis. Following a period of approximately 4 hr, the string and any adsorbed gastrointestinal fluid is withdrawn through the mouth.

Any Strongyloides larvae, Giardia trophozoites, or Cryptosporidium, or Cystoisospora (Isospora) oocysts that are present will also adhere to the string and will be pulled up with the string when it is removed.

Any bile, blood, or mucus attached to the string is examined under the microscope as a wet preparation for the presence of intestinal parasites (organisms/eggs) or as a permanent stained smear.

For the patient:

You may be asked not to eat or drink anything for 12 hours before the test. You may find it difficult to swallow the string, and you may feel an urge to vomit when the string is being removed.

When the test is performed?                 

Entero-Test (string test) is performed when a physician suspects a parasite infection, but no parasites were found in a stool sample. As its sensitivity is comparable to duodenal aspirate, it eliminates the need of duodenal intubation.

Procedure

1. Attach the string protruding from the capsule to the corner of the patient’s mouth.
2. Ask the patient to swallow the capsule with water (in the stomach, gelatin capsule is dissolved and the weighted string is carried by peristalsis into the duodenum.)
3. After approximately 4 hours, withdraw the string (during withdrawal, the small steel weight which is attached to the distal end of the string detaches and is eliminated in the stool).
4. Remove the bile-stained mucus clinging to the string by pulling the string between thumbs and finger and collect it in a small petri dish.
5. Examine the specimen by a wet mount technique to detect the presence of motility of the organisms or specific morphological forms (trophozoite of Giardia lamblia and larvae of Strongyloides stercoralis).
6. Iodine may be added later to facilitate the identification of any organism present.

Results:

• Normal test result: No presence of blood, parasites, fungus, or abnormal cells is normal.
• Abnormal test result: Presence of giardia or another parasitic infestation.

Report the organism and stage (trophozoite, cyst, oocyst, etc.) without using abbreviations. For example Giardia lamblia (G. duodenalis, G. intestinalis) trophozoites, Strongyloides stercoralis larvae. Confirmation of some species may require a permanent stained smear.

Uses

String test is a reliable and non-invasive diagnostic method of obtaining intestinal fluid samples for the detection of Giardia lamblia and other enteric pathogens such as Cryptosporidium, Cystoisospora (Isospora).

References

1. Sastry A.S. & Bhat S. (2014) Essentials of Medical Parasitology. Jaypee Brothers Medical Publishers (P) Ltd
2. Gracia, L.S. (2016). Diagnostic Medical Parasitology. ASM Press.