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Giardiasis Workup

  • Author: Hisham Nazer, MB, BCh, FRCP, , DTM&H; Chief Editor: Burt Cagir, MD, FACS  more...
Updated: Feb 15, 2016

Approach Considerations

The traditional basis of diagnosis is identification of Giardia intestinalis trophozoites or cysts in the stool of infected patients via a stool ova and parasite (O&P) examination. However, in more recent years, the advent of more objective techniques (eg, immnoassays, nucleic acid amplification techniques [NAATs]) has led to an increase in their use versus those that rely on subjective microscopic examination of fecal specimens for Giardia cysts.[52]

Stool examination may be performed on fresh specimens or after preservation with polyvinyl alcohol or 10% formalin (with appropriate staining). Ideally, 3 specimens from different days should be examined because of potential variations in fecal excretion of cysts. G intestinalis is identified in 50-70% of patients after a single stool examination and in more than 90% after 3 stool examinations.

Stool O&P testing aids in the diagnosis of giardiasis in 80-85% of patients. It remains the diagnostic method with which other tests are compared. Aspiration of duodenal contents and demonstration of trophozoites also have been used for diagnosis but this is more invasive than stool examination and, in direct comparison studies to stool microscopy, may have a lower diagnostic yield.

Stool antigen enzyme-linked immunosorbent assays also are available.[53] These tests are similar to the stool O&P test in terms of cost and have a sensitivity of 88-98% and a specificity of 87-100%. These tests are best used as a screening test in high-incidence settings such as day-care centers or for identification of subjects during an epidemic, but they should not take the place of stool microscopy.

If the results from 3 O&P tests are negative and giardiasis is still suspected, stool antigen enzyme-linked immunosorbent assay (ELISA) may be helpful. If both of these methods result in negative findings but the patient has symptoms consistent with small bowel diarrhea/malabsorption, upper endoscopy with biopsies and duodenal aspirate is a reasonable alternative.

Stool culture is not routinely used because of the difficulty of reproducibly isolating Giardia from patient fecal samples. However, stool cultures are beneficial in ruling out other pathogens as the cause of a patient's symptoms.

Routine laboratory tests (eg, CBC count, electrolyte levels) usually show normal results. Eosinophilia is an uncommon feature of infection.

Because immunoglobulin G (IgG) levels remain elevated for long periods, they are not beneficial in making the diagnosis of acute giardiasis. Serum anti-Giardia immunoglobulin M (IgM) can be beneficial in distinguishing between acute infections and past infections.


Stool Examination

Stool examination (see the images below) for trophozoites or cysts is the traditional method for diagnosing giardiasis. At least 3 stools taken at 2-day intervals should be examined for ova and parasites. Trophozoites may be found in fresh, watery stools but disintegrate rapidly. If the stool is not fresh or is semiformed to formed, trophozoites will not be found.

Cysts are passed in soft and formed stools. Fresh stool can be mixed with an iodine solution or methylene blue and examined for cysts on a wet mount. If not immediately examined, stool should be preserved in polyvinyl, alcohol, or 10% formalin, with subsequent trichrome or iron hematoxylin staining. Cyst passage is extremely variable, not related to clinical symptoms, and may lag behind the onset of symptoms by a week or more.

Cysts are smooth walled and oval, measuring 8-12 μm long and 7-10 μm wide. Iodine stains the cysts brown and accentuates their intracystic structures, especially their curved median bodies, axonemes, and nuclei. By focusing through the plane of the sample, 4 nuclei may be visualized, representing 2 daughter trophozoites.

Trophozoites are leaf-shaped, measuring 9-21 μm long and 5-15 μm wide. Stained organisms have a characteristic facelike image with 2 nuclei and 4 pairs of flagella.[22]

Because many antibiotics, enemas, laxatives, and barium studies mask or cause the disappearance of parasites from the stools, microscopic examination should be postponed for 5-10 days following these interventions.

Fecal leukocytes should not be visualized in stool samples of patients with giardiasis.

Giardia intestinalis trophozoites on stool examina Giardia intestinalis trophozoites on stool examination from a patient with diarrhea.
Giardia cyst. Giardia cyst.
Giardia trophozoite Giardia trophozoite

Stool Antigen Detection

Several tests to detect Giardia antigen in the stool are commercially available.[12, 33, 54] These utilize either an immunofluorescent antibody (IFA) assay or a capture enzyme-linked immunosorbent assay (ELISA) against cyst or trophozoite antigens. These tests have a sensitivity of 85-98% and a specificity of 90-100%.

Polymerase chain reaction (PCR) techniques may detect giardia in stool samples with parasites concentrations as low as 10 parasites/100 mcL. PCR may also be a valuable tool for screening of water supplies.[55] Real-time PCR has also the advantage of being able to detect both mild and asymptomatic infections.[56]

While more sensitive than stool examination, these examinations are limited to the detection of Giardia; isolated use might result in missing an alternative or concurrent parasitic infection.

A 2009 study evaluated a screening test for Giardia and Cryptosporidium on 136 fecal samples. The results showed the test to be 98.4% sensitive and 100% specific; the positive and negative predictive values were 98.7% and 99.3%, respectively.[57]


Abdominal Radiography

No imaging studies are required in the workup of giardiasis. Small-bowel plain films may show nonspecific thickening and distortion of the mucosal folds of the duodenum and jejunum, hypersecretion, and hypermotility.[10, 33] These changes are reversible with therapy. Barium studies should be avoided because barium can obscure identification of parasites for as long as 10 days.


String Test

The string test (Entero-test) consists of a gelatin capsule containing a nylon string with a weight attached to it. The patient tapes one end of the string to his or her cheek and swallows the capsule. After the gelatin dissolves in the stomach, the weight carries the string into the duodenum.

The string is left in place for 4-6 hours or overnight while the patient is fasting. After removal, it is examined for bilious staining, which indicates successful passage into the duodenum. The mucus from the string is examined for trophozoites in an iodine or saline wet mount or after fixation and staining.


Other Tests

Fecal fat quantification or a qualitative fecal fat analysis with Sudan stain may confirm steatorrhea. Serum carotene, folate, and vitamin B-12 levels may be variably depressed as a result of malabsorption. The findings from D-xylose absorption tests may be abnormal.

Disaccharidase deficiency is common during and after treatment and can be diagnosed with the aid of a lactose tolerance breath test.[58]

Serum electrophoresis can help diagnose immunoglobulin A, immunoglobulin M, and, occasionally, immunoglobulin G deficiency states.

Serologic studies are being investigated.


Endoscopy and Biopsy

Esophagogastroduodenoscopy (EGD) may be employed in patients in whom the diagnosis is suspected but unproven after stool microscopy and ELISA. This also may be used for patients who continue to manifest symptoms of malabsorption after adequate therapy.

Endoscopy may be used to assess the small bowel architecture and obtain a duodenal aspirate or biopsy. While rarely necessary, duodenal biopsy may be the most sensitive test. This has several benefits over the string test. An aspirate can be cultured to assess for overgrowth of the small intestine. Other small bowel parasites, such as microsporidia and cryptosporidia, may be detected in biopsy samples. Spruelike lesions, which may occur with giardiasis, can be detected with this technique.

Biopsy can also be used to visualize changes in histologic features. Intestinal biopsy shows flattened, mild lymphocytic infiltration and trophozoites on the surface.[19, 25]

Biopsy specimens from duodenum are often teeming with sickle-shaped Giardia trophozoites, which are tightly bound by the concave attachment disc to the villus surface of the intestinal epithelial cells. In cases where trophozoites are difficult to recognize in biopsy samples, specific anti-Giardia immunoperoxidase stains aid in the detection of the organisms, although these stains are not readily available.

Many patients exhibit atrophy of the small intestinal villi with a mixed inflammatory infiltrate in the lamina propria. The brush borders of the surface absorptive epithelial cells are irregular, and virtual absence of villi may be noted, as in celiac disease.


Histologic Findings

No classic universal histologic abnormalities result from infection with G intestinalis. Patients with giardiasis who undergo endoscopy and small bowel biopsy are likely to have similar findings compared to controls.

Patients with immunoglobulin deficiency states and giardiasis may demonstrate various degrees of villous atrophy that bear a striking resemblance to celiac sprue. These conditions can be differentiated from sprue by the absence of plasma cells in the lamina propria.

Contributor Information and Disclosures

Hisham Nazer, MB, BCh, FRCP, , DTM&H Professor of Pediatrics, Consultant in Pediatric Gastroenterology, Hepatology and Clinical Nutrition, University of Jordan Faculty of Medicine, Jordan

Hisham Nazer, MB, BCh, FRCP, , DTM&H is a member of the following medical societies: American Association for Physician Leadership, Royal College of Paediatrics and Child Health, Royal College of Surgeons in Ireland, Royal Society of Tropical Medicine and Hygiene, Royal College of Physicians and Surgeons of the United Kingdom

Disclosure: Nothing to disclose.

Chief Editor

Burt Cagir, MD, FACS Clinical Professor of Surgery, The Commonwealth Medical College; Attending Surgeon, Assistant Program Director, Robert Packer Hospital; Attending Surgeon, Corning Hospital

Burt Cagir, MD, FACS is a member of the following medical societies: American College of Surgeons, American Medical Association, Society for Surgery of the Alimentary Tract

Disclosure: Nothing to disclose.


Manoop S Bhutani, MD Professor, Co-Director, Center for Endoscopic Research, Training and Innovation (CERTAIN), Director, Center for Endoscopic Ultrasound, Department of Medicine, Division of Gastroenterology, University of Texas Medical Branch; Director, Endoscopic Research and Development, The University of Texas MD Anderson Cancer Center

Manoop S Bhutani, MD is a member of the following medical societies: American Association for the Advancement of Science, American College of Gastroenterology, American College of Physicians, American Gastroenterological Association, American Institute of Ultrasound in Medicine, and American Society for Gastrointestinal Endoscopy

Disclosure: Nothing to disclose.

Brooks D Cash, MD, FACP Director of Clinical Research, Assistant Professor of Medicine, Division of Gastroenterology, National Naval Medical Center

Brooks D Cash, MD, FACP is a member of the following medical societies: Alpha Omega Alpha, American College of Gastroenterology, American Gastroenterological Association, and American Society for Gastrointestinal Endoscopy

Disclosure: Nothing to disclose.

Steven C Dronen, MD, FAAEM Chair, Department of Emergency Medicine, LeConte Medical Center

Steven C Dronen, MD, FAAEM is a member of the following medical societies: American Academy of Emergency Medicine and Society for Academic Emergency Medicine

Disclosure: Nothing to disclose.

Michelle Ervin, MD Chair, Department of Emergency Medicine, Howard University Hospital

Michelle Ervin, MD is a member of the following medical societies: American Academy of Emergency Medicine, American College of Emergency Physicians, American Medical Association, National Medical Association, and Society for Academic Emergency Medicine

Disclosure: Nothing to disclose.

Glenn Fennelly, MD, MPH Director, Division of Infectious Diseases, Lewis M Fraad Department of Pediatrics, Jacobi Medical Center; Clinical Associate Professor of Pediatrics, Albert Einstein College of Medicine

Glenn Fennelly, MD, MPH is a member of the following medical societies: Pediatric Infectious Diseases Society

Disclosure: Nothing to disclose.

Murat Hökelek, MD, PhD Technical Consultant of Parasitology Laboratory, Professor, Department of Clinical Microbiology, Ondokuz Mayis University Medical School, Turkey

Murat Hökelek, MD, PhD is a member of the following medical societies: Turkish Society for Parasitology

Disclosure: Nothing to disclose.

Mark H Johnston, MD Associate Professor of Medicine, Uniformed Services University of Health Sciences; Consulting Staff, Lancaster Gastroenterology Inc

Mark H Johnston, MD is a member of the following medical societies: American College of Gastroenterology, American College of Physicians, American Gastroenterological Association, and Christian Medical & Dental Society

Disclosure: Nothing to disclose.

Sandeep Mukherjee, MB, BCh, MPH, FRCPC Associate Professor, Department of Internal Medicine, Section of Gastroenterology and Hepatology, University of Nebraska Medical Center; Consulting Staff, Section of Gastroenterology and Hepatology, Veteran Affairs Medical Center

Sandeep Mukherjee, MB, BCh, MPH, FRCPC is a member of the following medical societies: Royal College of Physicians and Surgeons of Canada

Disclosure: Merck Honoraria Speaking and teaching; Ikaria Pharmaceuticals Honoraria Board membership

Michael D Nissen, MBBS, FRACP, FRCPA Associate Professor in Biomolecular, Biomedical Science & Health, Griffith University; Director of Infectious Diseases and Unit Head of Queensland Paediatric Infectious Laboratory, Sir Albert Sakzewski Viral Research Centre, Royal Children's Hospital

Michael D Nissen, MBBS, FRACP, FRCPA is a member of the following medical societies: American Academy of Pediatrics, American Society for Microbiology, Pediatric Infectious Diseases Society, Royal Australasian College of Physicians, and Royal College of Pathologists of Australasia

Disclosure: Nothing to disclose.

Andre Pennardt, MD, FACEP, FAAEM, FAWM Clinical Associate Professor of Emergency Medicine, Medical College of Georgia; Assistant Professor of Military and Emergency Medicine, Uniformed Services University of the Health Sciences; Consulting Staff, Departments of Emergency Medicine, Aviation Medicine and Dive Medicine, Womack Army Medical Center

Andre Pennardt, MD, FACEP, FAAEM, FAWM is a member of the following medical societies: American Academy of Emergency Medicine, American College of Emergency Physicians, Association of Military Surgeons of the US, International Society for Mountain Medicine, National Association of EMS Physicians, Special Operations Medical Association, and Wilderness Medical Society

Disclosure: Nothing to disclose.

Barry J Sheridan, DO Chief Warrior in Transition Services, Brooke Army Medical Center

Barry J Sheridan, DO is a member of the following medical societies: American Academy of Emergency Medicine

Disclosure: Nothing to disclose.

Russell W Steele, MD Head, Division of Pediatric Infectious Diseases, Ochsner Children's Health Center; Clinical Professor, Department of Pediatrics, Tulane University School of Medicine

Russell W Steele, MD is a member of the following medical societies: American Academy of Pediatrics, American Association of Immunologists, American Pediatric Society, American Society for Microbiology, Infectious Diseases Society of America, Louisiana State Medical Society, Pediatric Infectious Diseases Society, Society for Pediatric Research, and Southern Medical Association

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

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Giardia lamblia, cyst form.
Giardia lamblia trophozoites in culture.
A Giardia intestinalis cyst.
Giardia intestinalis trophozoites on stool examination from a patient with diarrhea.
Giardia trophozoite
Giardia cyst.
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