Giardia Lamblia

Giardia Lamblia Specific Antigen Testing
by Renee A. Watase, M.T. (A.S.C.P), MBA, August 1995
Giardia lamblia is one of the most frequently identified stool parasites in the U.S. In addition to transmission from contaminated water supplies, Giardia is easily and frequently spread by the fecal-oral route from both asymptomatic carriers and symptomatic patients. The fecal-oral route is responsible for the not uncommon outbreaks in daycare centers.

Clinical Manifestations

Infection may result in asymptomatic cyst passage (5-15%); acute self-limited diarrhea (25-50%); and a chronic syndrome of diarrhea, malabsorption, and weight loss (35-70%). The incubation period is 1-2 weeks after ingestion of the cysts. The detection of trophozoites and cysts in the stool usually starts after the onset of symptoms and may last as long as 6 months.

The acute syndrome consists of diarrhea, abdominal cramps, bloating, flatulence, malaise, nausea, loss of appetite, and profuse watery stools progressing to greasy, foul-smelling stools. Most patients present at 7-10 days and consequently may have lost as much as 10 pounds.

Patients who progress to chronic diarrhea frequently have profound malaise. The periods of diarrhea may alternate with constipation and normal stools. In children there may be failure to thrive as a result of the prolonged infection.

The epidemiology of Giardiasis has been strongly associated with overseas travelers and wilderness hikers. However, the frequency of urban outbreaks in association with poor fecal-oral hygiene and contaminated urban water supply has increased significantly in the United States.


Diagnosis has been traditionally based on an O&P exam X 3. The first specimen is positive in 50-70% of cases. With three specimens, 90-95% of cases are detected. Since trophozoites and cysts are not excreted immediately upon infection, stool specimens collected early in the infection or during the early onset of symptoms may be negative.

The latest diagnostic development is an enzyme immunoassay (EIA) for Giardia Specific Antigen (GSA 65). The sensitivity is improved over the O&P. A number of studies has reported the sensitivity and specificity to be 95-98%. Consequently, fewer stool samples are required to achieve the 95-100% diagnosis rate.

With the availability of the GSA 65 EIA test, diagnosis by collecting specimens from the upper intestine via a string test and an upper endoscopy will probably be less frequently indicated. Serologic studies are most useful in epidemiological studies since they remain elevated for prolonged periods. Culture has been performed in the research setting but is difficult to use in the clinical setting. Susceptibility testing is also difficult. However, with the availability of a more sensitive and specific diagnostic test, clinical susceptibility studies should become feasible.

In a case of suspected Giardiasis, the most effective test now appears to be the EIA Giardia Specific Antigen test. The same applies to monitoring the effectiveness of treatment and diagnosing Giardia infections in aymptomatic family members. The diagnostic value of the O&P is not in its sensitivity but in its broad screening for a wide variety of pathogens. The O&P will remain a critical test for other causes of parasitic diseases.


The medications for treatment of Giardiasis have varied effectiveness and a number of side effects. Quinacrine has been considered by some to be the drug of choice, but it is no longer being produced in the USA. Metronidazole has an efficacy of 80-95%, and although has never received FDA approval for Giardiasis, is frequently used for Giardiasis. Furazolidone is advocated as an alternative, particularly for pediatrics, and has an efficacy of approximately 80%. Therapy during pregnancy is difficult due to the possible side effects on the fetus. Prevention is the best alternative with proper water treatment facilities and careful hygiene especially in day care centers.

DLS Experience

In comparison with traditional O&P, DLS has found the Giardia Specific Antigen by EIA to be as sensitive as the traditional O&P. The new test is easier to perform. As of August 1995, DLS Microbiology is performing the Giardia Specific Assay.

DLS Specimen Requirements

Fresh stool, unpreserved and refrigerated up to 48 hours, 10% formalin-fixed, or preserved in Cary-Blair or Stuart’s transport medium are acceptable. PVA or MIF treated specimens are not suitable.

DLS TEST SCHEDULE: Monday, Wednesday, and Friday


  1. Addiss, D.G. Evaluation of commercially available enzyme-linked immunoassay for Giardia lamblia antigen in stool. J. Clin. Micro. 1991 29(6):1137-42.
  2. Rosoff, J.D. and Stibbs, H.H. Isolation and identification of a Giardia lamblia-specific stool antigen (GSA 65) useful in diagnosis of giardiasis. J. Clin. Micro. 1986 23(5):905-910.
  3. Rosoff, J.D., et al. Stool diagnosis of giardiasis using a commercially available enzyme immunoassay to detect Giardia-Specific Antigen 65 (GSA65). J. Clin. Micro. 1989 27(9):1997-2002.
  4. Schieven, B.C. and Hussain, Z. Evaluation of an enzyme immunoassay test kit for diagnosing infections with Giardia lamblia. Serodiag. and Immunother. 1990 4:109-113.
  5. Hill, David R. Giardia lamblia, Chapter 59, Priniciples and Practices of Infectious Diseases, 4th edition, G.I. Mandell, J.E. Bennett, and R. Dolin, eds. Churchill Livingstone, 1995.