Investigations
1st investigations to order
direct fluorescence antibody (DFA) test
Test
Gold standard for diagnosing giardiasis.[60] The test involves incubation of stool sample with antibodies tagged with fluorescent markers. On examination under fluorescent microscope, Giardia cysts appear as green, glowing ovoid objects.[22] When compared with ova and parasite examination, this test is more sensitive and specific and has enhanced sensitivity and faster turnaround time than conventional stool microscopy methods.[61][62] Cost and specificity is usually comparable. Many of the commercially available assays can detect both Giardiaand Cryptosporidium species simultaneously.[22][63]
Result
presence of green, glowing ovoid objects indicates Giardia cysts
enzyme-linked immunosorbent assay
Test
ELISA can detect soluble stool antigens.[60] When compared with ova and parasite examination, this test is more sensitive and specific and has enhanced sensitivity and faster turnaround time than conventional stool microscopy methods.[61][62] Cost and specificity is usually comparable. Many of the commercially available assays can detect both Giardia and Cryptosporidium species simultaneously.[22][63]
Result
positive for parasite antigens and/or cyst wall
nucleic acid amplification test, polymerase chain reaction of stool sample
Test
Nucleic acid amplification test (NAAT) diagnostics offer greater sensitivity than microscopy or DFA for Giardia detection.[61] Polymerase chain reaction (PCR) of stool samples detects parasite concentrations as low as 10 parasites/100 microlitres of stool.[64] A single stool sample is often sufficient for complete parasitological diagnosis.
Often available on a multiplex platform, allowing testing for multiple pathogens.[65][66] Does not confirm viable parasite infection.
Result
positive
stool microscopy
Test
Conventional initial diagnosis of giardiasis is made by stool microscopy.[59] Inexpensive, but requires microscopic expertise and specialised laboratories. Stool samples should be examined fresh for trophozoites and cysts or placed immediately in a preservative. Up to three specimens from different days should be examined.[1] Some authorities have issued specific guidelines for collection of stool samples.[69]
Result
presence of cysts and trophozoites
FBC
Test
Should be ordered as baseline and to assist with differential diagnosis. Indicated in all patients with chronic diarrhoea to screen for immunocompromise and anaemia, which may co-occur with Giardia.
Result
usually normal with no eosinophilia
Investigations to consider
Lateral flow immunoassay (e.g., ImmunoCard STAT!)
Test
Completed in 10-12 minutes. Differentiates between Giardia duodenalis and Cryptosporidium parvum.
Result
presence of antigen
duodenal aspirates and biopsies
Test
Invasive test. Reserved for people with indeterminate, refractory diarrhoea to rule out other potential aetiologies such as coeliac sprue or Crohn's disease. Biopsy allows microscopic examination of small bowel architecture.
Result
presence of cysts and trophozoites
string test (EnteroTest)
Test
Involves swallowing a gelatin capsule attached to a long string. The end of the string remains outside the mouth and is taped to the cheek. The capsule dissolves in the stomach and the string passes into the duodenum. The string is left in place for 4-6 hours or overnight. It is then withdrawn and mucus from the string is examined for trophozoites in an iodine or saline wet mount or after fixation and staining.
Result
presence of trophozoites
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