Approach

A history of contact with contaminated water (such as swallowing water while swimming, drinking untreated tap water, eating lettuce, and contact with recreational fresh water) should be noted. Contact with children in diapers is an identified risk factor.

Absence of a known risk factor should not preclude diagnostic evaluation in patients with symptoms compatible with giardiasis.

Clinical evaluation

Presenting symptoms include diarrhea, abdominal bloating with cramps, frequent belching with a sulfur smell, nausea, anorexia, and/or fatigue. Children and immunocompromised people may present with weight loss, malnourishment, and faltering growth or other growth delays, even in the absence of other symptoms.[1]​​​[54]​​​​​ If a history of chronic diarrhea coexists with recurrent respiratory tract infections, common variable immunodeficiency should be considered.[55]

Investigations

Given the frequent mild presentation and overlapping symptomatology with other enteric pathogens, pathogen-directed testing is required to diagnose Giardia and to avoid delays in anti-giardial therapy.[56]

Part of the initial workup should include a complete blood count (indicated in all patients with chronic diarrhea to screen for immunocompromise and anemia, which may co-occur with Giardia), but this is likely to be normal with no eosinophilia.

Stool microscopy

Conventional initial diagnosis of giardiasis is made by stool microscopy.[57] Stool samples should be examined fresh for trophozoites and cysts or placed immediately in a preservative. There is variation in fecal excretion of cysts so it is recommended that up to three specimens from different days be taken.[1]​​[58]

Direct immunofluorescence, enzyme immunoassay, and molecular-based detection

These methods are increasingly replacing microscopy, and experienced microscopists are not always available. Microscopy with direct fluorescent antibody (DFA) testing is the gold standard for diagnosing giardiasis.[58]​ Enzyme-linked immunosorbent assay that can detect soluble stool antigens may be requested. When compared with ova and parasite examination, these tests are both more sensitive and specific. These methods, in general, have enhanced sensitivity and faster turnaround time than conventional stool microscopy methods.[59][60]​​ Cost and specificity are usually comparable. Many of the commercially available assays can detect both Giardia and Cryptosporidium species simultaneously.[20][61]

Other tests that can be performed include polymerase chain reaction (PCR)-based assays, rapid immunochromatographic cartridge assays, or microscopy with trichrome staining.​​​​[58]

Nucleic acid amplification test (NAAT) diagnostics offer greater sensitivity than microscopy or DFA for Giardia detection.[59] PCR of stool samples detects parasite concentrations as low as 10 parasites/100 microliters of stool.[62] A single stool sample is often sufficient for complete parasitological diagnosis. According to one study, application of real-time PCR improved the diagnostic yield by 18%. Although PCR-based testing of stool samples for Giardia has not yet been widely standardized, Giardia is commonly represented on clinically-available NAAT multiplex detection platforms capable of identifying multiple viral, bacterial, and parasitic targets.[63][64]​ Inability to distinguish viable parasites from non-living parasite DNA is a limitation of PCR-based diagnostics.

Other investigations to consider

  • Lateral flow immunoassay (e.g., ImmunoCard STAT!): detects, and distinguishes between, Giardia duodenalis and Cryptosporidium parvum in aqueous extracts of human fecal specimens.[65]

  • Duodenal aspirate and biopsy: invasive, but may be superior for detecting trophozoites and other enteric pathogens, such as microsporidia and cryptosporidia. These tests may be done when other tests have failed to reveal the diagnosis.​[66]​​

  • String test (EnteroTest): 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 the end is examined under the microscope for parasites that have become attached. The string test may be done if other methods (especially exam of stool samples and antigen tests) have failed to detect giardiasis and endoscopy is unavailable or contraindicated.

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