Cysts will not be detectable in clinical samples from
all human giardiasis cases, and the absence of trophozoites and cysts in repeated submissions of samples
from symptomatic individuals does not necessarily
indicate the absence of infection. Apart from recuperating immunocompetent cases, where trophozoite
and cyst abundance can be low, low cyst abundance
can occur because of fl uctuations in trophozoite abundance and intestinal motility, resulting in sporadic
cyst excretion. In these instances, and particularly
when clinical suspicion is high, trophozoite and cyst
negative stool samples should be subjected to antigen
and/or PCR-based detection, as sufficient Giardia antigen or DNA from trophozoites should be present in
faeces. For PCR-based methods, nested PCR methods, being more sensitive than direct PCR methods,
are likely to have a higher diagnostic index, particularly when single copy gene loci are used.
When trophozoites or cysts, Giardia antigen or
DNA cannot be found following extensive examination of repeat stool samples, and giardiasis is suspected clinically, more invasive procedures, such as the
examination of duodenal or jejunal fluid and/or tissue
biopsy may be indicated. Both are invasive procedures, requiring close liaison with other health professionals and should be used with caution. Duodenal or
jejunal fl uid can be sampled by endoscopy or the Enterotest® (a rubber-lined weighted gelatine capsule
containing a nylon string which is swallowed whilst
the free end of the string is taped to the side of the
mouth. The Enterotest® is left in place for 4–8 h, by
which time the string extends to its full length, and its
distal half becomes saturated with bile-stained mucus.
Once removed, the mucus is scraped off the string