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Rosalie Jalbert asks if " you must be off all antacids,
>i.e. PeptoBismal before the tests [for giardia] can be done"?.

I've had lots of stool exams done for parasites over the last 20 years and
no one has ever suggested that antacids might interfere.  According to
Markell and Voge's text, Medical Parasitology, Giardia lamblia "is one of
the most easily recognized intesinal protozoa."  They go on to write (p.74)
that "Giardia does not appear consistently in the stools of all patients.
Danziger and Lopez (1975) describe three patterns of excretion: high, with
parasites present in nearly all stools, low, with small numbers of parasites
present in only about 40% of stool specimens; and a mixed pattern with one
to two weeks of a high excretion rate alternating with a shorter period of
low excretion.  With this in mind, it is well to collect specimens at
intervals some days apart, if initially they are negative."

It is pretty standard practice to submit three stool specimens whenever a
parasite infection is suspected.  My insurance company recently refused to
pay for the two of the three until I got my doctor to send them a "medical
justification" for submitting three.

> My next question would then be--how long does untreated
>giardia stay in your intestine and what effects would it cause?

Many people in poor countries such as Mozambique, who have poor access to
treatment and clean water, live all their lives with Giardia.  I don't know
how significant a shortening of potential lifespan might be involved.

As for effects, Markell and Voge write (pp.74-75) Biopsy of the jejunum
...reveals in some patients shortening and blunting of the villi, reduction
in height of the columnar epithelial cells of the mucosa, and
hypercullularity of the lamina propria. In others with severe symptoms, a
fairly normal villus architecture is seen. "