Fecal flotations

Charles T. Faulkner ctfaulkn at utkux.utcc.utk.edu
Sat Mar 13 23:32:29 EST 1999



On Sat, 13 Mar 1999, James L. Mau, Biologist wrote:

>	Two reasons for going to test methods like ELISA are: 1) cost and 2)to
>	help eliminate errors in identifying organisms by less skilled eyes. Our

I think you are right on target with these observations. If a single ELISA
test can reliably detect Giardia or Cryptosporidium then cost containment
is a valid argument. Most studies suggest that 3 fecal samples submitted
over 3 days is necessary to achieve ~90% sensitivity for detection of
Giardia. This issue is a major consideration in human medicine where
infection with a single parasite (usually Giardia or Crypto) seems to be
the normal circumstance. Secondly, development of expertise in microscopic
detection of parasite eggs, larvae, oocysts, and cysts is also more often
an art than science.  Technicians require considerable practice and
familiarity with diagnostic stages of parasitic organisms to become
proficient at microscopic identification.  I'm not sure if Med Techs see
enough parasite cases in routine practice to become trully proficent at
identification of Giardia and Crypto.  In veterinary practice these
parasites are encountered on average 3-4 times a week, and oftentimes more
frequently than that. This enables our techs to develope a high degree of
confidence in their ability to identify the cysts in samples where the
parasites were not even suspected by the clinician. 

In our lab we routinely assess the prior probability (by intuition,
clinical presentation, etc.) that a fecal sample contains Giardia,
Cryptosporidium, Trichuris, hookworm, etc, and apply either the Sucrose or
ZNSO4 flotation technique to enhance our chances of detecting the parasite
of interest microscopically. I can appreciate that Med Techs do not enjoy
this flexibility in an environment that is dictated by insurance coverage,
liability, and threat of litigation. We have found that Giardia floats
2-3x better in ZnSO4 than in Sucrose, conversly Trichuris is recovered
better on Sucrose than ZnSO4. Sometimes we will use both techniques to
simultaneously cover all possibilities.  Although it is well recognized
that Sedimentation is necessary for recovery of Spirurid, Pseudophyllidian
tapeworms, and Trematode eggs, I don't understand why this technique has
predominated in human medicine where the laboratory work is narrowly
focused for detection of a single parasite species, rather than generally
focused for detection of any parasite species. In veterinary parasitology
we usually reserve this technique for special circumstances like zoologic
primate collections where Acanthocephalans or Spiruids like Trichospirura
are potentially implicated in a disease outbreak.  Judy's observation that
the risk of aerosolizing pathogens makes the most sense because the
sedimentation is performed in a closed centirfuge tube. The ELISA
procedures for Giardia are simply too expensive for routine use in
veterinary medicine.  Our flotation procedures cost approx $1.00 per
sample to perform. In contrast, the ELISAs cost between $15
and 25 per sample.  Because of this greater expense we reserve those
procedures for cases that continue to ellude us after microscopic
examination of several samples over the 3-4 day period first recommended
by Sawitz and Faust (1942), and others who have recently published on this
topic.

I think it is helpful for us to be able to share our different
perspectives on diagnostic parasitology. I hope others will feel free to
continue this thread as I would like to learn more about the human side of
the field. 

     ************************************************
     *  Charles T. Faulkner, Ph.D.                  *
     *  Clinical Parasitology Service               *
     *  Dept. of Comparative Medicine               *
     *  2407 River Drive                            *
     *  Knoxville, TN 37996-4500                    *
     *  Voice: (423) 974-5645  Fax: (423) 974-5640  *
     *  E-Mail: ctfaulkner at utk.edu                  *
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