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  #1  
Old 08-07-2012
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Bacteria Pseudomembranous Colitis Specific Diagnostic Test!

A 35-year-old woman underwent a surgery due to a ruptured ectopic pregnancy. On postoperative day 5, while still on a prophylactic antibiotic, she developed malaise, abdominal pain, nausea, anorexia, and watery diarrhea. Pseudomembranous colitis is suspected. Which of the following tests has the highest specificity for the causative organism?

a) Culture on selective medium
b) Cytotoxicity assay
c) Detection of specific antibodies against the organism
d) ELISA
e) Feeding of an experimental animal with the stool filtrate
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  #2  
Old 08-07-2012
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Pseudo membranous colitis, as I remember it's mostly commonly from C. difficile. Due to prior antibiotic use may be after ectopic pregnancy surgery.
For test I would go with stool culture for C. difficile.
So my answer is option A


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Old 08-07-2012
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Tests that are considered useless by gastroenterogists= C diff. culture for stool and H pylori serology.

To confirm, always do stool antigen for H pylori and toxin assay for C diff.
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Old 08-07-2012
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C.diff is a normal flora of git, so no pt doing culture..the colitis is due to d toxin, so i guess the answer is B...
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colonoscopy and sigmoidoscopy are still employed, stool testing for the presence of Clostridium difficile toxins is now often the first-line diagnostic approach. Usually, only two toxins are tested for - Toxin A and Toxin B - but the organism produces several others. This test is not 100% accurate, and there is a considerable false negative rate even with repeat testing.
Another, more recent two-step approach involves testing for the presence of C. Diff in the stool and then testing for toxin production. The first step is performed by testing for the presence of the C. Diff GDH antigen. If the first step is positive, a second test, a PCR assay targeting the toxin genes, is performed.

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Old 08-07-2012
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c.difficile has 2 toxin:
A:binding-cause diarhea
B: Exotoxin- cause pseudomembranous
so i choose b, detect it with cytotoxicity test
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Both cultures and toxins assay study used to detect C.diff, the culture is the most sensitive one, but clinically they do toxins studies more often than the culture.
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Default correct answer

The answer is b) Cytotoxicity assay

The definitive diagnosis of antibiotic-associated colitis is the cytotoxicity assay, with sensitivity ranging from 94% to 100% and specificity of 99%. In this assay, filtrate of the diarrheal stool is added to fibroblast tissue culture. C. difficile toxins exert a cytopathic effect characterized by rounding of the cells.

If you were thinking about B or D, the explanation beneath is a bit vague..

As for A) - Stool culture using selective media for primary isolation of Clostridium difficile, such as the cycloserine-cefoxitin-fructose agar (CCFA), is the most sensitive test. However, up to 25% of C. difficile strains are nontoxigenic and thus nonvirulent; therefore, the test lacks the desired specificity.

As for C) - C. difficile colonizes the large intestine of approximately 2% of healthy individuals and up to 20% of hospitalized patients. Approximately 25% of C. difficile are nonvirulent; thus detection of antibodies against the organisms is not diagnostic.

As for D) - ELISA is a rapid and more widely used assay with an excellent specificity (99%) and reasonable sensitivity (70% to 90%). Less than optimal sensitivity is due to the fact that ELISA allows the detection of toxin A (enterotoxin) but not toxin B. However, some strains may be negative for toxin A and produce only toxin B, leading to false-negative diagnosis.

As for E) -
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A 35-year-old woman underwent a surgery due to a ruptured ectopic pregnancy. On postoperative day 5, while still on a prophylactic antibiotic, she developed malaise, abdominal pain, nausea, anorexia, and watery diarrhea. Pseudomembranous colitis is suspected. Which of the following tests has the highest specificity for the causative organism?

a) Culture on selective medium
b) Cytotoxicity assay
c) Detection of specific antibodies against the organism
d) ELISA
e) Feeding of an experimental animal with the stool filtrate

B.



Watery diarhea with recent ABO use like this case - The antibiotic associated diarhear/C.difficile colitis should be R/O

Since theys ask for 'the most specificity' - The gold standard is stool assay

However, if they ask what to do next - The practical rapid screening tesis stool toxin A,B - which is ELISa to detect pathogenic toxin.

There are nonpathogenic strain of C.difficile made C) less likely

Although A) toxigenic C/S -culture on selective media and test for toxin also provide high specificity but it take too long time for result
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