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GI Stool Studies


iFOBT immunochemical; specific for human globin; fewer false pos
FOBT guidelines CLIA waived; 3 serial stools; avoid red meat & >250 mg vitamin C x 3 days; avoid ASA >325 mg x 7 days
FOBT timing s/b after 3 days; at least 3-5 min; within 14 days
FOBT other sources of false pos EtOH, anticoag, steroids, CTx, iodine prep
Fecal WBC: methylene blue; PMNs (poss macrophages); invasive organisms & IB disorders; >1 g spec clean dry container (Cary Blair); not done on pts in hosp >3 days
Fecal WBC: present for : shigella, campy, EIEC (poss salmonella, vibrio, yersinia, C diff)
Poss false negs in fecal WBC E. histolytica; CMV
Predominant causative enterotoxin for pseudomem colitis C diff toxin A; recent Abx tx; do ELISA test; may need serial stools
C diff toxin assay specimen criteria specimen >5 ml (fridge if >1 hr); pt 5 soft/liquid stools /24 hr; submit 3 specimens on 3 different days
Gold std for pseudomem colitis tissue cx (rarely done; usu do toxin assays); if done, need bx and anaerobic cx
3 most commonly screened bac pathogens Salmonella, Shigella & Campylobacter (4 other important patho: Vibrio, Aeromonas, Yersinia & E. coli O157:H7; all cx except E Coli: testing stool for toxin)
Bac stool cx specimen fresh/unpreserved <2 hr; or preserved (Cary-Blair) <96 hr; not in pts hosp > 3 days
O&P specimen parapak; if fresh: <2hr (liquid) or <4hr (formed); 1 spec/pt/wk or 1/hosp; reject if immunocompetent >6 yo or hosp >3 days
Acid fast stain for: crypto and cyclospora
IFA: for: Giardia (uses monoclonal Ab); also avail for Crypto
Rotavirus F/O trans; Jan-May; kids 6 mo-2 yr; 1-3 d incub, lasts 5-8 d; EIA: 1 mL stool, fridge if delay
PCR for Norovirus ONLY performed for epidemiologic reasons
Fecal fat screen for malabsorption dz; gold std: Quant: high-fat diet x2d before & during collection (72 hrs); normal <7g/24hrs; Qual: Sudan stain (high-fat diet): Pos = multiple fat drops seen (40x obj)
Tests for C diff: Cytotoxin B in stool is definitive (takes 24h); rapid ELISA for enterotoxin A & B faster but less sensitive
Positive FOBT should always be followed by: colonoscopy (and no more FOBTs needed) Negative FOBT tests: should be:
comma shaped GNR, sensitive to low pH V cholera; stims adenyl cyclase to overproduce cAMP
Shigella dx studies fecal WBC & RBCs, +stool cx; sigmoidoscopy: inflamed engorged mucosa, punctate lesions, ulcers
Created by: Abarnard
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