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GI Stool Studies
Gastroenterology
| Question | Answer |
|---|---|
| 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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