Salmonellosis laboratory tests
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2] Jolanta Marszalek, M.D. [3]
Overview
Symptoms of salmonellosis are indistinguishable from those caused by other gastrointestinal pathogens, therefore Salmonella must be identified with further studies. Stool culture in appropriate culture media, such as blood and MacConkey, is the most common study to identify the bacteria. When there are not enough bacteria on the sample, enrichment broths may be used to grown more bacteria. Serologic tests with polyvalent and specific antisera may be used to identify the serotype of Salmonella. The bacteria may also be identified in stool with microscopic examination and lactoferrin tests, however, due to the high dependence on the experience of the operator of the first, and cost and high rate of false-positives of the lactoferrin test, these are not commonly used. Imaging studies such as abdominal CT and ultrasound may be used to identify and monitor complications of salmonellosis, such as hepatic abscess. Colonoscopy may be used to rule out other causes of bloody diarrhea.
Laboratory Findings
The table below displays the nonspecific laboratory abnormalities associated with Salmonellosis, including:[1][2]
| Test | Findings |
|---|---|
| Serum chemistry | |
| Complete Blood Count | |
| Blood cultures |
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| Urinalysis |
Stool Cultures
Stool cultures in adequate culture media, allow the correct identification of the pathogen responsible for infectious diarrhea. The organism should be isolated from fresh stool. The sample should be planted in different selective and nonselective culture agar media, such as:[3]
- Blood
- MacConkey
- Bismuth sulfite
- Eosin-methylene blue
- Salmonella-Shigella
In cases where there is reduced number of pathogens, enrichment broths, such as tetrathionate or selenite, may be used prior to culture of the bacteria.[3]
The identification of the organism allows specific treatment of the disease, as well as appropriate follow-up recommendations.[4][5]
However, this test is not routinely performed due to its elevated cost, when compared with the accuracy of the results. Results from fecal cultures are often delayed and show an elevated rate of false-negatives.[6]
Stool culture may remains positive during 4 to 5 weeks, and in rare cases (chronic) for more than 1 year.[7]
Serologic Tests
After identification of Salmonella in stool cultures, the identification of the serologic type of the pathogen is given by serologic testing. Polyvalent and specific antisera should be used.[3]
For this classification the following bacterial structures are considered:[8]
- Capsular antigen
- Polysaccharide O antigens
- Flagellar antigens
Microscopic Examination
The examination of stool samples, after staining with methylene blue, helps in the diagnosis of acute diarrhea. This test allows the identification of leukocytes in feces, suggesting an inflammatory etiology for the diarrhea. When the results indicate a serious form of the disease, further studies, such as stool cultures are indicated.[6] Limitations of this test include:
- Smear must be obtained from a fresh specimen, preferably collected in a cup (sensitivity 95%) [9]
- The result is dependent on the experience of the operator
Lactoferrin Testing
Some studies advocate the importance to test for fecal lactoferrin. This test is considered more sensitive than simple microscopic examination of PMN. The Lactoferrin Latex Agglutination (LFLA) is an in vitro test able to detect a highly sensitive leukocyte marker, indicative of the presence of PMN.[6]
Its cost and high rate of false-positives in breast-fed children, limit its application.[4][10][6][11]
Colonoscopy
For some cases of bloody diarrhea of unknown origin, a colonoscopy is indicated. It contributes to the diagnosis by confirming or ruling out conditions such as ulcerative colitis or malignancy.[4]
Imaging Studies
For patients with complications, such as abscesses, CT and ultrasound may be used to identify and monitor these structures.
References
- ↑ Feldmann H, Geisbert TW (2011). "Ebola haemorrhagic fever". Lancet. 377 (9768): 849–62. doi:10.1016/S0140-6736(10)60667-8. PMC 3406178. PMID 21084112.
- ↑ Longo, Dan (2012). Harrison's principles of internal medicine. New York: McGraw-Hill. ISBN 007174889X.
- ↑ 3.0 3.1 3.2 "Salmonella".
- ↑ 4.0 4.1 4.2 Guerrant RL, Van Gilder T, Steiner TS, Thielman NM, Slutsker L, Tauxe RV; et al. (2001). "Practice guidelines for the management of infectious diarrhea". Clin Infect Dis. 32 (3): 331–51. doi:10.1086/318514. PMID 11170940.
- ↑ Longo, Dan (2012). Harrison's principles of internal medicine. New York: McGraw-Hill. ISBN 007174889X.
- ↑ 6.0 6.1 6.2 6.3 Choi SW, Park CH, Silva TM, Zaenker EI, Guerrant RL (1996). "To culture or not to culture: fecal lactoferrin screening for inflammatory bacterial diarrhea". J Clin Microbiol. 34 (4): 928–32. PMC 228919. PMID 8815110.
- ↑ Thielman NM, Guerrant RL (2004). "Clinical practice. Acute infectious diarrhea". N Engl J Med. 350 (1): 38–47. doi:10.1056/NEJMcp031534. PMID 14702426.
- ↑ Murray, Patrick (2013). Medical microbiology. Philadelphia: Elsevier/Saunders. ISBN 0323086926.
- ↑ Korzeniowski OM, Barada FA, Rouse JD, Guerrant RL (1979). "Value of examination for fecal leukocytes in the early diagnosis of shigellosis". Am J Trop Med Hyg. 28 (6): 1031–5. PMID 507279.
- ↑ Hines J, Nachamkin I (1996). "Effective use of the clinical microbiology laboratory for diagnosing diarrheal diseases". Clin Infect Dis. 23 (6): 1292–301. PMID 8953074.
- ↑ Miller JR, Barrett LJ, Kotloff K, Guerrant RL (1994). "A rapid test for infectious and inflammatory enteritis". Arch Intern Med. 154 (23): 2660–4. PMID 7993149.