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[edit]

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 agars, is the most common study to identify the bacteria. When there is not enough bacteria on the sample, enrichment broths may be used to grown more salmonella. Serologic tests with polyvalent and specific antisera may be used to identify the serotype of Salmonella. Blood results from patients with salmonellosis may show unspecific findings, such as: leukocytosis and elevated LFTs on the CBC; hypokalemia and hyper or hyponatremia; elevated inflammatory markers; and bacterial growth on blood cultures. Urinalysis may show hypercalciuria, hypocitraturia and increased ammonium excretion. The bacteria may also be identified in stool by microscopic examination, and lactoferrin tests, however, due to the high dependence on the experience of the operator, in microscopic examination, and high cost and rate of false-positives, of the lactoferrin test, these are not commonly used.

Laboratory Findings[edit]

The table below displays nonspecific laboratory abnormalities associated with Salmonellosis, including:[1][2][3][4][5]

Laboratory findings
Test Findings
Complete Blood Count
Electrolytes
Inflammatory Markers
  • CRP is often elevated
  • ESR may be elevated
Blood cultures
  • Indicated in the presence of prolonged or recurrent fever
  • May reveal presence of the bacteria, in case of bacteremia
  • Commonly positive in endovascular infections
Urinalysis

Stool Cultures[edit]

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:[6]

  • 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 bacteria.[6]

Fecal leukocytes may also be identified in the specimen.[7]

The identification of the organism allows specific treatment of the disease, as well as appropriate follow-up recommendations.[8][9]

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.[10]

Stool culture may remain positive during 4 to 5 weeks, and in rare cases (chronic) for more than 1 year.[11]

Serologic Tests[edit]

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.[6]

For this classification the following bacterial structures are considered:[12]

Microscopic Examination[edit]

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.[10] Limitations of this test include:

  • Smear must be obtained from a fresh specimen, preferably collected in a cup (sensitivity 95%) [13]
  • The result is dependent on the experience of the operator

Lactoferrin Testing[edit]

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.[10]

Its cost and high rate of false-positives in breast-fed children, limit its application.[8][14][10][15]

Gallery[edit]

References[edit]

  1. "Diarrhoea and Vomiting Caused by Gastroenteritis".
  2. Agarwal R, Afzalpurkar R, Fordtran JS (1994). "Pathophysiology of potassium absorption and secretion by the human intestine". Gastroenterology. 107 (2): 548–71. PMID 8039632.
  3. Wang F, Butler T, Rabbani GH, Jones PK (1986). "The acidosis of cholera. Contributions of hyperproteinemia, lactic acidemia, and hyperphosphatemia to an increased serum anion gap". N Engl J Med. 315 (25): 1591–5. doi:10.1056/NEJM198612183152506. PMID 3785323.
  4. Welbourne T, Weber M, Bank N (1972). "The effect of glutamine administration on urinary ammonium excretion in normal subjects and patients with renal disease". J Clin Invest. 51 (7): 1852–60. doi:10.1172/JCI106987. PMC 292333. PMID 4555786.
  5. Batlle DC, von Riotte A, Schlueter W (1987). "Urinary sodium in the evaluation of hyperchloremic metabolic acidosis". N Engl J Med. 316 (3): 140–4. doi:10.1056/NEJM198701153160305. PMID 3796685.
  6. 6.0 6.1 6.2 "Salmonella".
  7. Granville LA, Cernoch P, Land GA, Davis JR (2004). "Performance assessment of the fecal leukocyte test for inpatients". J Clin Microbiol. 42 (3): 1254–6. PMC 356889. PMID 15004086.
  8. 8.0 8.1 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.
  9. Longo, Dan (2012). Harrison's principles of internal medicine. New York: McGraw-Hill. ISBN 007174889X.
  10. 10.0 10.1 10.2 10.3 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.
  11. Thielman NM, Guerrant RL (2004). "Clinical practice. Acute infectious diarrhea". N Engl J Med. 350 (1): 38–47. doi:10.1056/NEJMcp031534. PMID 14702426.
  12. Murray, Patrick (2013). Medical microbiology. Philadelphia: Elsevier/Saunders. ISBN 0323086926.
  13. 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.
  14. 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.
  15. 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.
  16. 16.0 16.1 16.2 16.3 16.4 16.5 16.6 16.7 "Public Health Image Library (PHIL)".


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