Shigella Infection Workup

Updated: Mar 03, 2023
  • Author: Jaya Sureshbabu, MBBS, MRCPCH(UK), MRCPI(Paeds), MRCPS(Glasg), DCH(Glasg); Chief Editor: Russell W Steele, MD  more...
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Laboratory Studies


The total WBC count reveals no consistent findings. A shift to the left (increased number of band cells) in the differential WBC count in a patient with diarrhea suggests bacillary dysentery. Leukopenia or leukemoid reactions are occasionally detected.

In HUS, anemia and thrombocytopenia occur.

Bacteremia is rare, even in severe disease, possibly due to the superficial nature of Shigella infection; the organism rarely penetrates beyond the mucosa.

Blood culture should be obtained in children who appear toxic, very young, severely ill, malnourished, or immunocompromised because of their increased risk of bacteremia.

Stool examination

Isolation of Shigella from feces or rectal swab specimen is diagnostic but lacks specificity. Routine microscopy may reveal sheets of leukocytes on methylene-blue stained stool smear, which is a sensitive test for colitis but not specific for Shigella species.

In approximately 70% of patients with shigellosis, fecal blood or leukocytes (confirming colitis) are detectable in the stool.

Stool culture

A sample for stool culture should be obtained in all suspected cases of shigellosis.

The yield from stool cultures is greatest early in the course of disease. Guidelines for obtaining specimens to improve the yield are as follows:

  • Process specimens immediately after collection.

  • If processing is delayed, use a transport medium (eg, buffered glycerol saline).

  • Collect more than one stool or rectal (not anal) swab and inoculate them promptly on at least 2 different culture media.

  • Specimens should be plated lightly onto MacConkey, xylose-lysine-deoxycholate, Hektoen enteric, or Salmonella-Shigella, or eosin-methylene blue agars.

If processing is delayed, a rectal-swab sample can be placed in Cary-Blair transport medium or buffered glycerol saline.

After overnight incubation, colorless, nonlactose-fermenting colonies may be tested by means of latex agglutination to establish a preliminary identification of Shigella infection.

Antimicrobial susceptibility tests of all confirmed isolates should be performed by using the agar diffusion technique. The agar and broth-dilution methods are also widely used. The new Epsilometer strip method (E test) is used to accurately determine the minimum inhibitory concentration (MIC).

Despite meticulous care in obtaining and processing specimens from patients infected with Shigella species, approximately 20% may fail to yield Shigella organisms.

Enzyme immunoassay

An enzyme immunoassay for Stx is used to detect S dysenteriae type 1 in the stool.

Rapid techniques

With rapid techniques, gene probes or polymerase chain reaction (PCR) primers are directed toward virulence genes (invasion plasmid locus).

Other testing modalities

Other testing modalities, such as fluorescent antibody test and enzyme-linked DNA probes, are available in research laboratories.

A systematic review and meta-analysis by Tickell et al reported that the sensitivity of dysentery for laboratory-confirmed Shigella infection ranged from 1·9% to 85·9% in the time period between 1977 and 2016, with sensitivity decreasing over time (p=0·04). [20]


Other Tests

Additional diagnostic tools, such as gene probes and PCR analysis of stool for specific genes such as ipaH, virF, or virA can detect cases not diagnosed by culture but are usually available in research laboratories. [21]