Brucellosis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Raviteja Guddeti, M.B.B.S. [2] Danitza Lukac

Overview

A positive culture or presence of Brucella antibody in serological tests are diagnostic of brucellosis.

Laboratory Findings

• Complete blood count:
  • Mild leukopenia
  • Mild anemia
  • Relative lymphocytosis
  • Thrombocytopenia
• Liver function tests:
  • Mild transaminasemia

Bacterial Isolation:

• The isolation and identification of Brucella can confirm a diagnosis of brucellosis. Brucella is most commonly isolated from blood cultures.
• It can also, however, be isolated from:
  • Bone marrow
    • Gold standard
  • Cerebrospinal fluid
  • Wounds
  • Purulent discharge
  • Joint fluid

Serological tests:

• There are two types of serological tests, based on:
  • "Antibody production against lipopolysaccharide"
  • "Antibody production against other bacterial antigens"
• For a diagnosis to be made using serology, two serum samples are required.
  • The first serum sample should be taken when a person is acutely ill (≤7 days after symptom onset)
  • The second serum sample should be drawn 2-4 weeks later to check for a rise in antibodies (a fourfold or greater rise in antibodies would bean an individual is positive for brucellosis).
  • If submission of paired sera is not possible, a probable diagnosis can be made with a single serum sample.
• Brucella microagglutination test (BMAT):
  • A modified version of the serum (tube) agglutination test (SAT), that can detect antibodies to Brucella species: abortus, melitensis or suis.
  • There is no serological test available to detect antibodies to B. canis.
  • An agglutination titre greater than 1:160 is considered significant in nonendemic areas.
  • An agglutination titre greater than 1:320 is considered significant in endemic areas.
  • Due to the similarity of the O polysaccharide of Brucella to that of various other Gram-negative bacteria (e.g. Francisella tularensis, Escherichia coli, Salmonella urbana, Yersinia enterocolitica, Vibrio cholerae, and Stenotrophomonas maltophilia) the appearance of cross-reactions of class M immunoglobulins may occur.
  • False-negative SAT may be caused by the presence of blocking antibodies (the prozone phenomenon) in the α2-globulin (IgA) and in the α-globulin (IgG) fractions.
  • Serology is not currently available to monitor persons for RB51 vaccine exposure or for Brucella canis exposure.
• Rose Bengal:
  • Rose bengal has a positive predictive value is approximately 99% for patients with acute and chronic brucellosis.
  • It measures IgM and IgG antibodies.
• 2-mercaptoethanol (2-ME):
  • It measures IgG antibodies.
• Antihuman globulin (Coombs):
  • Used in chronic brucellosis patients with negative seroagglutination because they have IgG non-agglutinating antibodies.
• Indirect enzyme linked immunosorbent assay (ELISA):
  • ELISA typically uses cytoplasmic proteins as antigens.
  • It measures IgM, IgG, and IgA with better sensitivity and specificity than the SAT in most recent comparative studies.
• Dipstick assays:
  • New and promising, based on the binding of Brucella IgM antibodies, and found to be simple, accurate, and rapid.
• Brucellacapt test:
  • A single-step immunocapture assay for the detection of total anti-Brucella antibodies, is an increasingly used adjunctive test when resources permit.
• PCR:
  • Is fast and specific.
  • Many varieties of PCR have been developed (e.g. nested PCR, realtime PCR and PCR-ELISA) and found to have superior specificity and sensitivity in detecting both primary infection and relapse after treatment.
  • Unfortunately, these have yet to be standardized for routine use, and some centres have reported persistent PCR positivity after clinically successful treatment, fuelling the controversy about the existence of prolonged chronic brucellosis.

Gallery

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  Brucella abortus bacteria grown on a medium of sheep’s blood agar (SBA) 72hrs. From Public Health Image Library (PHIL).

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  Brucella abortus bacteria grown on a medium of sheep’s blood agar (SBA) 72hrs. From Public Health Image Library (PHIL).

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  Brucella abortus bacteria grown on a medium of sheep’s blood agar (SBA) 72hrs. From Public Health Image Library (PHIL).

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  Brucella suis bacteria grown on chocolate medium 72hrs. From Public Health Image Library (PHIL).

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  Brucella sis bacteria grown on chocolate medium 72hrs. From Public Health Image Library (PHIL).

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  Brucella bacteria grown on MacConkey agar (MAC) medium 24hrs (10x mag). From Public Health Image Library (PHIL).

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  Brucella suis bacteria cultured on chocolate agar medium 48hrs (10x mag). From Public Health Image Library (PHIL).

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  Brucella suis bacteria cultured on SBA 24hrs (10x mag). From Public Health Image Library (PHIL).

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  Brucella suis bacteria cultured on SBA 72hrs (10x mag). From Public Health Image Library (PHIL).

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  Brucella suis bacteria cultured on SBA 48hrs (10x mag). From Public Health Image Library (PHIL).

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  Brucella suis bacteria cultured on Thayer-Martin (TM) agar medium 48hrs (10x mag). From Public Health Image Library (PHIL).

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  Brucella suis bacteria cultured on chocolate agar medium 24hrs (10x mag). From Public Health Image Library (PHIL).