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Chapter 9: Laboratory Diagnosis (contains culture media and methods
extra informtion)
APPROACH TO LABORATORY DIAGNOSIS:
The specimen selection and collection are typically the responsibility of the medical staff, thus
clinicians should,
- understand the pathogenesis of the infection and ensure proper collection of an adequate
quantity of specimen from the body site that is most likely to yield growth of the
infecting organism, while avoiding contamination from the normal flora.
GENERAL / TRADITIONAL APPROACH TO DIAGNOSIS:
1. Culture, microscopic and phenotypic characterization of an organism
2. Serologic testing.
Serologic testing? in which the organism is identified by the detection of organismspecific
antibodies in the patient’s serum.
SUMMARY:
1. Specimen from infected site
2. Staining to guess genus of organism and begin empiric therapy
3. Culture and incubation in presence or absence of oxygen
4. Appropriate tests and special features
5. Antibiotic susceptibility tests performed
3. Serologic methods:
a. Identification of organism with known Antiserum: Slide Agglutination test, Latex
Agglutination test, Enzyme linked immunosorbent assay, Fluorescent antibody
test
b. Identification of serum antibodies with known Antigen: Slide or Tube
Agglutination test, Serologic test for Syphilis, Cold Agglutinin test
4. Molecular Diagnostic Methods: Genomic test, Proteomic test
Microscopic Examination:
, - For Sterile body sites staining methods like Gram-stain or Acid fast stain used
Sterile body sites? Do not harbor a “background” of normal flora, such as sterile tissues, cerebral
spinal fluid, joint fluid, or urine.
- If bacteria are seen in the specimen:
a. shape (e.g., cocci or rods), size, and arrangement (e.g., chains or clusters)
b. Gram positive, gramnegative, or acid fast
c. determine whether only one or more than one type of bacteria is present.
RESULT: Cannot definitively identify an organism, but often allows an educated guess to be
made regarding the taxonomic classification (genus) of the organism and thus guides empiric
therapy that can be initiated without waiting for growth of the organism.
Culture Based Methods:
For diagnosing bacterial or fungal infections: require the suspected pathogen to be isolated in
pure culture from a properly obtained clinical specimen.
Can be done by: agar-based medium, for instance, blood agar plates, and streaking the specimen
over the agar surface in a manner to obtain wellisolated colonies. The agar plates are then
incubated under atmospheric conditions that will support the growth of a variety of different
microorganisms.
Selective and Differential Medias:
- “selective” containing compounds that only allow certain bacteria to grow (e.g.,
antibiotics, salts, or dyes)
- “differential” because they contain other compounds that allow one type of bacteria to be
distinguished from another based on a biochemical reaction (e.g., detecting hemolysis on
blood agar plates or pigment formation).
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