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43 Cards in this Set
- Front
- Back
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Define Sensitivity
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The smallest amount of substance that can be detected. Also known as the "detection limit."
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What is a "false negative"?
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Failure to detect the target when it is present in a sample.
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Define specificity.
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The ability to detect a target substance rather than a non-target substance in a sample.
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What is a "false positive"?
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Detecting a non-target substance
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What are the five approaches to laboratory diagnosis of disease?
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culture, microspcopy, antigen detection, antibody detection/serology, and nucleic acid detection.
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What is a culture?
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Isolation and growth of the organism in pure culture.
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What is the disadvantage to cultures?
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Some pathogens cannot be isolated in culture media--particularily intracellular parasites. These pathogens may be cultured in living systems such as embryonated eggs, animals, or cell cultures. Also, may be slow to grow.
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What is the advantage of growing cultures?
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Able to test for antibiotic susceptibility testing, identification of virulence genes, or experimental models for pathogenicity.
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What are the two different types of microscopy?
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Brightfield and darkfield.
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What is brightfield microscopy?
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Light passes through a specimen which is magnified.
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What are some disadvantages to growing a culture?
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Some bacteria cannot be cultured because they must grow in living organisms, and some may be slow to grow.
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What is brightfield microscopy?
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Light passes through a specimen which is magnified up to 1000 fold.
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What is the advantages and disadvantages of brightfield microscopy?
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Poor resolving power so not useful for very small bacteria but the resolving power gives the ability to see two closely spaced objects.
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What is resolving power?
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the ability to separate two closely spaced objects.
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What is an acid fast stain used for?
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Used for detecting bacteria with a waxy cell wall such as Mycobacterium tuberculosis. (cannot be gram stained)
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Define darkfield microscopy.
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Specimen is illuminated brightly against a dark background.
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What is the advantage of darkfield microscopy?
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Useful for identifying bacteria that are too think to be seen by brightfield microscopy.
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What is electron microscopy?
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Beam of electrons focused on a sample.
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What is the advantage of electron microscopy?
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Better resolving power and magnification than brightfield and darkfield microscopy. Can be used to observe cell components such as pili and flagella.
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What are some general advantages of microscopy?
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Brightfield and darkfield microscopy are rapid and easy, can be used to detect organisms that cannot be cultured, and can be used to observe bacteria in situ.
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What are some general disadvantages of microscopy?
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Not specific, electron microscopy is highly skilled and expensive, the isolate is not available for further analysis, unlike cultured organisms.
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What is antigen detection?
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Antibodies are used to detect a specific antigen that is associated with the pathogen.
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What are the two methods of antigen detection?
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Immunofluorescence assays and enzyme immunoassays.
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What are two different types of immunofluorescence assays?
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Direct and indirect.
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Describe a direct immunofluorescence assay.
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Fluorescently labeled antibody binds to target antigen.
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Describe an indirect immunofluorescence assay.
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A primary antibody binds the target antigen. A secondary, fluorescently labeled antibody binds to the primary antibody.
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How is immunofluroescence detected?
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Microscopy. The microscope is equipped with attachments for fluorescence.
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What are some advantages of immunoflurescence?
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More specific, can be used to detect specific species, and also assess morhpology.
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What is a fluorescence spectrophotometer?
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This is an instrument that can be used to detect fluorescence.
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What is an enzyme immunoassay?
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An enzyme labeled antibody bind the antigen. A substrate is added and this is cleaved by the enzyme that is connected to the antibody. The result is a color change in the substrate.
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What are the general advantages of immunoassays?
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They are rapid and specific.
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What are the general disadvantages of immunoassays?
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The antibodies must be available for the pathogen. The isolate is not available for futher analysis, unlike cultered organisms.
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Define serology
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Detection of pathogen specific antibodies in the patient or detecting a host response to an infectious agent.
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What can serology be used for?
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Can be used to identify the time, course, and nature of the infection. Or whether it is a primary or secondary infection.
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What type of antibodies will be produced at the initial exposure of infection?
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IgM
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What type of antibodies will predominate late in the course of an infection?
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IgG
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What is an antibody titer?
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measure of the amount of antibody in the blood of a patient. The highest dilution of the patients serum that retains activity in an immunoassay.
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What are the advantages of serology?
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Can determine previous exposure, can determine the time course of an infection.
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What are the disadvantages of serology?
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Pathogen not directly identified. Only detecting a host response to an infection.
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What is the most common way of detecting nucleic acids?
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PCR
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Briefly describe how PCR works
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Primers target pathogen specific sequences. Pathogen specific sequences are amplified from a very low copy number to a very high copy number. Amplified product is visible following electrophoresis and staining.
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What are the advantages of PCR?
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Specific, Rapid, Can detect bacteria that can't be cultured, can detect virulence genes, useful for quickly characterizing the public health threat and identifying treatment options.
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What are the disadvantages of PCR?
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The isolate is not available for further analysis.
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