1. Specificity, Sensitivity, and Simplicity

Let's discuss these three principles of diagnostics...

Specificity

• Pathogenic vs. commensal organisms
  • Example: Salmonella spp.
• False positives
  • Example: B. anthracis

Sensitivity

• The limits of detection - what is a clean bill of health?

  Examples:

  "Testing blood for transfusion as well as for the preparation of blood products for therapy of patients requires the highest possible standard, and optimal conditions of investigation must be chosen to exclude even a minimal risk of infection. This is why the WHO Expert Committee on Biological Standardization (ECBS) has recommended that each donation be individually tested for infective agents." Source: World Health Organization

  "More recently, Roche has introduced a new version of the Amplicor test, which has been modified to make it more sensitive to very small amounts of HIV (50 copies/ml) ó although at the cost of making it less useful for measuring higher levels (greater than 40,000). This Ultra Sensitive assay is mainly used to monitor people whose viral load has decreased below the limit of detection using the standard Amplicor or Quantiplex tests." Source: Coming to grips with HIV viral load

• Calculations of the limits of a test.

Simplicity

• Pregnancy tests are simple...but...poor Mimi
• Xenodiagnosis of T. cruzi (not simple)
• String test for Giardia vs. ELISA
• Modern fast tests - e.g. S. pneumoniae

Resource Reading (to go beyond Table 9.1, p. 200): Rapid Methods in Diagnostic Microbiology, by Marie Schruff

2. Immunological Diagnostic Procedures

Common examples of tests that use antibodies to detect a target (or targets to detect an antibody)

• ELISA
• Radioimmune assay
• Agglutination test with latex beads
• Western blot

Local Industry Examples:

Specialty Laboratories Inc., Santa Monica - New tests

Diagnostic Products Corp., Los Angeles - Catalog

One Lambda Inc., Canoga Park - News

A couple of ways in which protein detection differs from DNA testing:

• The target being tested must be expressed, and cannot be masked as an epitope.
• The denaturation state of protein may be important for activity or antigenicity. For example, some antibodies may be used as a probe in a western blot while others may not.
• A epitope may be changed radically (or missing) following a single nucleotide substitution in the gene

Antibody methods

• Polyclonals and Monoclonals, and detection.

See also, fig. 10.5 (p. 236) from next week's readings

3. DNA Diagnostic Systems

DNA detected by southern analysis

• Detection by base-pairing
• Diagnostic methods
• Chips

Dr. Stan Metzenberg Department of Biology California State University Northridge