Neurology: Lab Procedures and Immunoassays

Lab Series Overview

• Upcoming Disease Topics: Infectious diseases, immunodeficiencies, immunoproliferation disorders (cancer), autoimmune hypersensitivity, and autoimmune disorders.
• Focus: Disease impact on the immune system, the body's immune response, and appropriate testing methods (antigen vs. antibody, procedure selection).
• Current Lab Focus: Agglutination (completed), ELISA (today's lab), HIV testing (today's lab).
• Primary serology tests: Agglutination or ELISA.

Labeled Immunoassays: General Concepts

• Detects antigen or antibody using a reactant with a detectable 'tag' or label.
• Common Labels: Color change, radioactivity, enzymes (producing light or color), fluorescence.
• Ligand: The molecule being measured/tested for in the patient sample (e.g., antigens, hormones, drugs, tumor markers, or antibodies).
• Receptor: Typically an antibody with a conjugated label from the company.
• One reactant (either ligand or receptor) is labeled with a marker for binding measurement.
• Label Characteristics: Must not alter molecule reactivity, remain stable, and have a good shelf life.
• Antibody (receptor) Characteristics: Must exhibit high affinity and specificity for the target antigen (monoclonal antibodies are preferred).
• Solid Phase: Essential for binding and enabling wash steps (e.g., polystyrene test tubes, microtiter plates, magnetic beads).
• Detection Methods: Geiger counter (for radioactivity), spectrophotometer/analyzer (for enzymes, fluorescence, chemiluminescence).

Quality Control: Blank vs. Negative Control

• Blank: Contains all reagents from the kit except the patient sample or the labeled component. Used to detect reagent contamination, ensure true negatives, and check for false positives from reagents.
• Negative Control: Contains an actual patient specimen known to be negative for the target. Confirms the procedure correctly yields a negative result for negative samples.
• Positive Control: Ensures the reagents are functional and the experiment worked as expected, yielding a positive result.

Radioimmunoassay (RIA)

• Principle: Uses radioactive substances in a competitive binding assay.
• Mechanism: Patient ligand competes with a radioactively labeled ligand for limited binding sites on a receptor/antibody.
• Results: Inversely proportional; more unlabeled patient ligand bound results in lower radioactivity, indicating a more positive patient result.
• Sensitivity: Extremely high, capable of detecting picograms per $ext{mL}$. For example, concentrations as low as $ext{1 pg/mL}$ are detectable.
• Disadvantages: Hazardous (radioactivity requires special handling, training, and equipment), costly disposal, and very short shelf life (typically only a few days).

Enzyme-Linked Immunosorbent Assay (ELISA)

• Principle: Utilizes enzymes to catalyze specific biochemical reactions, resulting in a measurable product (e.g., color change, light emission).
• Advantages: Cost-effective, readily available, long shelf life (months), adaptable to automation, uses inexpensive equipment, and non-hazardous disposal.
• Sensitivity: Very high due to enzyme amplification; one enzyme molecule can generate hundreds of product molecules.
• Specificity: Provided by the antibody's variable regions.
• Disadvantages: Potential for patient specimen inhibitors, enzyme label size interference, non-specific binding, and enzyme temperature sensitivity.
• Common Enzymes: Horseradish peroxidase, alkaline phosphatase.
• Assay Types: Classified as heterogeneous (requires wash steps) or homogeneous (no wash steps).
• ELISA does not have a 'Direct' assay type.
• Indirect (Noncompetitive) ELISA:
  • Purpose: Tests for antibody in the patient sample.
  • Procedure: Plate coated with specific antigen $<br /> ightarrow$ Add patient serum (patient antibody binds to antigen) $<br /> ightarrow$ Add enzyme-conjugated secondary antibody (which binds to the patient antibody) $<br /> ightarrow$ Add substrate, resulting in color/light product.
  • Results: Directly proportional; more color/light indicates more patient antibody.
• Sandwich/Capture (Noncompetitive) ELISA:
  • Purpose: Tests for antigen in the patient sample.
  • Procedure: Plate coated with specific capture antibody $<br /> ightarrow$ Add patient sample (patient antigen binds to capture antibody) $<br /> ightarrow$ Add enzyme-conjugated secondary antibody (which binds to the captured antigen) $<br /> ightarrow$ Add substrate, resulting in color/light product.
  • Results: Directly proportional; more color/light indicates more patient antigen.
• Competitive (Competitive) ELISA:
  • Purpose: Often for small molecules or when other formats are unsuitable.
  • Mechanism: Patient antigen competes with a labeled antigen for binding sites on a limited amount of antibody.
  • Results: Inversely proportional; more color/light indicates a negative patient result, while less color/light indicates a positive patient result (due to competition).

Immunofluorescent Assay (IFA)

• Principle: Uses antibodies conjugated with fluorescent tags (fluorophores or fluorochromes).
• Mechanism: These compounds absorb energy from an incident light and then emit light of a longer wavelength and lower energy.
• Detection: Requires specific optical filters matching the fluorophore's emission spectrum, often using a fluorescent microscope or a plate reader with internal filters.
• Common tags: Red, green, blue, yellow. Highly valuable for multiplex testing (simultaneously testing for multiple analytes).