ELISA for Virus Test
1.ELISA for HIV Test
The ELISA was the first screening test widely used for HIV because of its high sensitivity. In an ELISA, a person's serum is diluted 400-fold and applied to a plate to which HIV antigens are attached. If antibodies to HIV are present in the serum, they may bind to these HIV antigens. The plate is then washed to remove all other components of the serum. A specially prepared "secondary antibody" — an antibody that binds to other antibodies — is then applied to the plate, followed by another wash. This secondary antibody is chemically linked in advance to an enzyme. Thus, the plate will contain enzyme in proportion to the amount of secondary antibody bound to the plate. A substrate for the enzyme is applied, and catalysis by the enzyme leads to a change in color or fluorescence. ELISA results are reported as a number; the most controversial aspect of this test is determining the "cut-off" point between a positive and a negative result. A cut-off point may be determined by comparing it with a known standard. If an ELISA test is used for drug screening at workplace, a cut-off concentration, 50 ng/mL, for example, is established, and a sample that contains the standard concentration of analyte will be prepared. Unknowns that generate a signal that is stronger than the known sample are "positive." Those that generate weaker signal are "negative."
2.ELISA for West Nile Virus Test
The best laboratory test for diagnosing West Nile virus infection is an IgM antibody–capture enzyme-linked immunosorbent assay (MAC-ELISA) of serum or CSF collected eight to 21 days after the onset of symptoms. This simple, sensitive (95 percent) diagnostic test is available commercially and through local or state health departments. State public health laboratories usually can perform the test within 24 to 36 hours of submission. Testing of serum in the acute and convalescent phase (i.e., sera collected at least two weeks apart) may provide laboratory confirmation of infection. Since IgM antibody usually does not cross an intact blood-brain barrier, a single West Nile virus–specific IgM positive antibody titer in CSF can confirm central nervous system infection.
3.ELISA for Newcastle Disease Virus (NDV) Test
NDV causes a range of disease states from mild respiratory disease to severe diarrhea and death. The severity of the disease is determined by the infecting strain of NDV. Highly pathogenic strains (velogenic NDV) can cause swelling of the tissues around the eyes, diarrhea and death within 8 days after exposure. Moderately pathogenic strains (mesogenic NDV) produce acute repiratory tract infections and reductions in egg production. Milder strains (lentogenic NDV) produce an inapparent respiratory infection. ELISA is applied to monitoring NDV vaccination programs and detecting NDV infected flocks.