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Alias Names: Rabies Response / Rabies Vaccine Response / End-Point Titer
Methodology: Rapid Fuorescent Focus Inhibition
Edit Date: 10/30/2010
Performed: Monday, Thursday
Released: 11-12 days after setup at PeaceHealth Laboratories’ reference lab.
CPT Code: 86382-90
Specimen Collection Details
Collection: One 7.5 mL serum separator tube (SST). Also acceptable: One 4 mL lavender top tube (EDTA).
Handling: Allow to clot, centrifuge and separate serum or plasma from cells into a plastic vial. Refrigerate.
Stability: 7 days ambient, 17 days refrigerated, or 28 days frozen.
Standard Volume: 2 mL serum or plasma.
Minimum Volume: 1 mL serum or plasma.
Transport: Refrigerated.
Comments: Rapid Fluorescent Foci Inhibition Test (RFFIT) is a serum neutralization (inhibition) test, which means it measures the ability of rabies specific antibodies to neutralize rabies virus and prevent the virus from infecting cells. These antibodies are called rabies virus neutralizing antibodies (RVt4A). The RFFIT does not measure the amount of rabies specific antibodies but the neutralizing activity of the antibodies present in the serum. In the test, your serum (the non- cellular portion of your blood) is first diluted fivefold (1 part serum in 4 parts diluent). Further (serial fivefold) dilutions are performed, each of which contain less and less of your serum. These serum dilutions are mixed with a standard amount of live rabies virus and incubated. Whatever RVNA is present will neutralize the virus. Next, some tissue culture cells are added and the serum/virus/cells are incubated together. Whatever rabies via’s is left (i.e., that which has not been neutralized by the antibody in your serum), will infect the cells and this can be seen under the microscope through the use of specific staining. Calculation of the endpoint titer is made from the percent of virus infected cells observed on the slide. What does your result tell you? The RFFIT test can be used in two ways: to determine a RVNA titer (eg., 1:5) or to determine a value for RNA concentration (e.g., 0.1 IU/mL). The IU stands for international unit and is calculated from the titer by comparing it against a standard reference serum. We use the following formula: sample titer divided by the reference serum. Example: Your titer is 1:100, the reference serum titer is 1:200, and the value of the reference serum is 2.0 IU/mL. Using the formula: (100/200) X 2.0 IU/mL= 10 IU/mL the potency of RVNA in your serum is therefore 1.0 IU/mL.

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