What Is a Fluorescent Antibody?

A fluorescent antibody is a protein tagged with fluorescent dye that lights up under a microscope. This allows a technician to determine that there are antibodies in a sample presented for evaluation. The testing doesn’t enable counting of the antibodies and is purely qualitative in nature, allowing labs to provide a positive or negative result without specific information about how many antibodies are present. Testing can be useful for diagnosis of disease, including postmortem evaluation of patients with suspected infections.

The procedure for fluorescent antibody staining starts with fixing cells from the sample to a slide. Technicians wash the cells in a solution containing antigens bound to fluorescent tags. If the slide contains antibodies that the antigens fit with, they will lock on, attaching and remaining in place. Using a special filter, the technician can view the sample under the microscope and just see the fluorescence. If the slide lights up, it indicates that there are antibodies, and if it does not, the test is negative.

One reason to use this test is in the evaluation of suspected rabies cases. The virus cannot be identified in the bloodstream, and requires the examination of tissue from the brain. Technicians can take a small collection of cells and subject them to the fluorescent antibody test, looking for the tell-tale bright green indicative of rabies. Doctors may also request this testing on sputum from the lungs if they suspect the patient has a lung infection like Legionnaire’s disease.

Technicians use a number of failsafes to confirm the validity of the testing. This can include repeating positive tests to make sure antibodies are really there, and controlling samples to reduce the risk of cross-contamination. Detailed lab directions are followed to the letter when using the fluorescent antibody test to make sure it will be repeatable and follows a specific recognized protocol. Questionable samples can also be sent to an independent lab for verification.

Patients may hear this test described as DFA, for Direct Fluorescent Antibody. The price of the test can vary, depending on which organisms are being looked for and a lab’s pricing scheme. Insurance typically covers it and in some cases patients may be eligible for free testing through public health agencies. People involved in an outbreak of disease, for example, may receive extensive testing to learn more about the nature of the infection. This could be funded by certain governmental agencies, because it’s considered a public health service.