This test is performed after vaccination to document seroprotection in animal care workers. It is also used to determine exposure to rabies and in the diagnosis of rabies.
Identification and documentation of the presence of rabies-neutralizing antibody is important for veterinary health care workers and others who are at risk or may have been exposed to the rabies virus.
This test is performed on persons who are at great risk for animal bites (veterinarians and their staff, zoo workers, those who work with animals in laboratories) and on those who have received the human diploid cell rabies vaccine (HDCV). A rabies titer of greater than 1:16 is considered protective.
Rabies antibody is also used in diagnosing rabies in patients suspected of being exposed to the virus.
A fourfold rise in antibody titer over several weeks in a person not previously exposed to the HDCV indicates rabies exposure. If the patient has received HDCV and has been bitten by an animal suspected of having rabies infection, a very high antibody titer may support the diagnosis. The presence of antibody in the cerebrospinal fluid (CSF) is also supportive of the diagnosis, because usually there are not antibodies in the CSF after the HDCV vaccine, but there are antibodies after a bite from a rabies-infected animal. In patients who may have been exposed to rabies, the human rabies immunoglobulin (HRIG) is given after the antibody titers have been obtained. Half of the HRIG is given into the area of the bite, and half is administered as an intramuscular (IM) injection into the gluteal region. At the same time the first of the HDCV shots are administered to begin vaccination. Four subsequent IM injections are administered over the next 28 days. One can expect to see increases in rabies antibody levels in about 10 days, but protective levels may not be present for several weeks. Postexposure protocols exist to determine the proper handling of the patient and animal, depending on the real risk for the animal’s infection.
The rabies antibody is identified by the direct fluorescent antibody method. More recently immunofluorescence has been used.
PROCEDURE AND PATIENT CARE
Explain the procedure to the patient.
Tell the patient that no fasting or special preparation is required.
• Collect a venous blood sample in a red-top tube.
• Apply pressure or a pressure dressing to the venipuncture site.
• Assess the venipuncture site for bleeding.
TEST RESULTS AND CLINICAL SIGNIFICANCE
Exposure to rabies vaccine: This causes a relatively low titer of 1:16 or greater.
Recent bite exposure to rabies virus: This causes a progressive rise in titer to levels of 1:200 to 1:160,000.
Active rabies in patient or animal: Antibody titers are extremely high in patients who present with encephalitis and brain stem dysfunction. These patients rarely recover from the disease.