Austin Bat Hospital Blog

More on Rabies Titers (RFFIT vs ELISA)

Was your rabies titer determined by the ELISA test?  If so, you need to find another lab!  According to a rabies authority, the RFFIT type test is the only internationally approved procedure for measuring rabies neutralizing antibody.  The key word here is neutralizing. The ELISA test measures ANY antibody that can attach to rabies antigen and many antibodies that attach to a pathogen are not necessarily neutralizing antibody. So a positive ELISA test may not be a positive RFFIT test. This can be dangerous given the fatal nature of rabies. The RFFIT (rapid fluorescent foci inhibition test),  the FAVN (fluorescent antibody virus neutralizing), and the TC/SN (tissue culture serum neutralizing) are all tests that measure only that antibody that will kill live rabies virus.  Patient serum is diluted out, and each dilution is mixed with live rabies virus. The mixtures are then put onto cell culture to detect what virus was not killed by the serum.  All three mentioned neutralizing tests are comparable and acceptable in measuring serum neutralizing antibody. The CDC uses the RFFIT test.  Other labs use a TC/SN test that gives the exact same results. ELISA tests to measure rabies antibody are unfortunate. The reason the neutralizing tests are so far and few between is because they can only be done in labs approved to work with live rabies virus and those labs that have the practiced expertise to do these tests. I would definitely recommend Kansas State as the only commercial lab in the US that does a neutralizing test.

Understanding Rabies Titers

The RFFIT test can be used in two ways:  to determine a neutralization titer (e.g., 1/5) or to determine a value for rabies antibody concentration (e.g., 0.5 IU).  The IU stands for international unit and is measured against a standard reference.  So, you have a test serum (e.g., your blood) and you run it through the RFFIT.  The serum sample is mixed with a certain amount of rabies virus and incubated.  Whatever antibody is there will neutralize the virus.  Then you add some fresh cells and incubate it all together. Whatever rabies virus is left (i.e., that which has not been neutralized by the antibody in your blood), will infect the fresh cells and this can be seen under the microscope.  So, let's say you have a lot of antibody in your blood. The virus will all be neutralized and not infect the cells. Ok, how much do you really have?  Let's dilute your blood and then see if it still neutrralizes the virus.  Ok, we did a twofold dilution and it's still going strong--must have a lot of antibody.  Let's do a fivefold dilution--still going strong.  So, at a fivefold dilution (1/5) your blood still contains enough neutralizing antibody to be protective (by the definition of the standard 1/5).  If your blood petered out at say, a 1/2 dilution, you wouldn't be protected.  The bigger the denominator, the more antibody initially in your blood (e.g., a titer of 1/75 has more antibody than a titer of 1/5 because it still neutralized at a much higher dilution).  It is a dimensionless number because it just refers to the amount of dilution of the sample that will give a measured (i.e., standard) response.  It doesn't say how many antibodies are actually in your blood (i.e., it doesn't give a measurement of it).  The titer is a proxy for that value.  (The whole thing can be further complicated by serial dilutions in the amount of virus used too.)  But the RFFIT can be used to get the actual concentration of antibody (by working against the reference standards).  This is measured as IU/ml.  So, the 1/5 dilution is approximately equivalent to an actual antibody level of 0.5 IU/ml.

Thanks to Danny Brass for simplifying a complicated topic.

Evening Bat Baby

I have the sweetest little evening bat baby (Nycticeius humeralis).  He is a boy and is just now starting to eat meal worms parts.  Such a cutie!

Diseases in Nature Conference

I gave a presentation at the Texas Health Department's "Diseases In Nature" conference in College Station.  I talked about bat bites, and Meg Goodman of Texas Parks and Wildlife, talked about excluding bats from buildings.  We gave our powerpoint presentation in the Rabies session.