Tom Becket, DVM - to the members of Tick List
Titers" are derived in this way:
First the serum sample is diluted to some starting dilution, e.g., 1:20.
Then that dilution is diluted in a series of two-fold dilutions, e.g., 1:20,
1:40, 1:80, 1:160, 1:320, 1:640, 1:1,280, 1:2,520, 1:5,040, 1:10,080, etc.
(Note that if antibody is present in the serum, each successive dilution will contain one-half the amount of antibody per unit volume (e.g., per mL) as the preceding dilution.)
Then, starting with the lowest dilution (e.g., 1:20), a same-size sub-sample from each successive dilution is tested until a dilution is reached which does not show a reaction.  
 (Put differently: At some point the serum is so dilute that the sub-sample contains too little antibody to trigger a reaction.)
The highest dilution which DID show a reaction is reported as the "titer".
Results that differ by only one dilution (e.g., 1:640 and 1:1280) cannot be taken as evidence of any "real" difference. This is because the two samples could both have had near-borderline antibody levels.   If so, either cyclic variations in antibody production in the dog, or unavoidable small variations in test processes, could make the small difference required to move the result across the borderline from one dilution to the other.
For this reason, a change of at least 2 dilutions (a four-fold  or greater change in titer) is required as evidence for a significant change.
Anaplasma/Ehrlichia titers do decline slowly in *most* dogs following successful treatment to the point that negative results are obtained at 9-10 months post-treatment.  BUT, *most* is not *all*.   A significant minority of treated dogs do carry elevated titers indefinitely--some apparently for life--without ever showing a problem related to Anaplasma/Ehrlichia.
PCR testing for Anaplasma/Ehrlichia DNA offers one avenue for exploring whether such persistent-titer dogs still harbor Anaplasma/Ehrlichia. A limitation is that these organisms can "hide" in various tissues with only negligible amounts (or none) of their DNA in the blood to be detected by the PCR test.
The really essential procedure is to monitor the dog for a possible recrudescence of disease by watching for symptoms, optimumly abetted by periodic routine bloodwork (CBC and serum chemistries).  There is simply no substitute for "listening" to what the dog's behavior and condition is telling you.
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