WHICH SHOULD I USE—PCR OR SEROLOGY? CONSIDER BOTH
Most serologic tests document infection indirectly by detecting antibodies to the infectious agent. Demonstrating the presence
of antibody indicates exposure but not necessarily active infection. Therefore, it can be difficult to definitively attribute
clinical signs to infection, particularly in endemic areas.
PCR directly detects an organism's nucleic acid sequences. Recently, advances in molecular biology have made PCR panels easy
to perform and widely available. PCR makes it possible to directly test for the presence of multiple organisms with high sensitivity
and specificity, assuming appropriate laboratory controls are in place. However, organisms are not always circulating in blood
in high enough numbers to be present in a blood sample at the time clinical signs occur.
Considering whether the organism is likely to be circulating in blood at the time of presentation and the significance of
a positive or negative test result can help determine whether PCR or serology will be more sensitive or specific in a patient.
Table 1 summarizes this information for some common vector-borne disease agents that infect dogs.
Table 1: Some pathophysiologic considerations for determining which diagnostic test to use for select vector–borne disease
Is the organism in the sample? The clinical sensitivity of PCR
In order for PCR results to be positive, the organism (or the nucleic acid sequence that is the target of the PCR) has to
be present in the sample. Many current PCR assays have a high absolute sensitivity, meaning that they can reliably detect
the presence of only a few organisms in a sample. Therefore, if organisms are circulating in the blood in adequate numbers
at the time they cause clinical signs, it is very likely that a PCR test will detect infection.
A relatively large percentage of circulating blood cells are infected with organisms during the acute phase of infection with
A. phagocytophilum, B. gibsoni, B. canis, and E. canis in dogs. Thus, PCR is a sensitive test in a dog acutely infected with these agents.10,22,23 PCR can also detect infection in dogs with very low levels of parasitemia.
However, it is important to be aware that these organisms do not always circulate in adequate numbers in peripheral blood
to be detected using PCR. Babesia species and E. canis may circulate in blood in extremely low numbers or intermittently, particularly during chronic infection. Thus, although
highly sensitive, PCR results are not positive in all actively infected patients.7,10,24 Performing serologic testing in addition to PCR and conducting tests on additional blood samples using PCR and serology
can be important to facilitate a diagnosis of infection.
Some organisms never circulate in peripheral blood in high numbers. For example, Rickettsia rickettsii causes acute illness in dogs. Being endotheliotropic, these organisms circulate in relatively low numbers in peripheral blood
at the time clinical signs are occurring. Although PCR can be very useful in confirming acute infection, PCR is not as sensitive
as demonstrating a fourfold change in serologic titer.25
In dogs, Bartonella species circulate in blood in very low numbers. Furthermore, up to 50% of patients may be seronegative despite the presence
of infection.7 To document infection with Bartonella species, sterilely collected samples of blood and other specimens can be cultured in enrichment media before performing PCR
to increase sensitivity.7
Borrelia burgdorferi also does not circulate in peripheral blood in high numbers. Furthermore, clinical signs of Lyme borreliosis do not occur
in experimentally infected dogs until two to six months after tick exposure.26 Consequently, serologic testing rather than PCR is routinely used to document exposure to B. burgdorferi.