Rickettsial testing is used to determine whether a person with and a recent history of potential exposure to a specific rickettsia has been infected. Testing can help distinguish a rickettsial infection from other conditions causing similar symptoms, such as some other and infections and can help guide treatment. Testing may detect that your body has produced in response to the rickettsial infection or directly identify the organism by or by . Because these bacteria can only live inside animal cells they are difficult to culture and must be grown in tissue cultures or chicken embryos. This test is currently only performed at specialised laboratories.
When you have your blood drawn for diagnostic testing you usually have one tube and one EDTA (anticoagulant) tube collected. The serum tube goes towards the serology testing whereas the EDTA tube (plasma) is used for both DNA and culture testing. PCR testing may occasionally be performed on tissue.
With serology, the , or total antibody reactivity to rickettsial agents can be assessed. Usually, the IgM activity appears within a few days following infection, whereas the IgG appears later and usually as the disease is resolving. Total antibody screening may be performed instead which detects all rickettsial antibody activity. It is usually a good idea to have a follow up test performed in order for your doctor to confirm the diagnosis.
or PCR amplifies the genetic material of the rickettsia to detect the presence of the bacterium. Similarly, culture involves the placement of your blood onto living cells and monitoring these cells over time to see if a rickettsia can be isolated.
The serology testing will give your doctor information on the status of your infection and allow for the appropriate treatment to be administered. Both the PCR and culture tests look for the organism and aren’t sensitive tests since they require the organism to be circulating in your blood or present in tissue for the detection to occur. However, these tests are important as a positive result will definitively show the presence of the organism whereas the serology will only indicate that you have had an exposure to that organism. This may not always mean current infection.
Rickettsia testing is requested when a person has symptoms suggestive of a Rickettsial infection, particularly if they live in or have visited and area where Rickettisia infections are more common. A history of a tick bite, especially if there is an eschar (dark patch of dead tissue) on the skin at the site of the bite will suggest the need for testing. A history of camping, bushwalking or exposure to rodents in the previous one or two weeks are also important indicators.
Results of rickettsia testing require careful interpretation, taking into consideration the individual's and as well as risk of exposure. This is usually done best by the consulting doctor.
Antibody Tests
Antibody tests may be reported as reactive (positive) or non-reactive (negative), or may be reported as less than or greater than a certain . For example, if the established threshold is a titre of 1:128, then a result less than this is considered non-reactive while a titre greater than this is considered reactive.
If IgM (with or without IgG) rickettsial antibodies are detected in an initial blood sample, then it is likely that the person became infected with the rickettsia within the last few days or weeks. If the IgG is positive but the IgM is low or non-reactive, then it is likely that the person had a rickettsial infection sometime in the past. If the rickettsial IgG or total antibody titre increases four-fold between an initial sample and one taken 2 to 4 weeks later, then it is likely that a person has had a recent infection.
If an early test is negative for antibodies, the person may still have a rickettsial infection but it may just be that it is too soon after initial exposure to the organism and there has not been enough time to produce a detectable level of antibody. A negative result may also suggest that symptoms may be due to a different cause.
The following table summarises results that may be seen with antibody testing:
| IgM Result |
IgG Result |
Total Antibody Result |
Possible Interpretation |
| Reactive |
Non-reactive |
Reactive |
Current infection or false-positive IgM |
| Reactive |
Reactive |
Reactive |
Recent infection or false-positive IgM |
| Low or non-reactive or not tested |
Four-fold increase in samples taken 2-4 weeks apart |
Four-fold increase in samples taken 2-4 weeks apart |
Recent infection |
| Low or non-reactive |
Reactive or less than four-fold increase in samples taken 4-6 weeks apart |
Reactive or less than four-fold increase in samples taken 4-6 weeks apart |
Past infection |
| Non-reactive |
Non-reactive |
|
• Too soon after initial exposure for antibodies to develop. A reactive result on a second serum confirms the diagnosis of recent infection
• Symptoms due to another cause |
PCR Tests
If a PCR test is positive for rickettsia then it means that the DNA of this organism is present in your sample. This test cannot distinguish between living or dead bacteria. A positive test however does inform your doctor of the presence of these organisms and treatment can be adjusted accordingly.
The PCR may be negative for rickettsia if there are no bacteria present in the sample tested or if the bacteria are present in very low (undetectable) numbers. This is where the culture assay is a good backup test as it is more sensitive and can detect as few as 10 organisms. A negative test cannot be used to definitely rule out the presence of rickettsia.
Culture Tests
These are usually only performed in specialised laboratories. Positive results indicate the presence of the causative rickettsia whereas negative results may indicate that the organisms are present in low numbers or that you have not been exposed to a rickettsial agent. It must be noted that if you have started taking antibiotic treatment for a rickettsial disease, this test is likely to be negative even if you have been infected.