Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans through the bite of infected blacklegged ticks. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migrans. If left untreated, infection can spread to joints, the heart, and the nervous system. Lyme disease diagnosis is based on symptoms, physical findings (e.g., rash), and the possibility of exposure to infected ticks. Laboratory testing is helpful if used correctly and performed with validated methods. Steps to prevent Lyme disease include using insect repellent, removing ticks promptly, applying pesticides, and reducing tick habitat. The ticks that transmit Lyme disease can occasionally transmit other tickborne diseases as well.
How soon after a tick bite should you get tested?
In most cases, a patient who may have been exposed to Lyme disease should wait approximately 2 to 4 weeks after a tick bite to get tested. This allows time for antibodies against the Lyme bacteria to develop and become detectable in the blood. Getting tested too soon after a bite may lead to false negative test results. However, there are some situations where more prompt testing may be warranted:
- If a patient develops an erythema migrans rash within 3 to 30 days of a tick bite, no further testing is needed. The rash alone is diagnostic of Lyme disease and antibiotics should be started.
- If a patient develops symptoms like fever, headaches, joint pain or neurological issues within a few weeks of a bite, early Lyme testing may be done along with starting empiric antibiotic treatment.
- If a tick was attached for over 36 hours, testing may be done within 2 weeks as transmission risk is higher.
- If the bitten patient is pregnant, testing may be done more promptly due to risks of untreated Lyme disease to the fetus.
So in summary, while the standard window for serologic Lyme disease testing is 2 to 4 weeks after a tick exposure, certain scenarios warrant earlier testing based on symptoms, pregnancy status or extended tick attachment time. The decision to test sooner should be made on a case-by-case basis after discussing risks and benefits with the patient.
What kind of Lyme disease tests are available?
There are various tests that can be used to detect Lyme disease. The two main categories are antibody detection tests and polymerase chain reaction (PCR) tests:
Antibody Tests
- Enzyme-linked immunosorbent assay (ELISA): This is the most common initial screening test for Lyme disease antibodies. It detects if antibodies to Borrelia burgdorferi are present in the blood. If positive or equivocal, it is followed up by a Western blot test for confirmation.
- Western blot: This test looks for antibodies to specific proteins of the Lyme bacteria. It is used to confirm positive or equivocal ELISA results. The pattern of positive bands seen can indicate if infection is current or past.
- Indirect immunofluorescence assay (IFA): This can also detect Lyme antibodies like an ELISA. It is not as commonly used now.
Molecular Tests
- Polymerase chain reaction (PCR): This detects DNA of the Lyme bacterium itself. It is used in early Lyme disease when antibodies may not have developed yet. PCR is done on synovial fluid or, less commonly, blood or CSF.
In summary, serologic antibody tests like ELISA and Western blot are most useful for detecting Lyme disease after 2-4 weeks. PCR direct detection of Lyme DNA can sometimes diagnose Lyme earlier but is not routinely done on blood.
What are the CDC guidelines for Lyme disease testing?
The Centers for Disease Control and Prevention (CDC) has issued the following recommendations regarding Lyme disease laboratory testing:
- For patients with possible early localized Lyme (erythema migrans rash), no testing is needed. The rash can be clinically diagnosed.
- For patients with possible disseminated Lyme disease (multiple rashes, arthritis, neurologic issues), a two-tier test should be done – ELISA followed by Western blot for confirmation if ELISA is positive or equivocal.
- PCR testing of blood is not recommended, as presence of bacterial DNA in blood is variable. PCR of synovial fluid can be done in patients with Lyme arthritis.
- IgM Western blots are not recommended for patients with suspected Lyme who were previously treated. IgG blots only should be done as IgM may stay positive for prolonged periods.
- Testing for baseline antibodies is not recommended in asymptomatic patients after a tick bite without other Lyme risk factors.
- After suspected Lyme treatment, repeat testing is not recommended if the patient is asymptomatic. Serologic tests can remain positive for months to years after treatment.
In summary, the CDC endorses two-tier ELISA/Western blot antibody serologic testing for disseminated Lyme disease, but recommends against overuse of unproven tests or tests that do not accurately distinguish past treated infection from current active disease.
What factors lead to false results?
There are several factors that can interfere with Lyme disease test accuracy and lead to false positive or false negative results:
False positives
- Having another infection like syphilis, rheumatoid arthritis or mononucleosis which triggers an immune response that cross-reacts with the Lyme bacteria.
- Getting the Lyme vaccine in the past which primes the immune system.
- Taking certain antibiotics right before testing that cause a transient antibody spike.
False negatives
- Getting tested too soon, before the body has mounted a detectable antibody response (within first 1-2 weeks after a tick bite).
- Taking antibiotics prior to testing that may blunt the immune response.
- Having an impaired immune system that inhibits normal antibody production.
- Being tested too late, as antibodies may start to wane after infection has cleared.
To reduce inaccurate results, the two-tier ELISA/Western blot approach is used, and timing of tests in relation to possible tick exposure is important. However, no test is 100% foolproof. Clinical judgement must always factor into interpreting Lyme test results.
How accurate are the tests?
The accuracy of Lyme disease tests varies and depends on the type of test used and the stage of infection:
Sensitivity
This measures how often the test is positive when Lyme disease is truly present. Test sensitivity:
- ELISA: 29-40% sensitive in early localized disease, increasing to almost 100% in late disseminated disease.
- Western blot: 63% sensitive in early localized disease, 96-100% in late disseminated disease.
- PCR: Highly variable. More sensitive on synovial fluid (87-90%) than blood (29-45%).
Specificity
This measures how often the test is negative when no true Lyme infection exists. Test specificity:
- ELISA alone: 55% specific, so may have false positives without Western blot confirmation.
- Two-tier ELISA plus Western blot: 96-100% specific.
- PCR: Generally high specificity of around 95%.
In summary, the recommended two-tier ELISA/Western blot antibody testing protocol has high specificity but limited sensitivity in very early Lyme disease. No single test is perfect, so clinical judgement remains important.
Conclusion
In most routine cases, Lyme disease testing should be performed 2 to 4 weeks after a tick exposure to allow time for antibody development. However, certain scenarios like pregnancy or appearance of an erythema migrans rash warrant earlier testing. While no test is 100% accurate, the two-tier ELISA/Western blot approach is recommended by the CDC and has high specificity. Clinicians must use their judgement to interpret test results in the context of the patient’s symptoms and likelihood of tick exposure. Prompt and appropriate testing is important to avoid missing cases of Lyme disease and allow early antibiotic treatment when indicated.