Testing for Lyme Disease: CDC's Two-Step Serologic Process

For healthcare providers.

Borrelia burgdorferi under a microscope. CDC.

Laboratory diagnosis of Lyme disease rests on serologic tests for antibodies to Borrelia burgdorferi. When assessing a patient, consider:

• how likely it is that they were exposed to infected blacklegged ticks;
• whether their signs and symptoms fit Lyme disease;
• whether another illness could cause the same symptoms;
• laboratory results, where testing is indicated.

The two steps

CDC recommends a two-step serologic process, using assays cleared by the Food and Drug Administration (FDA). Both steps can be run on the same blood sample.

1. If the first test is negative, stop: no more testing is recommended.
2. If it is positive or indeterminate ("equivocal"), run the second.

The overall result is positive only when the first test is positive (or equivocal) and the second is positive (or, for some tests, equivocal).

• Standard two-tier testing (STTT) uses an enzyme immunoassay (EIA) first and a western blot second.
• Modified two-tier testing (MTTT), increasingly used by laboratories, uses two EIAs.

More: the updated CDC recommendation for serologic diagnosis of Lyme disease and APHL guidance on interpreting Lyme disease serologic results.

Points to remember

• Too early, a false negative. Antibodies take several weeks to develop, so tests can be negative in the first 4–6 weeks after infection.
• No test of cure. Once raised, antibody levels stay raised for months to years.
• Cross-reactions. False positives can occur in patients with other conditions, including relapsing fever, syphilis, rheumatoid arthritis and Epstein-Barr virus infection.
• IgM after a month. Some tests report IgM and IgG antibodies separately. Disregard a positive IgM result if the patient has been ill for more than 30 days.
• Early antibiotics. Patients treated early may be less likely to develop antibodies.

More serologic testing resources are available from CDC.

New tests

New tests may be developed to replace one or both steps. CDC will recommend a new test only after the FDA has cleared it.

Sources

• Centers for Disease Control and Prevention, "Clinical Testing and Diagnosis for Lyme Disease." https://www.cdc.gov/lyme/hcp/diagnosis-testing/index.html