Lyme disease is diagnosed through a two-step blood test that detects antibodies your immune system produces in response to the infection. Both steps can be run from a single blood draw, and both must be positive for the result to count. The process is straightforward, but timing matters: testing too early after a tick bite can produce a false negative.
The Two-Step Blood Test
The CDC recommends a two-tier testing approach for Lyme disease. The first step is a screening blood test called an enzyme immunoassay, or EIA. If this initial screen comes back negative, no further testing is needed. If it comes back positive or borderline, a second test is run to confirm the result.
In the traditional version, that second test is a Western blot, which looks for antibodies reacting to specific proteins from the Lyme bacterium. Increasingly, though, labs are switching to a modified approach that uses two different EIA tests instead of a Western blot. Both versions require FDA-cleared test combinations, and both follow the same basic logic: screen first, confirm second.
You don’t need to request a specific version. Your doctor orders the test, and the lab runs whichever two-tier method they have in place. The blood draw itself is no different from any routine lab work.
Why Timing Matters
These tests don’t detect the Lyme bacterium directly. They detect antibodies, which your immune system takes time to produce. During the first few weeks of infection, antibody levels may be too low to register, leading to a falsely negative result. FDA-cleared tests reach good sensitivity after four to six weeks have passed since infection.
This creates a frustrating gap. Early Lyme disease often announces itself with the characteristic bull’s-eye rash (called erythema migrans), which typically appears within days to a few weeks of a tick bite. At that point, a blood test may not yet be reliable. If you have the rash and a plausible history of tick exposure in an area where Lyme is common, doctors are advised to start treatment right away without waiting for lab confirmation.
If your initial test is negative but your doctor still suspects Lyme, retesting with a new blood sample in seven to fourteen days can sometimes catch antibodies that weren’t detectable the first time around.
How Accurate Are the Tests?
Test sensitivity depends heavily on how far along the infection is. In early Lyme disease, particularly when the rash is the only symptom, the traditional two-tier method catches only about 30 to 40 percent of true cases. That low number is largely because the Western blot step misses infections that haven’t triggered a full antibody response yet.
The modified two-tier approach, which swaps the Western blot for a second EIA, performs better in early disease but still typically falls under 80 percent sensitivity. In later-stage Lyme, when the infection has had weeks or months to provoke a strong immune response (as in Lyme arthritis or neurological Lyme), antibody tests are considerably more reliable.
The two-step design exists primarily to reduce false positives. The first screening test casts a wide net and can sometimes react to antibodies from other conditions. The second confirmatory test narrows the results to antibodies more specific to the Lyme bacterium. Running both steps together keeps the overall accuracy high enough for clinical decisions.
When You Don’t Need a Test
If you develop the bull’s-eye rash after a tick bite in an area where Lyme disease circulates, that rash alone is considered diagnostic. Treatment should start immediately. Waiting for blood test results in this situation can delay antibiotics unnecessarily, especially since the test is least reliable during exactly this window.
Testing becomes most useful when symptoms suggest Lyme disease but no rash appeared (or was never noticed). Joint swelling, facial nerve paralysis, heart rhythm changes, or neurological symptoms weeks after possible tick exposure are all situations where blood testing helps confirm the diagnosis.
What the Results Mean
A negative first-tier test means no antibodies were detected and no further testing is done. This is either a true negative or a test performed too early in the infection.
A positive or borderline first-tier test followed by a negative second-tier test means the initial result was not confirmed. There is no laboratory evidence of Lyme infection, though early infection can’t be completely ruled out if symptoms started within the past two weeks.
When both tiers come back positive, that confirms antibodies to the Lyme bacterium are present. One important caveat: antibodies can remain in your blood for months or even years after the infection has been successfully treated. A positive test does not necessarily mean you have an active infection right now, and repeat testing should not be used to track whether treatment is working. Your doctor will interpret the results alongside your symptoms and clinical history.
PCR and Other Testing Methods
In specific clinical situations, doctors may use a PCR test, which detects the bacterium’s genetic material directly rather than relying on your immune response. This is mainly useful for testing joint fluid when Lyme arthritis is suspected. PCR is not sensitive enough on other sample types (blood, spinal fluid) to be a reliable standalone test, and it is not part of routine Lyme screening. Some laboratories require special approval before running it.
Direct testing of tick specimens (sending in the tick that bit you) is available through some commercial labs but is not recommended as a basis for treatment decisions. Not every infected tick transmits the bacterium, and a positive tick test doesn’t confirm human infection.
Getting Tested
Any primary care doctor can order Lyme disease testing. The test requires a standard blood draw, and results typically come back within a few days to a week depending on the lab. If you’re in an area where Lyme is less common, your doctor may need to send the sample to a reference laboratory, which can add a few extra days.
If you’re considering testing, the most important thing to communicate to your doctor is the timeline: when you found the tick or noticed the bite, when symptoms started, and whether you’ve been in a region where Lyme-carrying ticks are active. That context helps your doctor decide whether to test now, treat empirically, or recommend retesting after a short wait.