Lyme disease is diagnosed through a combination of physical examination and blood testing, depending on how far the infection has progressed. If you have the characteristic expanding rash, a doctor can diagnose Lyme disease on sight, without any lab work. If you don’t have a visible rash, diagnosis relies on a two-step blood test that detects antibodies your immune system produces in response to the bacteria.
The Rash That Confirms Lyme Without a Blood Test
The hallmark rash of Lyme disease, called erythema migrans, appears in over 70 percent of infected people. When a doctor sees this rash and you have a history of possible tick exposure, that’s enough for a diagnosis and immediate treatment. No blood test is needed, and waiting for lab results would only delay antibiotics.
The rash doesn’t always look like the classic “bull’s-eye” you may have seen in photos. It can take several forms: a solid red or bluish oval plaque, an expanding circle with central clearing, a lesion with a crusty center, or a reddish-blue patch with no ring pattern at all. The key feature is that it expands over days, typically reaching at least 5 centimeters across. Some people develop multiple rashes in different locations, which signals the infection has started to spread. If you notice an expanding rash days to weeks after spending time in a tick-heavy area, that visual alone is the fastest route to diagnosis.
The Two-Step Blood Test
When there’s no rash, or when the diagnosis is uncertain, the CDC recommends a two-step blood test. Both steps can be run from a single blood draw. The tests don’t detect the Lyme bacteria directly. Instead, they look for antibodies your body makes to fight the infection.
In the standard version, the first step is a screening test called an enzyme immunoassay (EIA). If this comes back negative, no further testing is done. If it’s positive or borderline, the lab runs a second, more specific test called a Western blot. The Western blot looks for antibodies that react against specific proteins from the Lyme bacterium. For an early-stage IgM result to count as positive, at least 2 out of 3 target proteins must react. For a later-stage IgG result, at least 5 out of 10 target proteins must be present. Both steps must be positive for the result to be considered a confirmed case.
More labs are now using an updated approach called modified two-tier testing, which replaces the Western blot with a second, different EIA. This newer method uses FDA-cleared test pairs and tends to be faster to process. Borderline results on the second test are treated as positive, meaning they count as evidence of exposure to the Lyme bacterium.
Why Timing Matters for Accuracy
The biggest limitation of blood testing is that it can miss early infections. Your immune system needs time to build detectable levels of antibodies, and in the first two weeks after a tick bite, antibody levels may be too low for any test to pick up. If you’re tested very early and get a negative result but your doctor still suspects Lyme, the recommendation is to repeat the blood draw 7 to 14 days later.
The type of antibody detected also matters. IgM antibodies appear first, typically within the first few weeks of infection. IgG antibodies develop later and persist longer. Because of this timing, a positive IgM result is only considered meaningful if you’ve been sick for 30 days or less. After that window, IgM results should be disregarded because they’re more likely to be misleading. IgG results become the reliable marker for infections lasting more than a month.
What Can Cause a False Positive
The first-step screening test casts a wide net, which means it sometimes flags antibodies that aren’t related to Lyme at all. Several conditions can trigger a false positive on the initial screen, including syphilis, mononucleosis, periodontal disease, rheumatoid arthritis, and lupus. This is exactly why the two-step process exists. The second test is much more specific, filtering out most of these false alarms. If the first test is positive but the second is negative, the result is not considered evidence of Lyme disease.
Testing Beyond Standard Blood Work
In certain situations, doctors may use a test called PCR, which detects genetic material from the Lyme bacterium itself rather than antibodies. PCR is most useful when testing fluid drawn from a swollen joint. In that setting, a positive result is highly specific, meaning if it’s positive, the bacteria are almost certainly there. However, PCR performs poorly on blood samples and spinal fluid, where its sensitivity is too low to be reliable. One situation where PCR can be particularly helpful is in confirming persistent or recurring Lyme disease, since a positive result provides direct evidence of the bacterium’s presence.
What Your Test Results Won’t Tell You
Lyme antibody tests are designed to help with diagnosis, not to track recovery. After treatment, antibodies can remain in your blood for months or even years, so a positive result after completing antibiotics doesn’t mean you’re still infected. The CDC specifically notes that lab testing should not be used to monitor whether treatment is working. Instead, successful treatment is measured by whether your symptoms resolve. Repeat testing after treatment can create confusion and lead to unnecessary additional rounds of antibiotics, so most doctors rely on clinical improvement rather than follow-up blood work.
Conditions That Mimic Lyme Disease
Diagnosing Lyme can be complicated by other tick-borne illnesses that look similar. Southern Tick-Associated Rash Illness, or STARI, produces an expanding rash that can be nearly indistinguishable from a Lyme rash, but it’s caused by a different tick species found in the southeastern United States and doesn’t respond to the same testing. Researchers are currently working on metabolic biomarkers that could reliably distinguish Lyme from STARI and other lookalike conditions, but for now, geography and tick identification play a role in sorting out the diagnosis. If you were bitten in an area where Lyme-carrying ticks aren’t common, your doctor may consider STARI or other regional tick-borne infections instead.