Lyme disease is diagnosed through a two-step blood test that looks for antibodies your immune system produces in response to the bacteria. A single blood draw is all that’s needed, and both steps can be run from the same sample. However, the timing of the test matters enormously: antibodies take weeks to develop after infection, so testing too early is one of the most common reasons for a false negative. There’s also one important exception where no blood test is needed at all.
When a Blood Test Isn’t Required
If you develop the characteristic bull’s-eye rash (called erythema migrans), your doctor should diagnose and treat you based on that rash alone, without waiting for lab results. The CDC is clear on this point: serologic tests are often falsely negative during the first few weeks of infection, which is exactly when the rash typically appears. Waiting for a blood test in this situation can delay treatment unnecessarily.
The rash shows up in roughly 70% to 80% of people infected with Lyme. It usually appears 3 to 30 days after a tick bite and expands gradually over days, sometimes reaching 12 inches across. If you have possible tick exposure in an area where Lyme is common and a rash that fits this description, treatment should start right away.
The Standard Two-Step Blood Test
When there’s no rash or the diagnosis is uncertain, the standard approach is a two-tiered blood test. Your doctor orders a single blood draw, and the lab runs it through two stages.
The first step is an enzyme immunoassay (often called an ELISA). This test screens your blood for antibodies against the Lyme bacteria. If it comes back negative, testing stops there, and no further testing is recommended. If the result is positive or borderline, the lab moves to the second step.
The second step has traditionally been a Western blot, which looks for antibodies reacting to specific proteins from the Lyme bacteria. This confirmatory step exists because the first-step ELISA can produce false positives. You’re only considered positive for Lyme when both steps come back positive. A positive ELISA followed by a negative Western blot is reported as negative overall.
Labs increasingly use a newer approach called modified two-tiered testing, which replaces the Western blot with a second, different ELISA. Both assays must be FDA-cleared to be used together for this purpose. The modified approach is faster for the lab to run and produces results that are easier to interpret, since it eliminates the subjective band-reading involved in Western blots.
How the Western Blot Is Read
If your lab uses the traditional approach, the Western blot results depend on which type of antibodies are being measured. The test looks for your immune system’s reaction to specific bacterial proteins, and each reaction shows up as a visible “band” on the test strip.
For IgM antibodies (the type your body produces early in infection), at least 2 out of 3 specific protein bands must be present for a positive result. For IgG antibodies (produced later and lasting longer), at least 5 out of 10 specific protein bands must appear. Falling short of those thresholds means a negative result, even if some bands are visible.
One critical detail: IgM results are only considered meaningful if your symptoms started within the past 30 days. After that window, IgM results carry a high risk of being falsely positive, either from lingering antibodies after an old resolved infection or from cross-reactivity with something else entirely. Doctors are discouraged from relying on IgM results for patients who’ve been symptomatic for more than a month.
Why Timing Affects Accuracy
The two-tiered test detects antibodies, not the bacteria itself. Your body needs time to mount an immune response, which means testing too soon after a tick bite can produce a false negative. During the first one to two weeks of infection, antibody levels are often too low to detect.
IgM antibodies typically become detectable within two to four weeks of infection. IgG antibodies follow later and may take four to six weeks to reach detectable levels. If you test negative early on but your doctor still suspects Lyme based on your symptoms and exposure history, retesting a few weeks later is a reasonable next step.
This is why the rash-based clinical diagnosis matters so much. The window when the rash appears overlaps significantly with the window when blood tests are least reliable.
What Can Cause a False Positive
Several conditions can trigger a positive ELISA result even when Lyme bacteria aren’t present. This is one of the main reasons the second-step confirmatory test exists.
Active viral infections are a common culprit. Epstein-Barr virus, cytomegalovirus, and herpes simplex virus type 2 have all been linked to false positive Lyme results. These viruses can cause widespread activation of immune cells, producing antibodies that cross-react with the Lyme test. Other bacterial infections can cause the same problem, including syphilis (which is caused by a closely related spiral-shaped bacterium), as well as certain Rickettsia and Ehrlichia infections. Autoimmune conditions like thyroiditis have also been associated with false positives.
The two-step process catches most of these. A false positive on the ELISA alone doesn’t lead to a Lyme diagnosis as long as the confirmatory step is performed.
Testing for Neurological Lyme Disease
When Lyme disease affects the nervous system, a condition called neuroborreliosis, standard blood testing may not be enough. Doctors sometimes perform a lumbar puncture (spinal tap) to test cerebrospinal fluid directly.
The key measurement here is an antibody index, which compares the level of Lyme-specific antibodies in your spinal fluid to the level in your blood. Both samples need to be collected within 24 hours of each other. The lab calculates a ratio to determine whether antibodies are actually being produced inside your central nervous system or simply leaking in from your bloodstream. A high ratio indicates that your immune system is actively fighting the infection in your brain or spinal cord, not just in the rest of your body.
PCR Testing for Lyme Arthritis
For patients with Lyme-related joint swelling, doctors can test joint fluid directly for bacterial DNA using a PCR test. This test has the advantage of detecting the organism itself rather than relying on the immune response. In joint fluid, PCR is highly specific (virtually no false positives) with sensitivity ranging from 42% to 96%. It’s most useful as an add-on when standard blood tests are inconclusive but Lyme arthritis is suspected.
PCR testing of blood, by contrast, is unreliable for Lyme disease because the bacteria don’t circulate in the bloodstream at high enough levels to detect consistently.
Tests to Avoid
The CDC and FDA have issued warnings about unvalidated tests marketed by some specialty labs. These are often laboratory-developed (“home brew”) tests that haven’t gone through FDA clearance, meaning their accuracy hasn’t been independently verified. Reviews of some of these tests have raised serious concerns about false positives from laboratory contamination and the potential for misdiagnosis.
One distinction worth understanding: a lab being CLIA-certified means it meets basic quality standards for its operations. That’s not the same as the specific test being FDA-cleared. CLIA certification says nothing about whether a given test actually works. FDA clearance, on the other hand, means the test itself has been validated for accuracy. When you’re choosing where to get tested, look for labs running FDA-cleared assays using the standard two-tiered approach.
Cost and Access
The standard Lyme antibody test is widely available through most commercial labs and can be ordered by any primary care doctor. Out-of-pocket costs typically range from $19 to $116 depending on the lab and your location. Most health insurance plans cover Lyme testing when ordered by a physician, particularly in regions where the disease is endemic. Results usually come back within a few days, and since both tiers can be run from a single blood draw, you won’t need to return for a second sample.