Lyme Antibody (IgG) Test

If you have ever been bitten by a tick in an area where Lyme disease is common, or if you are experiencing unexplained joint pain, nerve problems, or fatigue, this test answers a specific question: has your immune system ever built a sustained response to the Lyme disease bacterium? A positive result does not automatically mean you are sick right now, and a negative result does not always mean you are in the clear. Understanding what this number actually tells you, and what it does not, is the difference between getting the right diagnosis and chasing the wrong one.

Lyme Antibody IgG (immunoglobulin G) measures a class of long-lasting antibodies your body produces against Borrelia burgdorferi, the spiral-shaped bacterium transmitted by blacklegged ticks. Unlike IgM antibodies, which appear early and fade within weeks, IgG antibodies represent a more mature immune response. They can remain detectable in your blood for years, sometimes decades, after the original infection has been treated and resolved.

How Your Body Builds a Lyme IgG Response

When Borrelia enters your bloodstream through a tick bite, your immune system first produces IgM antibodies as a rapid, first-wave defense. Over the following weeks, specialized immune cells called B cells undergo a process known as class switching, where they begin manufacturing IgG antibodies instead. These IgG antibodies target specific Borrelia proteins, particularly one called VlsE (a surface protein the bacterium uses to evade the immune system), and they bind the bacterium more precisely than the earlier IgM response.

The timing matters for testing. In very early Lyme disease, when a characteristic bull's-eye rash (erythema migrans) may be present, IgG antibodies often have not appeared yet. Only about 30 to 50% of people are seropositive (meaning they have detectable antibodies) at the time their rash first shows up. By three to four weeks after infection, that number rises. By the time Lyme disease has spread to joints, the nervous system, or the heart, IgG sensitivity reaches 97 to 100%.

This means a negative IgG result in the first week or two of symptoms does not rule out Lyme disease. It simply means your immune system has not had enough time to build that response yet.

What a Positive Result Means

A positive Lyme IgG tells you that your body has, at some point, mounted a full immune response to Borrelia. It does not tell you when the infection occurred or whether bacteria are still present. In a study following 79 patients for 10 to 20 years after treated Lyme disease, roughly 25% of those who had early Lyme and about 62% of those who had Lyme arthritis remained IgG positive years later, with no signs of active infection. A large registry study of over 34,000 patients in Sweden and Denmark confirmed that IgG antibodies persist longer than IgM, especially when initial levels are high, with median persistence stretching years.

This persistence is one of the most misunderstood features of the test. A positive result after treatment does not mean treatment failed. It means your immune system remembers the infection, the same way it remembers a vaccination. Repeating the test to see if it "turns negative" is generally not useful for monitoring treatment success.

What a Negative Result Means

A negative Lyme IgG result is most reliable when symptoms have been present for more than four weeks. If you have had joint swelling, facial nerve weakness, or heart rhythm problems for several weeks and your IgG is negative, Lyme disease becomes much less likely as the cause. For late manifestations like Lyme arthritis or late neurological Lyme, a negative IgG essentially rules out the diagnosis because nearly 100% of patients with these conditions are IgG positive.

The gap in sensitivity is at the very beginning. If you were bitten by a tick last week and have a rash, a negative IgG does not help you. Your doctor should diagnose erythema migrans based on the rash itself and start antibiotics without waiting for blood test confirmation.

The Two-Tier Testing System

Lyme IgG is not meant to be interpreted alone. Standard practice uses a two-tier approach: a screening test (usually an enzyme immunoassay, or EIA) followed by a confirmatory test (either a Western blot, which separates individual antibody targets on a test strip, or a second EIA targeting a different Borrelia protein). This layered approach exists because the screening test, while sensitive, can produce false positives from cross-reacting antibodies.

In a comparison across twelve clinical laboratories in Northern Europe, IgG results showed high agreement between labs using the same assay but lower agreement when different assay brands were used. This means your result can depend partly on which test your lab runs. A European meta-analysis reported overall sensitivity of about 73% and specificity of about 95% against healthy controls, though specificity dropped to around 80% in more realistic clinical populations where patients had other conditions that can mimic Lyme.

Newer modified two-tier strategies, which replace the Western blot with a second EIA (often targeting VlsE or C6 peptide), show slightly higher sensitivity, especially in early disease, while maintaining comparable specificity. These are increasingly adopted in clinical practice.

When Results Can Be Misleading

Several situations can produce a Lyme IgG result that does not reflect your actual infection status. Knowing these pitfalls can save you from unnecessary treatment or missed diagnoses.

• Active viral infections: Epstein-Barr virus (the cause of mononucleosis), cytomegalovirus, and BK virus can trigger antibodies that cross-react with Borrelia proteins, particularly flagellin (a whip-like structural protein also called p41) and OspC (a surface protein). In a study of 69 patients with active viral infections, false-positive Lyme results occurred frequently on both IgM and IgG screening tests.
• Intravenous immunoglobulin (IVIG) therapy: If you receive IVIG for an autoimmune or neurological condition, the pooled donor antibodies may include anti-Borrelia IgG from donors who were previously exposed. In one study, 39% of patients receiving a specific IVIG brand appeared to "seroconvert" to Lyme IgG positive, despite having no Lyme disease. This effect is brand-dependent and typically resolves within about three months after stopping the infusion.
• The hook effect at very high levels: In certain automated lab assays, extremely high IgG concentrations can paradoxically produce a falsely low reading. This technical artifact, identified in a study of over 5,600 samples, can cause late Lyme disease or Lyme arthritis to be missed. Labs manage this by automatically diluting samples that exceed certain thresholds.
• Assay variation between labs: Different manufacturers set different cutpoints. On the Liaison platform, the positive threshold is 17 AU/mL or higher; on the Enzygnost platform, it is 10 U/mL or higher. Comparing results across labs using different assays can be unreliable.

Lyme IgG and Joint Disease

Among the organ-specific associations, the link between Lyme IgG levels and arthritis is the most direct. In an Italian referral cohort of 54 confirmed Lyme cases, patients who presented with arthritis had dramatically higher IgG levels than those without joint involvement, averaging about 124 units compared to 25.5 units. This difference held even when comparing patients tested more than four weeks after a tick bite, suggesting the association reflects the disease pattern rather than just timing.

Cardiac and Neurological Associations

A large Danish registry study following over 400,000 individuals found that people who were tested for Borrelia antibodies (regardless of result) had markedly higher short-term rates of heart rhythm abnormalities called atrioventricular block, roughly 48 times more likely than the general population in the month surrounding testing. Over the long term, tested individuals had about a 30% higher rate of conduction disorders. When comparing seropositive to seronegative individuals specifically, those with both IgM and IgG antibodies had about twice the short-term risk of atrioventricular block.

For heart failure specifically, the same research group found that while being tested for Borrelia antibodies was associated with higher rates of heart failure diagnoses (likely because the test was ordered during a diagnostic workup), actual seropositivity itself was not associated with increased long-term heart failure risk. This is an important distinction: the test is often ordered when someone already has concerning symptoms, which inflates the apparent association.

For neuroborreliosis (Lyme disease affecting the nervous system), serum IgG has limited ability to confirm the diagnosis on its own. A nationwide Danish study of over 34,000 tested individuals showed that many patients with confirmed spinal fluid antibody production were IgG-negative in blood, and vice versa. When Lyme disease of the nervous system is suspected, spinal fluid testing is needed alongside the blood test.

Hematologic Cancers

The Danish registry cohort also examined cancer outcomes. Being tested for Borrelia antibodies was associated with about a 13-fold higher short-term rate of hematologic (blood) cancers, almost certainly because blood cancers cause symptoms that prompt antibody testing. More interesting was the long-term finding: individuals who were both IgM and IgG positive had roughly twice the rate of chronic lymphocytic leukemia (a type of blood cancer) compared to seronegative individuals. This association was adjusted for age and sex, but its clinical significance remains unclear, and it does not mean Lyme disease causes cancer.

Reference Ranges

Lyme IgG is interpreted as positive, borderline, or negative based on manufacturer-set cutoffs, not as a continuous scale with "optimal" or "healthy" zones. There is no level of Lyme IgG that is considered desirable for health or longevity. The cutpoints vary by assay platform and are determined by each manufacturer's internal validation against known positive and negative samples.

At very high levels (200 IU/mL or higher on the Enzygnost platform), the positive predictive value for true Borrelia infection approaches 99 to 100% in referred patients, and confirmatory Western blot testing may be unnecessary. Your lab report will include the specific cutpoints for the assay it uses. These thresholds do not change based on your age, sex, or ethnicity.

Tracking Over Time

For most biomarkers, serial trending reveals meaningful health trajectories. Lyme IgG is different. Because antibodies can persist for a decade or more after successful treatment, a stable positive result does not signal ongoing disease, and watching the number drift downward over months is not a reliable way to confirm treatment worked.

There are two scenarios where repeat testing has genuine value. First, if your initial test was negative but drawn very early (within the first two weeks of symptoms), retesting two to four weeks later can catch seroconversion, the point where your immune system has built enough IgG to be detected. A study of neuroborreliosis patients in Denmark found that about a fifth of individuals either seroconverted (developed new antibodies) or seroreverted (lost previously detectable antibodies) within two years, so timing matters.

Second, if you live in a highly endemic area and have a new clinical episode years after a prior infection, comparing a fresh result against your previous baseline (ideally run on the same assay at the same lab) can sometimes help determine whether a new immune response has occurred. But in most situations, once you have a confirmed positive Lyme IgG, retesting the same antibody adds little information. Clinical assessment drives the next steps, not the antibody level.

What to Do with Your Result

If your result is positive, the most important question is whether you have symptoms consistent with Lyme disease and whether you have had a plausible tick exposure. A positive IgG in someone with unexplained joint swelling, facial nerve palsy, or heart block in an endemic area strongly supports the diagnosis. A positive IgG in someone with vague fatigue and no clear exposure history is far more likely to reflect past resolved infection or cross-reactivity.

If your result is negative and you are more than four weeks from symptom onset, Lyme disease is unlikely for late manifestations. If negative and early in illness, retest in two to four weeks or pursue clinical diagnosis if a characteristic rash is present.

• Positive IgG with compatible symptoms: Discuss antibiotic treatment with an infectious disease specialist. For suspected neuroborreliosis, ask about spinal fluid testing to check for antibody production within the nervous system.
• Positive IgG without current symptoms: This likely reflects past exposure. No treatment is needed based on the antibody alone.
• Negative IgG within the first two weeks of symptoms: Retest in two to four weeks. If a classic bull's-eye rash is present, treatment should begin immediately without waiting for lab confirmation.
• Negative IgG after four or more weeks of symptoms: Lyme disease is unlikely for late manifestations. Consider alternative diagnoses, especially if joint, nerve, or heart symptoms persist. A rheumatologist or neurologist can help.