Lyme disease in children is diagnosed through a combination of clinical signs and blood tests, though the approach depends on how far along the infection is. If your child has the characteristic expanding red rash after a tick bite, a doctor can diagnose and begin treatment immediately, without waiting for blood work. If the rash isn’t present or symptoms appeared weeks later, blood tests become the primary diagnostic tool.
When Blood Tests Aren’t Needed
The bull’s-eye rash, called erythema migrans, is considered strong enough evidence on its own to diagnose Lyme disease and start antibiotics right away. This matters because blood tests are unreliable during the first few weeks of infection, which is exactly when the rash typically shows up. The CDC specifically recommends treating children with a likely rash and possible tick exposure in an area where Lyme is common, regardless of whether blood tests have been ordered.
The rash appears in roughly 70% to 80% of Lyme cases, usually within 3 to 30 days of a tick bite. It expands gradually over days, often reaching several inches across, and may feel warm but rarely itches or hurts. Not every Lyme rash forms a perfect bull’s-eye pattern. Some are uniformly red, and darker skin tones can make the rash harder to spot. If you notice any expanding red area after a known or possible tick bite, that’s reason enough to see a doctor promptly.
How Blood Testing Works
When a diagnosis isn’t obvious from symptoms alone, doctors rely on a two-step blood test recommended by the CDC. The first step is a screening test that detects antibodies your child’s immune system produces in response to the Lyme bacteria. If that screening comes back positive or borderline, a second, more specific test is run on the same blood sample to confirm the result. Your child only needs one blood draw for both steps.
The newer version of this two-step process uses two different screening tests back to back instead of following the first screen with the older confirmation method (called a Western blot). This updated approach catches more early infections. In studies, the newer method detected 61% to 74% of early localized cases, compared to just 29% to 48% with the older protocol. For early infections that have started to spread, the newer approach reached 100% sensitivity versus only 40% for the traditional method. By the time Lyme has progressed to later stages, both testing strategies detect virtually all cases.
The Timing Problem With Early Testing
The biggest limitation of Lyme blood tests is timing. Your child’s body needs 2 to 6 weeks after infection to produce enough antibodies for the test to pick up. Testing too early, particularly in the first couple of weeks, frequently produces a false negative. During early localized infection with just a single rash, the standard two-step test catches only about 14% of cases. That number jumps to around 65% if multiple rashes are present, and climbs to 96% to 100% once the infection has been present for weeks to months.
This is why doctors in areas where Lyme is common will often treat based on symptoms and exposure history rather than waiting for a blood test to turn positive. If your child was tested early and the result was negative but symptoms persist, your doctor may recommend retesting after several weeks.
Testing for Lyme Arthritis
Lyme arthritis is one of the most common late-stage symptoms in children, typically causing a swollen, painful knee or other large joint weeks to months after the initial infection. Blood antibody tests are highly accurate at this stage, catching nearly all cases. In most situations, a positive blood test combined with a swollen joint in a child from a Lyme-endemic area is enough to make the diagnosis.
In some cases, doctors may draw fluid from the affected joint. A specialized DNA-based test (PCR) can be run on that fluid to look for the Lyme bacteria directly. However, this test has significant limitations: in one study of children with confirmed Lyme arthritis, it detected the bacteria in only 29% of joint fluid samples. So a negative joint fluid test does not rule out Lyme arthritis, and doctors typically rely more heavily on blood antibody results.
Lyme arthritis in children is sometimes initially mistaken for a joint infection requiring emergency treatment or for juvenile rheumatoid arthritis. If your child develops unexplained joint swelling, especially in the knee, and you live in or have traveled to an area with ticks, make sure Lyme testing is part of the workup.
Testing for Neurological Lyme Disease
Lyme disease can affect the nervous system, and children are actually more prone to one particular complication: meningitis caused by the Lyme bacteria. The most recognizable sign is facial nerve weakness, where one or both sides of your child’s face droop. This occurs in roughly 55% of childhood neurological Lyme cases. Another 30% of children develop meningitis symptoms like headache, light sensitivity, nausea, and fatigue without obvious nerve involvement, which can make it easy to miss.
When a doctor suspects the infection has reached the nervous system, they may recommend a lumbar puncture (spinal tap) to analyze the fluid surrounding the brain and spinal cord. This test looks for signs of inflammation and antibodies produced locally in the nervous system. It’s more invasive than a blood draw, but it provides important information that guides treatment decisions for neurological symptoms.
Conditions That Look Like Lyme Disease
Several common childhood conditions can mimic Lyme disease, which is why accurate testing matters. The Lyme rash is frequently mistaken for cellulitis (a skin infection), a spider bite, or shingles. Joint symptoms from Lyme can look identical to a bacterial joint infection or juvenile rheumatoid arthritis. Even acute rheumatic fever, a complication of strep throat, can produce a ring-shaped rash and joint pain that closely resembles Lyme disease.
If your child has symptoms that could be Lyme but the blood tests are negative, your doctor may investigate these other possibilities. Conversely, if your child has been diagnosed with one of these conditions but isn’t improving with treatment, and you’re in a region where Lyme is common, it’s reasonable to ask whether Lyme testing has been done.
What to Expect at the Appointment
If you’re bringing your child in because you suspect Lyme disease, the doctor will first ask about tick exposure, outdoor activities, and where you live or have recently traveled. They’ll examine your child’s skin thoroughly for any rash, check joints for swelling, and look for neurological signs like facial weakness or difficulty with eye movements.
If blood tests are ordered, the results for the initial screening typically come back within a few days. If a confirmatory test is needed, expect another few days. The blood draw itself is the same as any routine lab test. No special preparation is needed, and your child doesn’t need to fast beforehand.
If your child has the characteristic rash, treatment will likely start at that first visit without waiting for lab results. For situations where the diagnosis is less clear, your doctor may start antibiotics based on clinical suspicion while waiting for test results, or may ask you to return if symptoms worsen or new ones develop.