The blood tests we have don’t test directly for the bacteria, but instead test for the body’s antibody response. When you hear people talk about sending a “Lyme test” or a “Lyme titer,” what they are sending is a two-tiered test, looking to see whether the body is making antibodies to that bacteria. For that reason, the test will be negative early on during the infection, because it takes time for the immune system to mount this defense.

So the Lyme test is not helpful in the earliest stages of infection — which is when you would ideally like to start treatment. Not only that, it takes a while to get the results.

“For me as an emergency room physician, none of the tests come back in rapid enough time to make a decision,” Dr. Nigrovic said. She makes treatment decisions on the basis of symptoms, such as meningitis or swollen joints, she said, and that also means deciding how aggressively to pursue alternative diagnoses — for example, deciding whether a swollen knee needs to be surgically drained or looking for other possible infections: “In my world, it’s like, O.K., do I tap the knee, let the orthopedist take the kid to the O.R. when it’s probably just Lyme?”

And the increase in Lyme should remind us that ticks can carry other infections as well.

If a different infection is actually causing the symptoms, starting antibiotics for Lyme can mean partially — that is, inadequately — treating something else that could be potentially dangerous. The acute presentation of Lyme includes facial nerve palsy, fainting and swollen joints, especially the knee, but also meningitis, with fever, headache and stiff neck.

“If you recall a tick bite, that’s an important thing to take into account,” Dr. Nigrovic said. So it’s routine to ask about that, in an emergency room or a doctor’s office. Even so, she said, in a study that she and her colleagues published this year, only one out of five children who end up diagnosed with Lyme could recall a tick bite. The nymphal stage ticks that transmit Lyme are so small, she said, that often they are not noticed. “If they don’t recall being bitten by a tick, we really should think about Lyme disease anyway if they’re in an endemic area.”