There were over 36,000 cases of suspected or confirmed Lyme disease in the United States in 2016, all beginning with the bite of an infected tick. But does every patient with a tick bite need a Lyme disease test? No, according to Robert Jones, MD, Medical Director for Infectious Diseases at Quest Diagnostics. Important considerations include tick identification, geography, symptoms and signs, and duration of tick attachment.

Lyme disease is caused by infection with Borrelia burgdorferi, carried by the deer tick. If the patient has removed the tick and is still in possession of it, the first question should be whether it is a deer tick (Ixodes scapularis) or a dog tick (Dermacentor variabilis). Deer ticks have a brown body with no white markings. Nymphs (juvenile deer ticks) are about the size of a pin head or poppy seed, while adults are 3-5 millimeters long. Dog ticks are larger than deer ticks, and have distinctive white markings on their backs. Dog ticks don’t carry Lyme disease.

Often, though, the patient will not have the tick, and may not even remember being bitten. When that’s the case, the next consideration is geography, Dr. Jones says. “Knowing the geography of the disease is helpful to determine whether your patient warrants testing. Lyme disease is an endemic disease. It is much more likely to be acquired in some areas than in others.” Those living in the Northeast and Upper Midwest are most at risk, while those in the Mountain States and the South are least at risk. For instance, in 2016, the incidence of confirmed Lyme disease was 2,455 per 100,000 in Pennsylvania, and zero in Oklahoma.

It is certainly possible to have Lyme disease in a state where it is not endemic, Dr. Jones adds, but this is most often because it was acquired through travel to a state with a high infestation. If your patient is outside of the endemic zone and has not recently traveled there, another diagnosis may be more likely.

The duration of tick attachment is also an important consideration, he added. “If the tick has been attached for less than 24 hours, and it has been removed appropriately, there is no risk. The Borrelia bacterium lives in the tick midgut, so the tick has to be attached, taking a blood meal, and the bacterium must migrate up to the salivary glands, and then enter the host. That takes time. If the tick is removed within 24 hours, some would say even longer, the risk of developing Lyme disease is little to none.”

The next consideration is symptoms and signs. The most characteristic sign of Lyme disease is the rash. The Lyme rash is at least 4 centimeters in diameter, Dr. Jones says, and may get larger. “It shouldn’t be confused with a small area of redness at the tick bite site.” The rash most commonly appears at the site of the bite, but may appear elsewhere. It may have a bulls-eye appearance, with resolution in the center surrounded by an area of redness. “The bulls-eye rash is diagnostic by itself,” he says.

However, Dr. Jones notes, up to 30% of patients infected with Borrelia never get a rash. Infection occurs most commonly during the warm months when ticks are most active. Patients typically present with flu-like symptoms, including fatigue, headache, and low-grade fever, along with joint pain, especially asymmetric knee pain with effusion. Infection is more likely in those who spend more time outdoors, whether gardening, golfing, hunting, or just in the back yard.

“If you suspect it may be Lyme disease, but it’s not quite clear that that’s what it is, that’s the time to get a Lyme disease test,” Dr. Jones says. Since there are other diseases also carried by the deer tick, which may be transmitted along with or instead of Lyme disease, it may be valuable to order these tests at the same time.