A 46-year-old woman comes to your clinic with a rash. Nine days ago, she spent a weekend camping and hiking in western Connecticut. Four days ago, she noticed a red spot at the back of her right upper arm. It has grown every day since. It is mildly warm and slightly itchy, but not painful.

She has felt more tired than usual for three days, with a mild headache and achy joints. She does not remember a tick bite.

Past medical history: None.

Medications: None.

Allergies: None known.

Pregnancy: Not pregnant; uses an IUD.

Vitals: Temperature 99.9°F; heart rate 78 bpm, regular; blood pressure 124/76 mmHg.

Physical exam: See image below. (Representative photograph of the lesion at the back of the right upper arm).

You suspect Lyme disease.

A. Start doxycycline now without Lyme laboratory testing

B. Send two-tier Lyme antibody testing, and start treatment if it is positive

C. Biopsy the edge of the rash and send it for Borrelia PCR

D. Send antibody testing now and again in 3 weeks, and treat if the second test turns positive

Choose your answer now before moving on.

An expanding red rash of this size, appearing days after time outdoors where Lyme disease is common, is erythema migrans. At this stage, the rash is the diagnosis. You treat it on sight.

The reason is the test, not the rash. Antibodies take weeks to form, and the rash appears before most patients have made them. In a review by leading Lyme researchers, standard two-tier testing during an acute erythema migrans rash had a sensitivity of only about 30%. Roughly seven in ten patients who truly have Lyme disease would test negative today.

A negative result would not exclude Lyme disease or change the need for treatment. It adds little diagnostic value, can falsely reassure you and the patient, and can delay treatment while the infection has a chance to spread. A positive test would not change what you do either. That is why the 2020 joint guideline from the Infectious Diseases Society of America, the American Academy of Neurology, and the American College of Rheumatology recommends diagnosing erythema migrans clinically rather than with laboratory tests in patients with a likely tick exposure.

Treatment is doxycycline 100 mg twice daily for 10 days. A randomized trial found 10 days worked as well as 20 days.

This is the most common reflex, and the trap. With about 30% sensitivity at this stage, most true cases test negative, so "treat if positive" means leaving most infected patients untreated. Testing has its place, but not here.

It is invasive and rarely needed outside research settings. The rash has already answered the question the biopsy is meant to ask. It adds cost and delay without changing treatment.

Antibody levels do rise over the following weeks, which is why convalescent testing 2-3 weeks later is more sensitive. It has a legitimate clinical role when a lesion is atypical for erythema migrans and initial serology is negative. However, it is the wrong option in this patient because she already has a clinically diagnostic presentation, so treatment should not wait. Waiting three weeks to treat a diagnosis you can already see gives the infection time to spread to the nerves, heart, or joints.

A diagnostic test is only worth ordering if its result could change what you do. When the clinical picture is classic, and the test is known to miss most early cases, a negative result can't rule out the disease, and a positive one won't change your plan. Treat what you see.

A positive Lyme test cannot tell you the infection is active. Antibodies can stay positive for months to years after successful treatment, and Borrelia DNA can persist after the bacteria are dead. Never order Lyme serology or PCR to "confirm cure."

Two-tier testing (a screening immunoassay followed by a confirmatory test) is very specific. In patients with acute erythema migrans, it is about 99% specific, but only about 30% sensitive. After treatment, during the convalescent weeks, sensitivity rises to roughly 60 to 70%. Early positives are usually IgM.

A newer approach, modified two-tier testing, replaces the second-step immunoblot with a second immunoassay. In a study of 228 Dutch patients with erythema migrans, the best modified algorithm reached about 78% sensitivity while maintaining about 96% specificity, significantly better than standard two-tier testing. Ask your lab which method it uses. It matters most in early cases without a typical rash, where you actually need the test.

In practice: Even with the better test, a classic rash still needs no testing. Save serology for the presentations in the Quick Reference table that change your decision.

Duration: In a randomized, double-blind trial of 180 patients with erythema migrans, 10 days of doxycycline worked as well as 20 days, providing evidence that extending treatment to 20 days did not improve outcomes. Complete response at 30 months was 90% versus 84%. Adding a single dose of IV ceftriaxone did not improve outcomes and increased diarrhea.

Alternatives: Amoxicillin or cefuroxime axetil work for erythema migrans when doxycycline isn't an option. Azithromycin is a backup for patients who can't take either class.

The quiet bonus of doxycycline: The same tick can carry Anaplasma phagocytophilum, the cause of human granulocytic anaplasmosis. Doxycycline treats it; amoxicillin and cefuroxime do not. If your patient has high fevers, low white cells or platelets, or raised liver enzymes alongside the rash, that difference matters. However, remember that doxycycline does not treat babesiosis, another relevant coinfection, so persistent fever should prompt reassessment.

Who needs an alternative: Pregnancy is one of the classic reasons to choose amoxicillin or cefuroxime. That's why the case states she isn't pregnant. Guidance on short doxycycline courses in young children has changed in recent years, so check current pediatric recommendations rather than older rules. For breastfeeding patients, a short course of doxycycline may still be allowed. LactMed considers short-term use unlikely to harm a nursing infant; treatment decisions still depend on the circumstances.

In a randomized trial in a high-risk area of New York, a single 200 mg dose of doxycycline given within 72 hours of removing an attached Ixodes scapularis tick reduced erythema migrans from 3.2% to 0.4% (an 87% relative reduction), with an absolute reduction of about 2.8 percentage points. Nausea was common (15% vs 3%).

Which bites matter: In the same trial, every case followed a bite from a nymph-stage tick, and most followed a nymph that was at least partly engorged with blood. 8 of 81 partly engorged nymphs led to erythema migrans, compared with none of 59 flat, unfed nymphs and none of 97 adult female ticks.

In practice: For an adult, one 200 mg dose of doxycycline is recommended if a deer tick was attached for at least 36 hours, the bite happened where Lyme disease is common, and the tick was removed within the past 72 hours, as long as the patient can take doxycycline.

If the patient has no symptoms, do not order Lyme antibody testing. The test is too early to detect a new infection, and a positive result could reflect an old infection or be a false positive.