Making Sense of Serologies: HIV, Lyme, Hepatitis, and TB Testing in Primary Care

 

NP CHRONICLES

Clinical Education for NP Students & New Graduates

Making Sense of Serologies: HIV, Lyme, Hepatitis, and TB Testing in Primary Care

Infectious Disease  |  Board Prep  |  Test Interpretation

Serologic testing looks straightforward until you're staring at a reactive HIV screen in a low-risk patient, a Lyme ELISA drawn three days after a rash appeared, or an isolated hepatitis B core antibody before an infusion. This post distills a CE lecture from Daniel A. Solomon, MD, FIDSA, of the Division of Infectious Diseases at Brigham and Women's Hospital, covering four serologies that come up constantly in primary care: HIV, Lyme disease, hepatitis B/C, and latent TB. The focus is interpretation — not just what to order, but what the result actually means once it's back.

HIV: The Algorithm Matters as Much as the Result

Modern HIV screening uses a 4th-generation antigen/antibody (Ag/Ab) combination assay, which detects both HIV antibody and the p24 antigen — meaningfully shortening the “window period” compared to older antibody-only assays. But a reactive screen is never a diagnosis on its own. It triggers a defined confirmatory sequence.

Step	Test	Purpose
1	HIV Ag/Ab combination assay (screen)	Initial screen; detects antibody and p24 antigen
2	HIV-1/2 differentiation assay	Confirms and distinguishes HIV-1 vs. HIV-2 if the screen is reactive
3	HIV RNA PCR (viral load)	Resolves discordant results; confirms diagnosis in suspected acute infection

 

CASE FROM PRACTICE A 26-year-old pregnant woman, mutually monogamous and asymptomatic, has a reactive HIV Ag/Ab screen on routine prenatal testing. The HIV-1/2 differentiation assay comes back negative, and HIV viral load is undetectable. Interpretation: false-positive screen, possibly related to cross-reactive antibodies that can occur during pregnancy. In low-prevalence populations, a false-positive screening result is genuinely more common than acute HIV — but a false positive on both the screen AND the differentiation assay is rare, occurring in roughly 1 in 200,000 cases.

That case illustrates a core teaching point: as HIV prevalence in the tested population drops, the proportion of reactive screens that are false positives rises, even with a highly specific test (99.4% specificity in the example used). At 10% prevalence, most reactive screens are true positives; at 0.4% prevalence, false positives become common. This is Bayesian reasoning applied directly to a test result NPs order constantly.

When Acute HIV Is the Real Concern

CASE FROM PRACTICE A 22-year-old man who has sex with both men and women presents with fever, myalgias, pharyngitis, diarrhea, and cervical lymphadenopathy. His last HIV test, one month prior, was negative, and he has had one new partner since. HIV Ag/Ab is reactive, but the differentiation assay is negative — a pattern that would normally suggest a false positive. Because acute infection was clinically suspected, viral load was sent anyway: >10,000,000 copies/mL, with a CD4 count of 256 (13%). Diagnosis: acute HIV. He was started on ART at diagnosis and has since achieved an undetectable viral load with excellent adherence.

The clinical lesson: a reactive screen with a negative differentiation assay is not automatically a false positive if acute infection is on the differential. Antibody hasn't yet fully developed during the acute window, so the differentiation assay can be falsely negative even as the patient is acutely, highly infectious. Any time acute HIV is a real possibility — a compatible “mononucleosis-like” syndrome, recent high-risk exposure, or a discordant result in a symptomatic patient — send the viral load.

•     Acute HIV syndrome is highly variable: approximately 40% of cases are asymptomatic. Among those who are symptomatic, the most common features are fever (75%), fatigue (68%), myalgia (49%), rash (48%), headache (45%), pharyngitis (40%), and cervical adenopathy (39%).

•     Diagnosing acute HIV matters because the window period, while small, still exists; the syndrome resolves spontaneously (creating real risk of loss to follow-up if missed); acute infection is a period of maximum infectiousness; and early treatment reduces transmission risk, preserves CD4 counts, targets a less diverse viral population, and reduces the size of the HIV reservoir.

NUANCE TO FLAG Lifetime HIV risk is not evenly distributed, and the data are worth knowing for both clinical index of suspicion and for framing risk discussions without bias. Estimated lifetime risk in heterosexual men: 1 in 2,713 (White), 1 in 437 (Latinx), 1 in 97 (Black/African American). Among men who have sex with men: 1 in 11 (White), 1 in 5 (Latinx), 1 in 2–3 (Black/African American). These disparities reflect structural and systemic factors, not individual behavior differences, and should inform equitable testing practices — offering testing broadly rather than only to patients who disclose risk factors.

CLINICAL BOTTOM LINE — HIV False-positive HIV screens are common in low-prevalence populations. The standard confirmatory test is the HIV-1/2 differentiation assay, but any positive screen — and any case where acute infection is clinically suspected, regardless of differentiation assay result — should be confirmed or clarified with an HIV RNA viral load.

Lyme Disease: Match the Test to the Stage

Lyme testing performance depends heavily on how long the infection has been present, which is why clinical staging should drive both diagnosis and test selection rather than reflexively ordering serology on every rash or joint complaint.

Stage	Time from Bite	Key Features	Diagnostic Approach
Early localized	7–28 days	Erythema migrans; fatigue, headache, myalgias, fever	Clinical diagnosis preferred — serology often still negative
Early disseminated	Weeks to months	Meningitis, CN palsy, radiculopathy, peripheral neuropathy; Lyme carditis, AV block	Antibody test with reflex to confirmatory testing
Late disease	Months to years	Knee arthritis, acrodermatitis chronica atrophicans, encephalopathy	Antibody test with reflex to confirmatory testing
Post-treatment Lyme disease syndrome	3–12 months post-treatment	Headache, fatigue, arthralgias	Clinical diagnosis; no antibiotics indicated

 

Standard two-tier testing (an ELISA with reflex to Western blot, or a modified two-tier approach using two sequential EIAs) is designed to be used in early disseminated or late disease — not early localized disease, where sensitivity is genuinely poor.

Stage	Sensitivity	Specificity
Early localized	14–17%	>99%
Early disseminated	89.7%	>99%
Late disseminated	99.4%	>99%

 

CLINICAL BOTTOM LINE — LYME In early localized disease (classic erythema migrans, 7–28 days post-bite), diagnose clinically — serology sensitivity is only 14–17% at this stage and a negative result should not delay treatment. Reserve two-tier serologic testing for early disseminated or late disease, where sensitivity exceeds 89%. Don't use antibody testing as a “test of cure” — many patients remain seropositive long after successful treatment, and diagnosing reinfection serologically can be genuinely difficult.

Hepatitis B: The Triple Screen, Explained

CASE FROM PRACTICE A 37-year-old woman with no past medical history, born in Ho Chi Minh City and living in the US since 2007, presents for a routine visit. Should she be screened for hepatitis B? Yes — and per current CDC guidance, so should essentially every adult.

CDC now recommends universal hepatitis B screening: every adult aged 18 and older should be screened at least once in their lifetime, regardless of disclosed risk factors. Pregnant patients should be screened for HBsAg during every pregnancy, regardless of vaccination history or prior testing. Periodic risk-based testing should additionally cover incarcerated individuals, people with a history of STIs or multiple partners, people with hepatitis C, and anyone who simply requests testing.

The “triple screen” — HBsAg, HBsAb, and HBcAb — accomplishes three distinct goals at once, and understanding what each component tells you is the key to interpreting the panel correctly.

Goal	Test	Action if Positive/Result
Identify chronic HBV	HBsAg	Monitoring and/or antiviral treatment
Identify susceptibility	HBsAb (anti-HBs)	If negative — vaccinate
Identify reactivation risk	HBcAb (anti-HBc)	If positive and patient becomes immunosuppressed — monitor or give prophylaxis

 

The reactivation risk identified by an isolated positive HBcAb is not theoretical. HBV flares can occur even in patients who appear to have fully resolved infection, are often preceded by a rise in viral DNA, and classically occur at the start or withdrawal of immunosuppressive therapy. The 2025 AGA Clinical Practice Guidelines stratify management by reactivation risk category:

•     High risk: antiviral prophylaxis is recommended over monitoring alone (strong recommendation, moderate-certainty evidence).

•     Moderate risk: antiviral prophylaxis is suggested over monitoring alone (conditional recommendation, moderate-certainty evidence).

•     Low risk: monitoring alone is suggested over antiviral prophylaxis (conditional recommendation, moderate-certainty evidence).

CASE FROM PRACTICE A 42-year-old woman with newly diagnosed multiple sclerosis is about to start ocrelizumab, an anti-CD20 agent. Her hepatitis B panel: HBsAg negative, HBsAb negative, HBcAb positive. Interpretation: this is an isolated core antibody pattern — no current active infection, no protective surface antibody, but evidence of past exposure with a real risk of reactivation on B-cell-depleting therapy. Recommended action: preventive antiviral therapy for roughly one month before starting ocrelizumab, then continuing through the full treatment course.

Hepatitis C: One Antibody Test, Different Meanings Over Time

Hepatitis C follows a different logic than hepatitis B, and it trips people up for a specific reason: the antibody result doesn't change even after the virus is long gone.

•     HCV antibody remains positive for life once a person has been infected — whether they spontaneously cleared the virus, were successfully treated, or have ongoing chronic infection.

•     A positive HCV antibody does not confer immunity to reinfection. Patients with ongoing risk factors need periodic screening for reinfection using an HCV viral load, not repeat antibody testing.

•     If HCV antibody is positive, the next step is an HCV viral load (RNA). A negative viral load in an antibody-positive patient indicates either spontaneous clearance or prior successful treatment — not an active infection needing treatment now, but a patient who should be counseled on reinfection risk and re-screened as needed.

•     A positive viral load confirms current infection requiring further workup and treatment consideration.

Left untreated, chronic HCV follows a long natural history: roughly 20–25 years from infection to cirrhosis in a subset of patients, and 25–30 years to hepatocellular carcinoma, end-stage liver disease, or death in those who progress — underscoring why identifying chronic infection early, even in asymptomatic patients, changes the trajectory.

CLINICAL BOTTOM LINE — HEPATITIS B & C For hepatitis B, order the triple screen (HBsAg, HBsAb, HBcAb) to simultaneously assess for active infection, immunity, and reactivation risk — an isolated positive HBcAb before immunosuppressive therapy is a common, clinically important finding that changes management. For hepatitis C, remember the antibody is lifelong and does not indicate current infection or immunity; use HCV viral load to determine current infection status and to screen for reinfection in patients with ongoing risk factors.

Latent TB: TST vs. IGRA, and Why Pre-Test Probability Matters

TB has not disappeared from US practice — cases in foreign-born individuals have exceeded those in US-born individuals every year since 2001, and globally TB remains a major cause of morbidity and mortality. Age at immigration matters because it affects the likelihood of prior BCG vaccination and the duration and context of potential TB exposure, both of which shape how test results should be interpreted.

Tuberculin Skin Test (TST)

The TST involves an intradermal injection of purified protein derivative (PPD), read at 48–72 hours and measured in millimeters of induration — never simply recorded as positive or negative. The cutoff for a positive result depends on the patient's individual risk profile:

Induration Cutoff	Applies To
≥5 mm	HIV or other immunosuppression, close contacts of active TB cases, fibrotic changes on chest x-ray
≥10 mm	Other high-risk groups (e.g., recent immigrants from high-burden countries, congregate settings, certain comorbidities)
≥15 mm	Everyone else (no known risk factors)

 

TST has real limitations in both directions. False positives can result from non-TB mycobacteria exposure or prior BCG vaccination (more likely to cause persistent false positives with older age at vaccination) or simple misinterpretation. False negatives can occur with poor technique, the two-visit requirement (loss to follow-up), very recent infection, active TB disease itself, immunosuppression or anergy, very young age (under 6 months), live virus vaccination, or recent viral infection — measles being the classically described culprit.

Interferon-Gamma Release Assays (IGRAs)

IGRAs (QuantiFERON-TB Gold and T-SPOT.TB are the two commercially available options) measure interferon-gamma production after incubating whole blood with M. tuberculosis-specific antigens. Advantages over TST include requiring only one visit, more objective interpretation, and no confusion from prior BCG vaccination. Downsides include higher cost, the need for whole blood processing, variable availability, and cross-reactivity with M. marinum and M. kansasii.

Test	Sensitivity	Specificity	Negative Predictive Value
TST	90–100%	29–39%	99–100%
IGRA	80–90%	56–83%	99–100%

 

Note the specificity gap: TST has meaningfully lower specificity than IGRA, largely driven by BCG cross-reactivity, which is precisely why IGRA is often preferred in BCG-vaccinated populations. TST and IGRA can be discordant in either direction, and when they disagree, incorporating pre-test probability — not just picking whichever result you trust more — is the correct approach. The Online TST/IGRA Interpreter (tstin3d.com, or search “Online IGRA Interpreter”) is a free calculator designed for exactly this scenario, incorporating risk factors like HIV status, immunosuppression, chest x-ray findings, and age at TB exposure to estimate active TB risk from a discordant or borderline result.

Why Biologics Change the Calculus

TNF-alpha is critical to both in vitro and in vivo control of TB, which is why anti-TNF biologics carry a well-documented TB reactivation risk. Compiled data cited in the lecture found active TB rates of roughly 10.9 per 100,000 in the general population versus 1,503 per 100,000 in patients on biologics during the pre-screening era — a more than 100-fold difference. Among patients on TNF inhibitors who develop active TB, median time to onset is 3–6 months (closer to 12 months for etanercept specifically), disseminated disease occurs in roughly a third of cases (versus ~6% in the general population), and fatality is 12–18% (versus under 5% in patients not on biologics).

CASE FROM PRACTICE A patient with a positive TST, a borderline IGRA, and a history of BCG vaccination is being considered for etanercept. Using the online TST/IGRA interpreter with the patient's specific risk profile (including planned TNF-alpha inhibitor use) to estimate active TB risk, the recommended answer was: give preventive (latent TB) therapy for approximately one month before starting etanercept, then continue the full course of treatment concurrently. This reflects the standard approach of treating LTBI before or concurrent with initiating anti-TNF therapy rather than withholding the biologic indefinitely.

CLINICAL BOTTOM LINE — LATENT TB TST and IGRA measure different things and can legitimately disagree — TST's main weakness is lower specificity (BCG cross-reactivity), while IGRA trades some sensitivity for improved specificity and single-visit convenience. Interpret either test using the patient's pre-test probability, not in isolation. Before starting any anti-TNF biologic, screen for LTBI and treat it — reactivation risk on these agents is dramatically elevated and carries higher rates of disseminated disease and fatality than TB in the general population.

Putting It All Together

The thread connecting all four serologies is the same: a single test result rarely stands on its own. It has to be read against the pre-test probability the clinical picture creates — prevalence and exposure history for HIV, time since tick bite for Lyme, immunosuppression status for hepatitis B, and TB exposure risk for TST/IGRA interpretation. Ordering the right test, at the right time, for the right stage of disease is most of the battle; the rest is resisting the urge to treat a lab value as a verdict rather than one more piece of the clinical picture.

 

Reference

Solomon, D.A. (2026). Making sense of serologies: Ordering and interpreting HIV, hepatitis, Lyme, and TB tests in primary care. Pri-Med Institute CE activity.

 

Tick Bite Workup: Geography Drives the Differential, and Timing Determines the Test

Don’t order Lyme titers the day after a tick bite. They’ll be negative. And don’t order them at all if the patient was bitten in Oklahoma.

Tick-borne illness is one of primary care's most geography-dependent diagnoses. The tick species, the pathogen it carries, the region where the bite occurred, and the timing of your lab work all determine whether you'll catch the diagnosis or miss it entirely. This post maps out which diseases to suspect based on location, when to test, what tests to order, and—critically—when to treat empirically before labs come back.

Step One: Identify the Tick and the Geography

Tick Species	Common Name	Primary Geography	Diseases Transmitted
Ixodes scapularis	Blacklegged tick / Deer tick	Northeast, upper Midwest, mid-Atlantic (CT, MA, NY, NJ, PA, MN, WI)	Lyme disease, Anaplasmosis, Babesiosis, Powassan virus, B. miyamotoi relapsing fever
Ixodes pacificus	Western blacklegged tick	Pacific Coast (CA, OR, WA)	Lyme disease (lower incidence than East Coast), Anaplasmosis
Amblyomma americanum	Lone Star tick	Southeast, south-central US (TX, OK, AR, MO, TN, NC, VA, FL)	Ehrlichiosis, STARI (Southern Tick-Associated Rash Illness), Heartland virus, Bourbon virus, Alpha-gal syndrome
Dermacentor variabilis	American Dog tick	East of Rocky Mountains, Pacific Coast	Rocky Mountain Spotted Fever (RMSF), Tularemia
Dermacentor andersoni	Rocky Mountain Wood tick	Rocky Mountain states (MT, WY, CO, ID)	RMSF, Colorado Tick Fever, Tularemia
Amblyomma maculatum	Gulf Coast tick	Gulf Coast states, expanding northward	Rickettsia parkeri rickettsiosis (milder cousin of RMSF, often with eschar at bite site)

Geography Is Your Differential

A febrile patient with a tick bite in Connecticut = think Lyme, Anaplasmosis, Babesiosis. A febrile patient with a tick bite in North Carolina = think RMSF, Ehrlichiosis, STARI. A febrile patient with a tick bite in Oklahoma or Arkansas = RMSF and Ehrlichiosis dominate; Lyme disease is exceedingly rare. Always ask where the bite occurred, not just where the patient lives. Travel history changes everything.

The Major Tick-Borne Diseases: Lab Workup and Timing

Lyme Disease (Borrelia burgdorferi)

Stage	Timing	Clinical Presentation	Lab Approach
Early Localized	3–30 days post-bite	Erythema migrans (EM) — expanding bull's-eye rash ≥5 cm. May have flu-like symptoms.	DO NOT order serology. Diagnose CLINICALLY. EM is pathognomonic. Serology is negative in up to 60% of early localized disease. Treat empirically: doxycycline 100 mg BID × 10–21 days.
Early Disseminated	Weeks to months	Multiple EM lesions, facial palsy (CN VII), carditis (heart block), meningitis, radiculopathy	Two-tier testing: EIA/ELISA (screening) → if positive/equivocal, confirmatory Western blot (IgM if <30 days of symptoms, IgM + IgG if >30 days). OR the newer modified two-tier (MTTT): EIA #1 → EIA #2 (no Western blot). Serology is ~70–80% sensitive at this stage.
Late Disseminated	Months to years	Lyme arthritis (large joint, especially knee), late neurologic Lyme (encephalopathy, polyneuropathy)	IgG serology is almost always positive at this stage (>95% sensitivity). IgM alone is NOT sufficient for late disease—IgM positivity without IgG in late Lyme is likely a false positive.

The Timing Trap

Lyme serology takes 2–6 weeks to become positive after infection. Ordering Lyme titers the day after a tick bite—or even the day the EM rash appears—will be negative and gives false reassurance. In early localized Lyme (EM rash), the diagnosis is clinical and the treatment is empiric. Serology is for disseminated disease when the clinical picture is compatible but the EM rash was missed or absent.

Prophylaxis After Tick Bite (The 72-Hour Window)

Single-dose doxycycline 200 mg PO can be offered for Lyme prophylaxis if ALL of the following are met:

• The tick is identified as Ixodes scapularis (deer tick)
• The bite occurred in a Lyme-endemic area
• The tick was attached for ≥36 hours (engorged appearance)
• Prophylaxis can be given within 72 hours of tick removal
• No contraindication to doxycycline (pregnancy, age <8—though recent evidence supports doxycycline in shorter courses for children)

If any criterion is not met, observe without prophylaxis. Monitor for EM rash for 30 days.

Rocky Mountain Spotted Fever (Rickettsia rickettsii)

Treat First, Test Later

RMSF has a case fatality rate of 20–25% if untreated and can kill within days. Do NOT wait for lab confirmation to treat. If the clinical picture is compatible (fever + headache + myalgia ± rash in a tick-exposed patient in an endemic area), start doxycycline immediately—adults AND children of any age. The rash (petechial, starting on wrists/ankles, spreading centrally) may not appear until day 3–5 or may be absent in 10–15% of cases. Delaying treatment until the rash appears or until labs confirm increases mortality dramatically.

• Diagnosis: IFA (Indirect Fluorescent Antibody) for R. rickettsii IgG—paired sera (acute + convalescent 2–4 weeks later). A 4-fold rise confirms the diagnosis. The acute sample is almost always negative. Diagnosis is typically retrospective.
• CBC pattern: Thrombocytopenia, leukopenia (or normal WBC), elevated LDH, hyponatremia, elevated transaminases
• Geography: Despite the name, most cases occur in the southeast and south-central US (NC, TN, OK, AR, MO), not the Rocky Mountains. Also seen in tribal lands of the Southwest (Arizona).
• Treatment: Doxycycline 100 mg BID (adults) or 2.2 mg/kg BID (children) for 5–7 days or until afebrile ≥3 days.

Anaplasmosis (Anaplasma phagocytophilum)

• Vector: Ixodes scapularis (same tick as Lyme—co-infection is common)
• Geography: Northeast, upper Midwest (overlaps Lyme belt)
• Presentation: Fever, headache, myalgia, malaise. NO rash (or rash is rare). 1–2 weeks after bite.
• Lab pattern: Leukopenia + thrombocytopenia + elevated transaminases—this triad in a febrile patient from an endemic area is Anaplasmosis until proven otherwise.
• Diagnosis: Peripheral blood smear showing morulae (intracytoplasmic inclusions in neutrophils)—seen in 20–80% of cases. PCR (most sensitive in first week). IFA serology (paired sera; same timing issues as RMSF).
• Treatment: Doxycycline. Empiric treatment for the leukopenia/thrombocytopenia/transaminitis triad while awaiting confirmation.

Ehrlichiosis (Ehrlichia chaffeensis)

• Vector: Amblyomma americanum (Lone Star tick)
• Geography: Southeast, south-central US
• Presentation: Clinically identical to Anaplasmosis. Fever, headache, myalgia. Rash more common than Anaplasmosis (~30%) but still not reliable.
• Lab pattern: Same triad: leukopenia + thrombocytopenia + elevated transaminases
• Diagnosis: Morulae in monocytes (not neutrophils—that's the distinction from Anaplasmosis). PCR, IFA serology.
• Treatment: Doxycycline.

Babesiosis (Babesia microti)

• Vector: Ixodes scapularis (co-infection with Lyme and Anaplasmosis possible)
• Geography: Northeast and upper Midwest (concentrated in southern New England and NY)
• Presentation: Fever, fatigue, hemolytic anemia. Can be severe in asplenic, immunocompromised, or elderly patients.
• Lab pattern: Hemolytic anemia (elevated LDH, low haptoglobin, elevated indirect bilirubin, reticulocytosis) + thrombocytopenia
• Diagnosis: Thin blood smear showing intraerythrocytic ring forms (look like malaria—pathology must be alerted to consider Babesia). PCR. IFA serology.
• Treatment: Atovaquone + azithromycin (mild/moderate). Clindamycin + quinine (severe). NOT doxycycline—Babesia is a parasite, not a bacterium.

The Co-Infection Problem

Same Tick, Multiple Pathogens

Ixodes scapularis can carry Lyme, Anaplasmosis, and Babesiosis simultaneously. A single tick bite can transmit more than one pathogen. Suspect co-infection when a Lyme patient doesn't respond to doxycycline as expected, has persistent high fevers (uncommon in Lyme alone), has hemolytic anemia (Babesiosis), or has the leukopenia/thrombocytopenia/transaminitis triad (Anaplasmosis). When treating Lyme in the northeast, have a low threshold to check CBC, smear, and liver enzymes for co-infection.

Alpha-Gal Syndrome: The Lone Star Tick Surprise

The Lone Star tick (Amblyomma americanum) can sensitize patients to alpha-gal, a carbohydrate found in mammalian meat (beef, pork, lamb). This causes a delayed IgE-mediated allergic reaction 3–6 hours after eating red meat.

• Presentation: Urticaria, angioedema, GI symptoms, or anaphylaxis occurring 3–6 hours after eating mammalian meat. The delay makes the connection non-obvious.
• Diagnosis: Alpha-gal IgE (specific IgE to galactose-alpha-1,3-galactose). Total tryptase if anaphylaxis suspected.
• Geography: Southeast US (Lone Star territory), expanding northward
• Management: Avoidance of mammalian meat and products (some patients also react to dairy, gelatin, or medications in gelatin capsules). EpiPen prescription for those with history of anaphylaxis.

STARI: The Lyme Look-Alike of the South

Southern Tick-Associated Rash Illness (STARI) presents with an EM-like expanding rash after a Lone Star tick bite in the southeastern US. It is NOT Lyme disease—B. burgdorferi is not transmitted by Lone Star ticks. The causative agent remains unidentified. STARI is milder than Lyme, does not cause the disseminated complications (carditis, arthritis, neurologic disease), and is generally treated empirically with doxycycline. The clinical pearl: an EM-like rash in Georgia or Texas is STARI, not Lyme. Don't order Lyme serology in non-endemic areas.

Timing Summary: When to Order What

Scenario	Lab Approach	Timing
Asymptomatic tick bite, tick just removed	No labs. Observe for 30 days. Prophylaxis if criteria met (see above).	N/A
EM rash present	No serology needed. Diagnose clinically. Treat with doxycycline.	Day of diagnosis
Fever + tick exposure, endemic for RMSF/Ehrlichiosis	CBC, CMP, start doxycycline empirically. Draw acute serology (IFA) but don't wait for results. Convalescent titers in 2–4 weeks.	Treat immediately. Labs are retrospective.
Suspected disseminated Lyme (facial palsy, carditis, arthritis)	Two-tier serology (EIA/ELISA → Western blot or MTTT). Sensitivity ~70–95% depending on stage.	≥2–4 weeks after infection for IgM; ≥4–6 weeks for IgG
Suspected Anaplasmosis/Ehrlichiosis	CBC (leukopenia + thrombocytopenia + elevated LFTs), blood smear for morulae, PCR (first week), IFA (paired sera)	PCR best in first week of illness. Serology requires convalescent sample.
Suspected Babesiosis	Thin blood smear, PCR, CBC (hemolytic anemia pattern), IFA serology	Smear and PCR at presentation. Serology takes weeks.
Suspected alpha-gal	Alpha-gal specific IgE	Anytime after sensitization (weeks to months post-bite)

The Pitfalls

• Ordering Lyme serology too early: The #1 error. Serology is negative in the first 1–3 weeks. EM rash is a clinical diagnosis. A negative titer at day 5 rules out nothing.
• Ordering Lyme serology in non-endemic areas: Lyme is concentrated in the Northeast and upper Midwest. A positive Lyme titer in a patient bitten in Alabama is almost certainly a false positive. Geography first, labs second.
• Waiting for labs to treat RMSF: This kills people. Doxycycline is the treatment for all suspected rickettsial disease, and it should be started on clinical suspicion alone. The labs confirm the diagnosis after the fact.
• Forgetting co-infection: A Lyme patient in Connecticut who isn't improving on doxycycline may have concurrent Babesiosis (which doesn't respond to doxycycline). Check the smear.
• IgM-only positivity in late Lyme: If symptoms have been present for >30 days and IgG is negative but IgM is positive, this is most likely a false positive IgM, not late Lyme disease.
• Checking “Lyme titers” as a screening test for chronic fatigue: Lyme serology is not a screening tool for nonspecific symptoms. It has a 5% false positive rate. In low-probability patients, a positive result is more likely false than true (Bayesian reasoning).
• Doxycycline in children: The AAP and CDC now endorse doxycycline for children of ALL ages for tick-borne illness. Short courses (≤21 days) do not cause dental staining. Do not withhold doxycycline from a child with suspected RMSF.
• Missing the Lone Star tick expanding range: Amblyomma americanum is moving northward due to climate change. Ehrlichiosis and alpha-gal are appearing in areas that were previously non-endemic. Stay current on your local tick maps.

Bottom Line

Tick-borne illness workup starts with two questions: where was the patient bitten, and when? Geography determines the differential. Timing determines the test. EM rash = treat without labs. RMSF suspicion = treat before labs. Lyme serology is useless in the first 2 weeks and useless outside endemic areas. The leukopenia/thrombocytopenia/transaminitis triad points to Anaplasmosis or Ehrlichiosis. Hemolytic anemia after an Ixodes bite = Babesiosis. And doxycycline is safe for children. Know your ticks, know your geography, and when in doubt, give the doxycycline.

Stay sharp out there.