In a patient with a 10‑day tick exposure, a three‑day course of doxycycline, and a history of type 1 diabetes mellitus, who now presents two weeks later with an itchy rash on the legs and chest and decreased energy, what laboratory tests should be ordered for tick‑borne illnesses?

Laboratory Testing for Suspected Tick-Borne Illness

Draw acute serology (IgG IFA) for Lyme disease, anaplasmosis, and ehrlichiosis NOW, and order whole blood PCR for Anaplasma and Ehrlichia species, along with a complete blood count with differential to look for thrombocytopenia, leukopenia, and morulae in white blood cells. 1

Rationale for Testing Strategy

Your patient's presentation—rash developing 2 weeks after tick exposure despite initial doxycycline—raises concern for multiple tick-borne pathogens. The 3-day doxycycline course was inadequate for treatment (though appropriate for prophylaxis if given within 72 hours of tick removal) 2. The rash, fatigue, and timing suggest possible:

• Lyme disease (erythema migrans can appear 3-30 days post-bite)
• Anaplasmosis or ehrlichiosis (typically present within 1-2 weeks)
• Southern tick-associated rash illness (STARI) if in appropriate geographic area 3

Specific Laboratory Tests to Order

Immediate Testing (Acute Phase)

Serologic Testing:

• IgG IFA for Lyme disease (Borrelia burgdorferi)
• IgG IFA for Anaplasma phagocytophilum
• IgG IFA for Ehrlichia chaffeensis

Molecular Testing:

• Whole blood PCR for Anaplasma and Ehrlichia species (3-5 mL EDTA-anticoagulated blood) 1
  • This is most sensitive during the first week of illness and before or within 48 hours of starting doxycycline 1
  • Since you restarted doxycycline today, draw this IMMEDIATELY

Hematologic Testing:

• Complete blood count with differential looking for:
  • Thrombocytopenia (low platelets)
  • Leukopenia (low white blood cells)
  • Morulae in monocytes (ehrlichiosis) or granulocytes (anaplasmosis) 1

Hepatic Testing:

• Liver function tests (AST, ALT) - often elevated in anaplasmosis/ehrlichiosis 4

Convalescent Testing (2-4 Weeks Later)

Repeat IgG IFA serology for all three pathogens 1

• A fourfold or greater increase in antibody titer between acute and convalescent samples confirms acute infection 1
• Single elevated titers are never sufficient to confirm acute infection 1

Critical Timing Issues

Why PCR Must Be Drawn NOW

The PCR sensitivity diminishes rapidly after doxycycline initiation and within 1 week of collection 1. Since you already restarted doxycycline, you have a narrow window—ideally the blood should have been drawn before the first dose today.

Why Acute Serology May Be Negative

IFA assays are insensitive during the first week of rickettsial infection 1. Most patients seek care when antibodies haven't yet developed. This is why:

• You need paired acute and convalescent sera tested together 1
• A negative acute serology does NOT rule out tick-borne disease 1
• Treatment should never be delayed waiting for results 1

Common Pitfalls to Avoid

1. Don't rely on acute serology alone: The first week antibody response is often absent. Negative acute serology means nothing 1

2. Don't skip the convalescent sample: Without the 2-4 week follow-up serology, you cannot confirm the diagnosis retrospectively 1

3. Don't delay PCR: Every hour after doxycycline initiation reduces PCR sensitivity 1

4. Don't forget the blood smear: An experienced microscopist may identify morulae, providing rapid presumptive diagnosis, though sensitivity is low 1

5. Consider co-infection: Ixodes ticks can transmit multiple pathogens simultaneously (Lyme, anaplasmosis, babesiosis) 4

Geographic and Clinical Context

If this patient is in the northeastern or upper midwestern United States, Ixodes scapularis ticks transmit Lyme disease, anaplasmosis, and babesiosis. If in the southeastern United States, consider Amblyomma americanum (lone star tick) causing STARI, which mimics Lyme disease but tests negative for Borrelia burgdorferi 3.

The 10-day tick attachment is concerning—longer attachment increases transmission risk, particularly for Lyme disease where transmission typically requires 36-48 hours of attachment.

Treatment Considerations

You correctly restarted doxycycline 100 mg BID for 10 days 4, 1. This covers:

• Lyme disease (though 14-21 days is more standard for early disseminated disease)
• Anaplasmosis
• Ehrlichiosis

Monitor for clinical improvement within 24-48 hours 4. If no improvement, consider:

• Babesiosis co-infection (requires different treatment: atovaquone plus azithromycin)
• Alternative diagnosis
• Viral tick-borne illness (Powassan, Heartland virus—doxycycline ineffective) 5

The patient's type 1 diabetes doesn't alter testing strategy but may increase infection severity risk, warranting closer monitoring.