What laboratory test should I order for suspected Lyme disease?

Laboratory Testing for Lyme Disease

For patients with classic erythema migrans (EM) rash in an endemic area, do not order laboratory testing—make a clinical diagnosis and treat immediately. 1, 2

When to Order Testing vs. Clinical Diagnosis

No Testing Required:

• Erythema migrans present (expanding annular lesion >5 cm) + epidemiologic exposure to endemic area = clinical diagnosis alone 1, 2
• Approximately 70-80% of Lyme disease patients present with EM 2
• Testing during early EM is insensitive (30-40% sensitivity) and delays appropriate treatment 2

Testing IS Required:

• All suspected Lyme disease cases without erythema migrans 2
• Disseminated disease manifestations (neurologic, cardiac, arthritic) 1
• Atypical skin lesions suggestive but not diagnostic of EM 1

The Correct Test to Order

Order 2-tiered serologic testing: EIA/ELISA (or IFA) followed by reflex Western immunoblot if positive or equivocal. 2

How the 2-Tiered System Works:

First Tier:

• Enzyme immunoassay (EIA/ELISA) or immunofluorescence assay (IFA) 2
• Most labs use EIA because it's automated and provides quantitative results 2
• Measures overall IgM and IgG antibody response to Borrelia burgdorferi 2

Second Tier (Reflex Testing):

• Western immunoblot performed only if first tier is positive or equivocal 2
• Detects antibodies against specific B. burgdorferi protein antigens 2
• Increases specificity to >98%, reducing false positives from the first tier 2

Timing Matters for Test Interpretation:

For symptoms <30 days:

• Order both IgM and IgG Western immunoblots 2
• IgM appears first during early infection 2
• May need convalescent-phase testing 2-3 weeks later if initial test negative 1

For symptoms ≥30 days:

• Order IgG Western immunoblot only 2
• IgM becomes unreliable after 30 days and causes false positives

Test Performance Characteristics

Sensitivity:

• Early infection (acute EM): 30-40% (window period while antibodies develop) 2
• Disseminated disease: 70-100% 2

Specificity:

• 95% across all disease stages when 2-tiered testing used correctly 2

What NOT to Order

Avoid these tests:

• PCR or culture of blood/serum—spirochetes transiently enter bloodstream in small numbers, making direct detection impractical 2
• PCR or culture of skin lesions—antibody testing preferred even for atypical EM 1
• Single-tier testing alone—insufficient specificity 2

For Neuroborreliosis:

• Order serum antibody testing, NOT CSF PCR or culture 1
• Serum serology remains the preferred diagnostic approach even for CNS/PNS involvement 1

Critical Pitfalls to Avoid

1. Testing patients with low pretest probability (<20%): Results in more false positives than true positives—don't test without appropriate clinical suspicion and endemic exposure 3

2. Testing too early: Antibodies take time to develop. If clinical suspicion high but initial test negative, obtain convalescent sample 2-3 weeks later 1

3. Ordering tests on patients who don't need them:

   • Routine screening in developmental/behavioral/psychiatric disorders—do not test 1
   • Typical ALS, relapsing-remitting MS, Parkinson's, dementia, new-onset seizures—do not test 1
   • Nonspecific brain MRI white matter changes without other clinical support—do not test 1

4. Misinterpreting positive IgM after 30 days: IgM becomes unreliable and causes false positives in late disease—only interpret IgG after first month 2

Modified 2-Tiered Testing (Newer Alternative)

A 2-EIA algorithm (replacing Western blot with a second EIA, such as C6 peptide) is FDA-approved as of 2019 and performs equivalently or better than standard 2-tiered testing 4, 5. This offers faster results, lower cost, and slightly enhanced sensitivity in early disease 4, 5. However, the standard 2-tiered approach with Western blot remains widely used and acceptable 2.