What does the CDC Lyme serologic test detect, and how does the Igenex PCR test differ?

CDC Lyme Testing vs. Igenex: What They Detect

The CDC-recommended two-tiered serologic test detects antibodies (IgM and IgG) against Borrelia burgdorferi antigens, while Igenex offers both serologic testing with modified interpretation criteria and PCR-based molecular testing that directly detects bacterial DNA rather than antibodies.

CDC Two-Tiered Serologic Testing

The standard CDC approach uses a two-step antibody detection system 1:

First Tier

• EIA (ELISA) or IFA measures total antibody response (IgM and IgG) to B. burgdorferi antigens
• Most commonly uses whole-cell sonicate preparation containing multiple bacterial antigens
• Highly sensitive but less specific due to cross-reactive antigens
• Newer FDA-cleared tests use specific antigens like VlsE or C6 peptide for improved specificity 1

Second Tier (if first tier positive/equivocal)

• Western immunoblot detects antibodies against specific B. burgdorferi proteins
• Increases specificity to >98% by reducing false positives from the first tier 1
• Uses standardized interpretation criteria established at the 1994 Dearborn conference

Performance characteristics: 70-100% sensitivity and >95% specificity for disseminated Lyme disease 1. This is the standard of care endorsed by IDSA/AAN/ACR guidelines 2.

Igenex Testing Differences

Igenex offers fundamentally different testing approaches:

Modified Serologic Criteria

• Uses less stringent interpretation criteria than CDC standards
• In one study of 402 samples: only 70 met CDC criteria for positive Western blots, while 236 met Igenex criteria 3
• This increases sensitivity but at the cost of specificity and potential false positives

PCR-Based Molecular Testing

• Directly detects B. burgdorferi DNA rather than antibodies
• Can be performed on blood, urine, or other samples
• The Lyme Multiplex PCR-dot blot assay amplifies p66 and OspA gene fragments 3

Critical distinction: In one study, 23 patients (21.5%) who were antibody-negative by CDC criteria tested positive by Igenex PCR 3. However, this PCR approach is not FDA-cleared or CDC-recommended for routine diagnosis.

Key Clinical Pitfalls

Why Antibody Tests Can Miss Cases

• Early infection: Antibodies take time to develop; sensitivity is only 40-42% in acute early Lyme with erythema migrans 1
• Convalescent phase testing improves sensitivity to 57-61% 1
• Direct detection by PCR or culture is challenging because spirochetes only transiently enter bloodstream in small numbers 1

Problems with Non-Standard Testing

• Lack of standardization: PCR for B. burgdorferi has not been standardized for routine diagnostic use 4
• False positivity risk: Even in healthy populations from non-endemic regions, there is background false positivity 4
• Not guideline-recommended: IDSA/AAN/ACR guidelines recommend serum antibody testing over PCR or culture for Lyme arthritis assessment 2

When PCR May Be Useful

• Synovial fluid/tissue in seropositive patients when more definitive information is needed for treatment decisions 2
• This is the only guideline-endorsed PCR application - not blood or urine PCR

Practical Recommendations

Only order serologic testing when pretest probability is at least 20% based on epidemiologic and clinical factors 4. Testing low-probability patients increases false positives.

For suspected early Lyme disease, recognize that negative serology does not rule out infection - clinical diagnosis of erythema migrans should prompt treatment without waiting for serology 1.

Avoid non-FDA-cleared PCR tests on blood or urine for routine diagnosis - they lack validation and are not supported by major guidelines 2, 4.