Does a negative blood PCR exclude Borrelia infection?

A blood PCR is a direct test for Borrelia nucleic acid in the sampled blood at that moment. For an 83-year-old man, a negative result therefore weakens evidence of current detectable spirochetemia, but should not be used alone to exclude Lyme borreliosis or other Borrelia infection.

Clinical evidence

• Blood/plasma PCR for Borrelia burgdorferi has limited and variable sensitivity, generally about 30–50% (reported estimates roughly 34–62%), so a negative result has poor rule-out value.
• In suspected Lyme neuroborreliosis, reported blood-PCR sensitivity is only approximately 1–28%. The 2020 IDSA/AAN/ACR guideline characterizes blood PCR sensitivity as unacceptably low for routine diagnostic exclusion.
• Detection differs by species and timing. In Borrelia miyamotoi disease, serial sampling found a median PCR-detectable period of 4 days after illness onset; in one intensively sampled subgroup, tests were consistently negative from day 6, although occasional later positivity occurred.

Mechanistic explanation

• Spirochetemia is often low-level and transient. Organisms may rapidly localize to tissues, leaving little circulating DNA for a blood assay to detect.
• Thus, a negative result can reflect sampling after the short period of bloodstream detectability rather than absence of infection. Assay platform, blood versus plasma matrix, handling, and prior antibiotic exposure can further reduce detection.

Clinical implications

• For suspected Lyme disease, appropriately timed serologic antibody testing is generally preferred over routine blood PCR. Depending on the manifestation, PCR of a relevant specimen—such as skin for erythema migrans or synovial fluid/tissue for arthritis—may be more informative.

Bottom line

• A negative blood PCR means no Borrelia DNA was detected in that blood specimen, not that infection is excluded. Its main limitation is stage-dependent, transient spirochetemia, particularly once infection has moved beyond early systemic circulation.