Does Beyfortus Cause Positive RSV PCR?
No, Beyfortus (nirsevimab) does not interfere with RT-PCR-based RSV diagnostic assays and will not cause a false positive RSV PCR result. 1
Mechanism and Diagnostic Testing
Nirsevimab is a monoclonal antibody that targets the prefusion conformation of the RSV fusion (F) protein at the highly conserved site Ø. 2 The FDA drug label explicitly states that nirsevimab does not interfere with reverse transcriptase polymerase chain reaction (RT-PCR) or rapid antigen detection RSV diagnostic assays that employ commercially available antibodies targeting antigenic site I, II, or IV on the RSV fusion protein. 1
Clinical Implications for Testing
RT-PCR testing remains accurate in infants who have received nirsevimab, as the antibody does not interfere with nucleic acid amplification methods used in PCR assays. 1
If immunological assay results are negative when clinical observations are consistent with RSV infection, the FDA recommends confirming the diagnosis using an RT-PCR-based assay. 1 This guidance applies to all patients, not specifically those who received nirsevimab.
The presence of nirsevimab in the bloodstream (which can persist for approximately 150 days after administration) does not affect the ability to detect RSV genetic material via PCR. 3, 4
Important Caveats
Nirsevimab provides protection, not immunity: While nirsevimab reduces the risk of medically attended RSV lower respiratory tract infection by 74.5% 3 and hospitalization by 83.2% 5, breakthrough infections can still occur, particularly near the end of the 150-day protection window. 4 In these cases, PCR testing will accurately detect RSV infection.
The antibody targets the virus, not the diagnostic test: Nirsevimab works by binding to the RSV F protein and preventing viral entry into cells, but this mechanism does not interfere with PCR detection of viral RNA in respiratory specimens. 2