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Baloxavir marboxil Influenza A Antiviral Resistance PCR

Test Code: 2700
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Clinical Utility

Resistance can be a clinically significant problem with baloxavir marboxil (trade name: XOFLUZA™), with reported incidences of 9-20% for H3N2, 3-5% for H1 and 1-3% for flu B viruses. However, mutations isolated only by influenza A (H3 and H1 subtypes) confer resistance levels >10-fold, making this a key factor to target with an assay. From Hayden et al. (N Engl J Med 2018; 379:913-923), “emergence of polymerase acidic protein variants with I38T/M/F substitutions conferring reduced susceptibility to baloxavir occurred in 2.2% and 9.7% of baloxavir recipients in the phase 2 trial and phase 3 trial, respectively.”


Extraction of influenza A RNA from nasopharyngeal swab specimens followed by amplification and detection of resistance determinants at the 38th codon (isoleucine) of the pol gene using real-time, qualitative PCR. Nucleotide changes at the 38th codon resulting in amino acid substitutions to methionine, phenylalanine and threonine result in highly elevated resistance and are detected by this assay. An internal control is added to ensure the extraction was performed correctly and the PCR reaction was not inhibited. This test has not been cleared or approved for diagnostic use by the U.S. Food and Drug Administration.

Same day (within 8-12 hours from receipt of specimen), Monday through Saturday.

Specimen Type Order Code CPT Code NY Approved Volume Assay Range Special Instructions
NP swab 2725 87502 / 87503x2 No

2 mL (min 1 mL)


• Place sterile swab placed in 2 mL sterile saline, M4, or viral transport media in a sterile, screw top tube.
• Do not use calcium alginate swab or wood shafted swab.
• Can be shipped at ambient or frozen temperature Monday through Friday.
• Specimens shipped at ambient temperature must be received within 6 days of collection.


Ship Monday through Friday. Friday shipments must be labeled for Saturday delivery. All specimens must be labeled with patient's name and collection date. A Viracor Eurofins test requisition form must accompany each specimen. Multiple tests can be run on one specimen. Ship specimens FedEx Priority Overnight® to: Viracor Eurofins, 1001 NW Technology Dr, Lee's Summit, MO 64086

Causes for Rejection

Specimen received outside stability, insufficient specimen volume, or specimen types other than those listed.

Specimens are approved for testing in New York only when indicated in the Specimen Information field above.

The CPT codes provided are based on Viracor Eurofins' interpretation of the American Medical Association's Current Procedural Terminology (CPT) codes and are provided for informational purposes only. CPT coding is the sole responsibility of the billing party. Questions regarding coding should be addressed to your local Medicare carrier. Viracor Eurofins assumes no responsibility for billing errors due to reliance on the CPT codes illustrated in this material.


Center for Disease Control and Prevention. (2018, Dec 3). Influenza (Flu). Retrieved from:

The New England Journal of Medicine, "Baloxavir Marboxil for Uncomplicated Influenza in Adults and Adolescents" September 6, 2018 Vol. 379, No. 10

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