Influenza A and B Antigen

Influenza virus can be detected by rapid antigen detection and conventional virus culture. The rapid flu test differentiates between types A and B. The differentiation allows a choice of therapy between the newer neuraminidase inhibitors, which are active against both strains, or use of the older, less expensive antivirals for influenza A only.

The ideal time to collect respiratory specimens is within 48 to 72 hours after onset of symptoms when the mean viral titers in nasopharyngeal specimens are highest. Because respiratory viruses replicate in the epithelium of the nasopharynx, nasopharyngeal washes or aspirates provide more reliable specimens than either throat or nasopharyngeal swabs. Sensitivity is reported to be 89-97%, with the best performance obtained from nasopharyngeal (NP) washes or swabs.

Nasopharyngeal aspiration is recommended because it is less traumatic than either swabs or washes and provides a more reliable specimen. The optimal NP wash volume is 2-3 mL. Larger volumes may decrease test sensitivity. The recommended collection procedure is as follows:

Wearing gloves, aspirate 3 mL sterile saline into a 5 mL syringe.
Cut a 6 inch length of #5 French feeding tube & attach to the syringe.
Flush the tubing with 2 mL saline.
Introduce approximately 1 inch of tubing into one nostril to the back of the nose.
Aspirate secretions into the tubing; if no secretions are obtained, then flush a small amount of saline into the nasopharnx & aspirate back into the tubing.
Remove (do not cut) the tubing from the syringe & place tubing into a sterile container, such as a urine cup.
Place container in ice & transport immediately to the laboratory.

Nasopharyngeal swabs may be submitted with or without viral transport media or in saline. Swabs with wooden shafts, cotton tips, or calcium alginate swabs may interfere with results and should not be used. Viral culture can be performed subsequently on the same sample, if requested.

Intra-nasal vaccine (FluMist) may give falsely positive results for influenza antigen A, B, or both within the first week after vaccination.

Reference value is negative.




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