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Influenza Update: Diagnostic Test, Drug Resistance, and Possible Drug Interaction

By Gigi Kwik Gronvall, Ph.D. and Luciana L. Borio, M.D., November 16, 2006

Diagnostic Test for Avian Influenza

A test is being developed that could revolutionize the way influenza is diagnosed. Researchers at the University of Colorado, the Centers for Disease Control and Prevention, and InDevR, LLC have designed a gene chip assay that can identify and subtype Influenza A virus, distinguishing, for example, between H3N2 and H5N1 in less than 7 hours. And advances in ‘lab-on-a-chip’ technology may reduce this time even further to less than one hour. Compared with current technologies, which can take days or weeks, this test could be very useful for determining whether a patient is infected with influenza, and even more useful for H5N1 surveillance. The cost to produce the test, which is called “MChip,” is less than  $10. With 95% sensitivity, this test is also considerably more sensitive than other rapid influenza tests, which typically have 70% to 75% sensitivity.

The MChip test was described in the November 15 issue of Analytical Chemistry [1]. The technology rests upon the slow-evolving Matrix gene, or “M gene,” which is present in all influenza A viruses. Each test has 15 probe sequences from the M gene embedded on a chip; viral RNA from patient samples is tested for hybridization to the chip. A fluorescent readout is produced based on the ability of a sample to bind to a specific M gene sequence and the relative intensity of binding. Once the readout is analyzed, the influenza A subtype can be identified. New data that will be published in the December 15 issue of Analytical Chemistry show that the M Chip correctly identified 21 out of 24 different H5N1 flu strains from humans, birds, and cats, and did not produce false positive results [2, 3].

Multidrug-Resistant Influenza A/H3N2 Virus in an Immunocompromised Child

In a study published online in the journal Clinical Infectious Diseases, Baz and colleagues characterize the molecular evolution of a multidrug-resistant influenza A/H3N2 virus that was shed over the course of one year by an immunocompromised child [4]. During that time, a total of 17 influenza A/H3N2 isolates were recovered by cell culture from the child, who had influenza pneumonitis. The child was treated sequentially with oseltamivir, then amantadine with zanamivir.

Drug susceptibility was determined by neuraminidase (NA) inhibition assays and the sequencing of key viral genes. An influenza A variant with NA mutations E59G, E119V, and I122V that conferred resistance to oseltamivir while preserving sensitivity to zanamivir and the investigational drug peramivir was first detected after 38 days of oseltamivir treatment. The E119V mutation persisted for 8 months after cessation of oseltamivir therapy, indicating that the mutation is stable in the absence of drug pressure in the immunocompromised host. Amantadine therapy also led to rapid emergence of resistance to that drug; resistant isolate was detected after only 5 days of amantadine therapy and persisted for more than 100 days after drug cessation.

Development of drug-resistant variants may arise and persist in immunocompromised children, even after cessation of treatment, resulting in treatment failure, chronic infection, and increased risk of transmission of resistant variant to others.

Is there a Possible Drug Interaction between Oseltamivir (Tamiflu®) and Clopidogrel (Plavix®)?

A study from the University of Rhode Island’s College of Pharmacy indicates that the anti-clotting drug clopidogrel may render oseltamivir ineffective; clopidogrel is used for long-term prevention of atherosclerotic events [5]. Hydrolysis is required for the activation of oseltamivir’s antiviral activity. It is thought that clopidogrel inhibits the hydrolysis of oseltamivir’s prodrug, as both drugs share the same hydrolytic liver enzyme, HCE1. In vitro, the hydrolysis of oseltamivir was inhibited as much as 90% in the presence of clopidogrel when both drugs were present at equal concentrations. Human pharmacokinetic trials are now necessary to assess the clinical significance of the observation, as it is not known whether the drug interaction observed in vitro would occur in vivo. The FDA has not issued any safety alerts to the public or to clinicians.


  1. Dawson ED, Moore CL, Smagala JA, et al. M Chip: a tool for influenza surveillance. Analytical Chemistry 2006;78:7610-7615. Accessed November 14, 2006 at

  2. Dawson EG, Moore CL, Dankbar DM, et al. Identification of A/H5N1 influenza viruses using a single gene diagnostic microarray. Analytical Chemistry 2006. DOI: 10.1021/ac061920o. Accessed November 14, 2006 at

  3. Fox Maggie. M protein gene chip test rapidly and cheaply identifies bird flu strains. Reuters. November 13, 2006.

  4. Baz M, Abed Y, McDonald J, Boivin G. Characterization of multidrug-resistant influenza A/H3N2 viruses shed during 1 year by an immunocompromised child. Clinical Infectious Diseases 2006;43. DOI: 10.1086/508777. Accessed November 14, 2006 at

  5. Shi D, Yang J, Yang D, et al. Anti-influenza prodrug oseltamivir is activated by carboxylesterase human carboxylesterase 1, and the activation is inhibited by antiplatelet agent clopidogrel. Journal of Pharmacology and Experimental Therapeutics 2006. DOI: 10.1124/jpet.106.111807. Accessed November 14, 2006 at