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Researchers Share Early Clinical Data on Performance of Quidel MDx Assays

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Researchers last week provided a glimpse of the clinical performance of a trio of extraction-free molecular assays currently being developed by clinical diagnostics firm Quidel.

Specifically, in a company-sponsored corporate workshop session held in conjunction with the Association for Molecular Pathology meeting in Phoenix, Ariz., the independent investigators presented data from clinical assessments of the Quidel Molecular Group A Strep, AmpliVue Group B/C/G Strep, and Quidel Molecular HSV 1&2/VZV assays.

Quidel currently has two flavors of extraction-free molecular products: Quidel Molecular, its portfolio of real-time PCR assays that can be used on multiple real-time PCR instruments but are marketed with two Life Technologies platforms under an agreement between the firms; and AmpliVue, its collection of instrument-free assays that leverage helicase-dependent amplification technology that Quidel acquired along with BioHelix.

In the first corporate workshop presentation, Blake Buchan, assistant professor of pathology and associate director of clinical microbiology at the Medical College of Wisconsin, discussed his group's involvement in clinical trials for the Quidel Molecular Group A Strep test, an instrument-agnostic real-time PCR assay to identify Group A Streptococcus and distinguish it from both Group G and Group C Strep.

Buchan said that his lab is seeking molecular alternatives to the gold standard of culture, which is sensitive and specific but not rapid enough. The lab is currently using an RT-PCR/FRET-based laboratory-developed test using Roche analyte-specific reagents with the LightCycler platform; Meridian Bioscience's Illumigene GAS test, a loop-mediated isothermal amplification-based assay that has US regulatory clearance; and the Quidel Molecular GAS test running on the Life Technologies ABI 7500 Fast Dx system, which is still research use only.

Buchan said that in a clinical assessment of more than 400 patient throat swabs, the Quidel test achieved 97.6 percent sensitivity and 99.4 percent specificity, comparing favorably with published sensitivities and specificities of the Roche ASR and Illumigene assays. All assays performed better than culture, he noted.

However, Buchan also said that the Quidel assay combines some of the desirable features of the other two molecular assays in that it is able to distinguish between the different Strep groups and takes about 75 minutes to perform. The Illumigene test takes about the same amount of time, but doesn't distinguish between the groups; while the Roche ASR test can distinguish Group A from Groups G and C, but takes at least 90 minutes to perform and is not direct from sample, i.e., it involves additional sample prep steps.

This ability to distinguish between the different Strep groups is important, Buchan noted, because Groups G and C are about 40 to 50 percent as common as Group A in patients with acute pharyngitis, and may require different antibiotic regimens.

The Quidel Molecular GAS assay is currently under review by the US Food and Drug Administration.

Meantime, Timothy Uphoff, molecular pathology section head for Marshfield Laboratories, a division of Wisconsin's Marshfield Clinic, presented early data from his lab's clinical assessment of Quidel's AmpliVue Group B Strep test.

Uphoff said that the lab has been using Quidel's AmpliVue C. diff assay "with good results" thus far, noting the attractiveness of the assay's simplicity — it only requires incubation in heating blocks at two different temperatures for about an hour, and the readout is performed visually on a lateral flow strip.

Uphoff's lab compared the AmpliVue C. diff assay to culture in more than 900 patient specimens, noting that the molecular assay achieved a sensitivity of 99.4 percent and specificity of 92.8 percent. Discordant samples that were actually positive but that the Quidel test indicated were negative were further tested using a BD Max GBS test, bumping specificity up to about 97 percent.

Quidel's test has a much faster turnaround time, is easier to implement, and demonstrated a better detection rate than culture, identifying 26 percent more positive results, Uphoff noted. Should the assay obtain FDA approval, it will likely fall into the "moderately complex" category.

Currently, the only FDA-approved Quidel AmpliVue assay is AmpliVue C. diff.

Finally, Alexandra Valsamakis, director of clinical virology and molecular microbiology at the Johns Hopkins Hospital, presented data from an assessment of the Quidel Molecular HSV 1&2/VZV assay, which is designed to detect and differentiate herpes simplex virus type 1 and type 2, and varicella-zoster virus.

Valsamakis noted that nucleic acid testing is quickly becoming the gold standard to test for such infectious agents because of its superior speed and sensitivity. She also noted that a triplex test for HSV 1, HSV 2, and VZV is attractive because the agents often get mistaken for one another in clinical diagnoses.

In their trial, Valsamakis and colleagues compared the Quidel Molecular assay running on three different real-time PCR platforms — Life Tech's QuantStudio Dx and ABI 7500 Fast Dx, and Cepheid's SmartCycler II — to conventional culture-based methods.

She noted that the assay hands-on time was about 15 minutes, and that the time to result was about 60 minutes total. The sensitivity and specificity of the Quidel molecular test exceeded 96 percent on all three instrument platforms, and achieved sensitivity and specificity of up to 100 percent on the QuantStudio Dx platform.

Further, Valsamakis noted, the Quidel Molecular assay picked up anywhere between 10 and 30 positive samples that were missed by culture.

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