Amplification Assay Effective at Detecting C difficile

Jim Kling

October 24, 2011

October 24, 2011 (Las Vegas, Nevada) — The illumigene loop-mediated isothermal amplification (LAMP, Meridian Bioscience) assay outperforms enzyme immunoassays (EIAs) in the detection of Clostridium difficile, which is a common cause of antibiotic-associated diarrhea and pseudomembranous colitis. This finding comes from a study presented here at the American Society for Clinical Pathology 2011 Annual Meeting.

Toxigenic culture remains the gold standard for diagnosing the presence of C difficile, but it has a long turnaround time. EIAs are faster but are limited by lower sensitivity and specificity.

Molecular assays have been reported to have similar sensitivity and specificity to toxigenic culture, with shorter turnaround times. The researchers conducted a study to compare LAMP with their facility's current EIA method (Premier Toxins A&B, Meridian Bioscience) to determine if LAMP could be adopted for use.

The LAMP assay uses amplification to detect the pathogenicity locus, which is universal to toxigenic C difficile strains. The team's EIA method detects toxins A and B. The study involved 95 nonformed raw stool specimens from hospitalized patients; these were tested using both methods. Disagreements between the 2 methods were resolved with a polymerase chain reaction (PCR) method to detect the tcdC regulatory gene.

In all, 82 of 95 specimens had negative results on both tests and 7 had positive results on both tests. The results of 6 were in disagreement. Four samples were positive with LAMP and negative with EIA; these tested positive on PCR. Two samples that were negative with LAMP and positive with EIA tested negative on PCR.

The EIA method had a 64% sensitivity and 98% specificity, compared with 100% sensitivity and 100% specificity for LAMP. The LAMP assay had a turnaround time similar to EIA.

"C difficile has been an ongoing problem in diagnosis. LAMP had the same turnaround time to run the test as our old assay, with much better results," Dawn Morong, MT, senior medical technologist at NorDx clinical laboratory, who presented the research, told Medscape Medical News.

The study confirms what others have found, which is that molecular assays tend to be more accurate than other approaches, according to Frederick S. Nolte, PhD, director of the molecular pathology laboratory at the Medical University of South Carolina in Charleston.

"[LAMP] is simple to do. It may not be quite as sensitive as the PCR-based assays, but it doesn't require any expensive equipment. [That might make it] more accessible to labs that may not have molecular pathology or molecular biology experience. It's a way of pushing that technology out to [settings] that aren't as sophisticated when it comes to molecular analysis," Dr. Nolte told Medscape Medical News.

Ms. Morong has disclosed no relevant financial relationships. Dr. Nolte reports receiving research funding and honoraria from BD Diagnostics, which markets a PCR-based assay for C difficile.

American Society for Clinical Pathology (ASCP) 2011 Annual Meeting: Abstract 171. Presented October 20, 2011.

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