Blood Culture Assay Generates Rare False Positives

A test kit for detecting methicillin-resistant Staphylococcus aureus (MRSA) can very occasionally give false-negative results.

The assay detects the presence of MRSA and S. aureus (SA) in a Gram-positive culture bottle (GPCB) in less than an hour. Other current culture-based testing methods for GPCB require an additional 18 to 48 hours for determination of MRSA or SA.

The kit is the Xpert MRSA/SA Blood Culture Assay, manufactured by Cepheid (Sunnyvale, CA, USA). The company has issued a voluntary recall due the increased reports of false negatives for MRSA. Cepheid stated that the test does not need to be returned to the company. The Class 1 recall covers all kits made and distributed from October 21, 2008, through June 21, 2010.

The company stresses that the product can still be used and report results of MRSA positive/SA positive as such. However, when customers obtain a result of MRSA negative/SA positive, the company recommends conducting further antimicrobial susceptibility testing using the U.S. Food and Drug Administration (FDA; Rockville, MD, USA) approved methods for an accurate MRSA finding.

Because the test has the rare potential to generate false-negative MRSA results, patients with MRSA infections may receive incorrect treatment or delayed care, according to an alert sent by MedWatch, the FDA's safety information and adverse event reporting program. So far, only one adverse event was reported to the agency through its medical device reporting process.

MRSA and SA are the causative pathogen in up to 35% of severe sepsis cases, which strikes 750,000 people annually in the U.S. Cepheid has identified the emergence of novel MRSA strain types as one cause of the false negatives in an ongoing "failure investigation." The investigation has yet to determine fully the remaining causes of the erroneous results.

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