Investigation of Suspected False-Positive Results for Bordetella Pertussis

Background:  Three clinics in two areas of Texas reported an unusual increase of positive PCR results for Bordetella pertussis in 2010. All clinics used the same laboratory (Lab A) for testing.  In other locales the collection methods and transport media used by this laboratory had been associated with false positives due to environmental contamination in the clinical setting. Essentially, environmental pertussis DNA contaminates the specimen collection materials and then is dispersed when exposed to liquid media during transport. Lab A also has a higher positivity rate than other labs that perform pertussis PCR testing for Texas patients.

Objectives: To assess whether Lab A’s pertussis results were concordant with results from a laboratory using dry swab and no media.

Methods:  Two clinics collected dual nasopharyngeal swabs on patients with symptoms suggestive of pertussis. Parallel testing was performed by Lab A and the Texas Department of State Health Services (DSHS) Laboratory with each lab using their standard protocols for collection and transport.

Results:  Of 64 samples, Lab A reported 16 (25.0%) positives, 28 (43.8%) negatives, 1 (1.6%) positive for parapertussis, and 19 (29.7%) equivocal results. The DSHS lab reported 63 (98.4%) negatives and 1 (1.6%) positive for parapertussis. The overall discrepancy rate was 54.7%.

Conclusions:  The discrepant results between the DSHS lab and Lab A, a high rate of positivity from Lab A when compared with pertussis PCR testing in other Texas laboratories, and the previous history of environmental contamination indicate that collection and transport issues should be investigated. A planned study of collection protocols and environmental testing for contaminants will address whether clinical contamination exacerbated by collection and transport methods is a factor in the discrepant results and the high proportion of positive and equivocal results.