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BACKGROUND: Shiga toxin-producing Escherichia coli (STEC) are a serious cause of gastroenteritis, with O157 being the most commonly isolated serogroup. Clinical laboratories have increasingly been adopting culture-independent methods for STEC testing, such as the Meridian Bioscience Incorporated’s Shiga toxin 1 and 2 (ST1, ST2) test ImmunoCard STAT! EHEC (ICS). The study objective was to evaluate the performance of ICS compared to other methods for E. coli O157 (O157) detection: culture using CT SMAC, culture augmented by immunomagnetic separation (IMS), and Shiga toxin genes (stx1, stx2) PCR.
METHODS: The Minnesota Department of Health (MDH) excludes children with O157 infection detected in surveillance (sporadic cases) from daycare until two consecutive stools collected at least 24 hours apart test negative for O157 or Shiga toxin. During O157 outbreak investigations, all daycare attendees and workers are tested until they have two consecutive negative results regardless of symptom history. During 2012, all stool specimens submitted for O157 testing at MDH as part of sporadic or outbreak O157 case exclusion from daycare were tested by ICS, culture, IMS, and PCR.
RESULTS: Ninety-three individuals provided 283 specimens; 17 (18%) individuals were sporadic O157 cases excluded from daycare, and 76 (82%) were part of six different O157 daycare outbreaks. When ICS was compared to a gold standard, defined as a positive culture, IMS, and/or PCR, the positive predictive value (PPV) of ICS was 97% and the negative predictive value (NPV) was 69%. PPV and NPV of ICS were also calculated at 4 increasing time intervals between symptom onset and specimen collection date: ≤7 days after symptom onset, 8-21 days after onset, 22-35 days after onset, and ≥36 days after onset. The PPV and NPV of ICS were 100% and 0% in the first time interval; 100% and 30% in the second interval; 100% and 59% in the third interval; and, 100% and 56% in the fourth interval. Sixty-six (24%) specimens were positive for stx2 by PCR, but only 29 (11%) were positive for ST2 by ICS.
CONCLUSIONS: Our results demonstrate that ICS has decreased ability to detect Shiga toxins and identify O157 infections compared to other standard methods, and remained positive a shorter time compared to other methods. Additionally, the ICS was able to detect ST2 in less than half of specimens that were positive for stx2 by PCR. When used for public health follow-up, this may lead to premature removal of exclusion orders for children attending daycare.