From June to August or September of 2001 and 2002, we prospectively evaluated the ProSpecT assay for detection of Shiga toxins 1 and 2 in stool specimens. The ProSpecT assay was a highly sensitive and specific test for E. coli
O157 in stool (seven of seven without false positives; 100%) compared to SMAC. The positive predictive value of the ProSpecT assay for the presence of O157 and non-O157 STEC serotypes combined was 93% (27 of 29 specimens). Performance of ProSpecT assay for the detection of O157 and non-O157 STEC, in our population, was similar to that observed in an evaluation by Kehl et al. (sensitivity and specificity, 99%) and comparable to studies of the Premier EHEC assay for detection of O157 STEC (sensitivity, 89 to 100%; specificity, 99.7%) and O157 and non-O157 STEC (sensitivity, 98 to 100%; specificity, 98%) (19
Many laboratories either fail to screen for STEC or screen only visibly bloody stools either because the local prevalence of STEC is considered to be too low or in an effort to contain laboratory costs (8
). The prevalence of STEC in our population (1.3%) was almost identical to that of an earlier multicenter U.S. study, in which STEC were as prevalent as Shigella
spp. (D. W. Acheson, K. Frankson, D. Willis, et al., Abstr. 98th Gen. Meet. Am. Soc. Microbiol., abstr. C-205, 1998). The prevalence of Shigella
spp. was slightly higher in our population (1.7%), so that STEC were the fourth most common enteric pathogen during the summer months. In an earlier large Belgian PCR-based study of more than 10,000 stool specimens, STEC were the third most prevalent enteric pathogens after Campylobacter
Having decided that the local prevalence of STEC warrants some form of screening, our findings emphasize again the importance of using a test that detects both O157 and non-O157 STEC serotypes. In the present study, as in others, non-O157 STEC were more prevalent than O157 STEC serotypes (D. W. Acheson et al., 98th Gen. Meet. Am. Soc. Microbiol.; L. J. Nims et al., 101st Gen. Meet. Am. Soc. Microbiol.). In the last decade, Australia, Europe, and North and South America have seen an increase in detection and reporting of non-O157 STEC as a cause of HUS (L. J. Nims et al., 101st Gen. Mtg. Am. Soc. Microbiol.) (1
). Similarly, in our population, the majority of STEC infections and half of the cases of hemorrhagic colitis and HUS were caused by non-O157 STEC serotypes. As in previous studies, in which virulence factor-based tests led to increases of 20 to 30% in detection of STEC-positive isolates, the ProSpecT assay detected twice as many STEC positives as SMAC culture alone (19
After deciding to screen for STEC and recognizing the increasing importance of non-O157 serotype STEC, the question arises as to which stool specimens to screen with a virulence-factor based test. Most U.S. laboratories that screen for STEC screen only visibly bloody stools (8
). However, presence of blood in the stool is a poor predictor of the presence of STEC (19
). In the present study, screening visibly bloody stools would have identified all patients with severe STEC-associated disease but would have missed the majority of STEC-positive patients (14 of 27 [52%] with nonbloody stools). In addition, screening only bloody stools may miss STEC-positive patients if the presence of blood is obscured in transport medium or if a history is not conveyed to the laboratory. Specifically, in this study, screening all stools, including nonbloody specimens, by ProSpecT assay detected almost twice as a many STEC-positive specimens. Previously in our laboratory, stools were screened for STEC by SMAC only if visibly bloody or upon request. Our present strategy is to screen all stools for STEC by the ProSpecT assay during the warmer summer months, when the frequency of STEC infection increases (22
). For the remainder of the year, in an effort to improve efficiency and cost-effectiveness of STEC screening, we test for STEC only if stools are bloody or upon request. For epidemiologic and public health purposes, all STEC-positive strains undergo SMAC and serotyping of sorbitol-negative colonies and are forwarded to the state laboratory.
Detection and identification of even a single O157 or non-O157 STEC infection have important benefits for the individual and for public health (5
). In searching for a cause of bloody diarrhea, physicians have performed unnecessary diagnostic imaging, colonoscopies, and surgeries on patients subsequently shown to be infected with STEC (15
). Similarly, seven of our STEC-positive patients (26%) had additional diagnostic procedures performed (computed tomography was performed on four patients, and colonoscopy was performed on three patients) prior to detection of STEC (data not shown). We suggest, and Park et al. (31
) have recently demonstrated, that an earlier diagnosis of STEC infection would have avoided such potentially harmful and expensive investigations in a significant number of our patients. In addition, establishing a specific diagnosis may prevent potentially deleterious antimicrobial or antimotility treatment (2
). It is a matter of concern that 30% of STEC-positive patients in the present study received empirical antimicrobials (6
) and/or antimotility agents (4
). The fact that quinolones were the antimicrobials prescribed in the majority of these cases is worrisome. Experimental evidence demonstrates that quinolones are potent inducers of Shiga toxin-encoding bacteriophages and toxin production in STEC (21
). Whereas the role of antimicrobials in the pathogenesis of HUS may be considered controversial by some, the potentially deleterious effect of antimotility treatment upon the evolution of STEC disease is well established (6
). Thus, it is particularly worrisome that several patients received antimotility agents prior to diagnosis of STEC infection. Our findings reinforce past data that physicians should not prescribe antimicrobials or antimotility treatment for patients known to have STEC infection. Furthermore, at a minimum, early detection permits identification and optimal monitoring of patients at risk of progression to HUS (38
). And finally, in the future, timely microbiologic diagnosis of STEC infection may permit specific anti-Shiga toxin prophylaxis to prevent severe STEC-associated disease (3
The public health benefits of diagnosing STEC infection are readily apparent in the setting of outbreaks of this potentially lethal agent (5
). Detection of a STEC-positive patient requires an epidemiologic investigation to assess whether a case is truly sporadic or part of an outbreak, to determine the source of the outbreak, and to prevent further spread. Early detection, rapid notification, and timely epidemiologic investigation of a STEC outbreak in the western United States led to recall of contaminated product and prevention of an estimated 800 additional cases of infection (5
). In the present study, familial and day care center transmission of non-O157 STEC (patients 24, 25, and 26) may have gone undetected if screening was limited to bloody stools or SMAC alone was performed.
Results of the present study should be interpreted in light of certain limitations of study design. Firstly, because a reference method that detects non-O157 STEC, such as PCR, was not used, it is not possible to assess sensitivity, specificity or negative predictive value for detection of non-O157 STEC serotypes. However, in the first part of the study, the ProSpecT assay demonstrated 100% sensitivity and specificity for detection of O157 serotype STEC, and we expect the performance characteristics to be similar for antigenically identical toxins of non-O157 serotypes. In addition, 11 of 12 non-O157 serotypes detected by ProSpecT assay were confirmed by PCR at the CDC. The one discordant non-O157 STEC-positive specimen (positive by ProSpecT assay but negative at the CDC) was from a young child (patient 26) that was temporally clustered with non-O157 STEC-positive isolates from two siblings (patients 24 and 25) attending the same day care center. Considering that the isolate was positive by both ProSpecT and Premier EHEC assays but negative by repeat Premier EHEC assay at the CDC, difficulty isolating STEC-positive non-sorbitol-fermenting colonies from SMAC at the state laboratory or loss of the Shiga toxin genes may account for the discordant result (17
). Secondly, many laboratories may consider the cost of screening by ProSpecT assay prohibitive. Our screening strategy cost almost $33,000 over two consecutive summers: $1,220 per positive test; and $16 per negative test. However, notwithstanding the potential societal or public health benefits of STEC outbreak detection and prevention efforts, we believe that the additional costs of screening are justified and may, in fact, be associated with significant cost savings. Although detection of STEC infection, by itself, does not prevent disease in the individual patient, it may prevent potentially deleterious treatments that cause progression to HUS or hemorrhagic colitis. We speculate that cost savings from the prevention of progression of even a single case of STEC infection justify the additional expense of our screening strategy. Specifically, admission and treatment of a single HUS case (patient 22) cost $97,977. And finally, our results may not be representative of populations with different prevalences of STEC infection. Although the frequency of STEC in our population was similar to those in previous multicenter studies, further study is required to determine if our screening strategy is applicable in other populations (23
; D. W. Acheson et al., 98th Gen. Meet. Am. Soc. Microbiol.).
In summary, the ProSpecT assay was a highly sensitive and specific test for E. coli O157 in stool (100%), with a high positive predictive value for O157 and non-O157 STEC serotypes (93%). Specifically, during the warmer summer months, screening all stools by ProSpecT assay detected twice as many STEC-positive patients as SMAC alone. In addition, testing all stools rather than only visibly bloody ones detected twice as many STEC-positive specimens. Thus, screening all stools for presence of STEC by a virulence factor-based test more accurately reflects the true prevalence of STEC infection, further emphasizes the contribution and pathogenic potential of non-O157 STEC, and provides important public health data. In addition, for the individual patient, tests such as ProSpecT assay may avoid unnecessary, expensive, and possibly harmful investigations and prevent potentially deleterious treatments and progression to severe disease. We conclude that virulence factor-based tests, such as the ProSpecT assay, which detect both O157 and non-O157 STEC serotypes in stool, are sensitive and should be more widely adopted by clinical microbiology laboratories.