To our knowledge, this is the first study to examine how current methods used to estimate the prevalence of CDI in the United States compare with the use of toxin assay results to estimate the prevalence of CDI. The results of this multicenter study suggest that the CDI prevalence measured using all ICD-9-CM
codes was higher than the CDI prevalence measured using the NIS, which, in turn, was higher than the CDI prevalence measured using the NHDS. These results were not surprising, given that the NIS captures more ICD-9-CM
codes than the NHDS but does not capture all ICD-9-CM
codes. In this study, the overall CDI prevalence measured by the NHDS criteria was the same as the CDI prevalence measured by the positive toxin assay results. Previous research indicates that ICD-9-CM
codes overestimate CDI prevalence [16
]. NHDS, by limiting the data set to the first 7 ICD-9-CM
codes in each discharge record, may eliminate patients who do not truly have CDI. However, in this study, the annual increase in CDI prevalence as measured by the NHDS criteria was greater than that revealed by positive toxin assay results, indicating at some point that the CDI prevalence identified by the NHDS criteria may become greater than the CDI prevalence identified by toxin assay results.
When comparing the CDI prevalence measured using the NHDS criteria with the toxin assay results at the hospital level, the CDI prevalence was higher by toxin assay results at hospitals A and B, whereas the prevalence was higher by the NHDS criteria at hospitals C and D. Hospital A’s results may be due in part to the fact that this institution became more vigilant to ensure that all medical records were adequately reviewed by medical coders in a timely fashion midway through the study period. Hospital B’s results may reflect laboratory practices at this institution. During the study period, this hospital’s microbiology laboratory tested formed stools for C. difficile
toxin. This practice is discouraged because testing asymptomatic patients may falsely elevate the CDI prevalence by 2 mechanisms: asymptomatically colonized patients without CDI can have positive toxin assay results, and testing for C. difficile
in low-prevalence populations will increase the number of false-positive test results [19
]. This may explain the higher CDI prevalence by toxin assay results than that by the NHDS criteria at this institution. Last, the NIS and NHDS databases do not exclude on the basis of stool consistency either. Therefore, it was unknown what effect this might have on our results.
There are limitations to the use of administrative data for disease surveillance purposes. The ICD-9-CM
codes are assigned by medical coders. Not all medical coders have the same level of training and certification, which may result in variable coding practices from coder to coder and facility to facility. For a patient to receive an ICD-9-CM
code, the diagnosis must be clearly stated in the medical records by a treating physician. Additional variability may occur if physician documentation is inconsistent. Patients who receive the ICD-9-CM
code for CDI but who do not have laboratory confirmation frequently have a history of CDI but lack ongoing symptoms of CDI [17
]. Furthermore, ICD-9-CM
codes are assigned after discharge, creating a time lag in the availability of data, and ICD-9-CM
codes do not provide any information about date or place of onset of CDI. Therefore, ICD-9-CM
codes alone are not ideal for CDI incidence surveillance.
Despite the limitations of ICD-9-CM
codes, there are limitations to the use of laboratory results on C. difficile
toxin tests for CDI surveillance as well. The “gold standard” to detect pathogenic C. difficile
from stool, toxigenic culture, is labor and resource intensive and takes several days until results are final. As a result, there are an increasing number of methods and algorithms to detect C. difficile
or its toxins in stool, all of which differ in sensitivity and specificity. Stool handling and processing can also affect the sensitivity and specificity of an assay. Testing practices vary in interpretation of positive results. Although this practice is uncommon, the diagnoses for some patients are made by means of endoscopy alone [18
]. Indiscriminate repeated testing for C. difficile
can falsely elevate CDI incidence by as much as 27% [20
]. Most importantly, CDI is a clinical diagnosis. Testing stool samples obtained from patients who do not have clinical symptoms compatible with CDI will result in positive test results for patients without CDI. In addition, the NIS and NHDS databases do not exclude patients with recurrent disease or patients with repeated toxin assay tests. To keep comparisons consistent, we did not exclude these patients either. As a result, this may have overestimated the true CDI prevalence by toxin assay.
An alternative CDI surveillance system already in use is that of the National Healthcare Safety Network (NHSN), which has been augmented by mandatory C. difficile
public reporting requirements of many states in the United States. Currently, the NHSN is collecting data on C. difficile
using 2 different reporting methods: (1) infection surveillance and (2) laboratory-identified events [21
]. To date, 166 facilities are participating in the NHSN C. difficile
infection surveillance reporting, 576 facilities are participating in the C. difficile
laboratory-identified event reporting, and 36 facilities are participating in both reporting methods (D. Sievert, PhD, personal communication, 4 June 2010). Review of data from the 36 facilities performing both methods of surveillance will be important to further our understanding as to whether use of laboratory data alone in the absence of clinical information from facilities that do not test formed stool for C. difficile
is a valid method for CDI surveillance.
This study indicates that current estimates of CDI prevalence in the United States based on ICD-9-CM codes may be falsely elevated. Fortunately, the NHSN is currently collecting data for CDI surveillance. The NHSN system provides a standardized method of CDI surveillance and will be able to assess the utility of laboratory-based CDI surveillance. Thus, the NHSN system may represent a substantial improvement in the quality of data available for hospital-based CDI surveillance, national CDI prevalence estimates, and interhospital CDI prevalence comparisons.