The need for nationwide public health surveillance of CO poisoning is recognized in the national Healthy People 2010
goal of “increasing the number of Territories, Tribes, and States, and the District of Columbia that monitor carbon monoxide poisoning from 7 to 51.”35
Using multiple data sources, we designed a public heath surveillance system with the ability to retrospectively describe the characteristics of those poisoned, detect and describe the magnitude of exposure events, assess trends in incidence over time, and identify and track risk factors for both disaster-related and routine cases of acute carbon monoxide poisoning. The system is being used to define the magnitude of the problem, as well as the relative contribution of various exposure sources; such a system is not only feasible, but also essential for planning and evaluating prevention efforts.
Thirteen states and New York City currently list acute CO poisoning as a reportable condition.36
Ninety percent of state health departments currently have access to hospitalization (IP) data;37
however, the relatively small magnitude of cases identified in the IP datasets means that, for public health jurisdictions with relatively small populations, a CO surveillance system could not be based solely on IP data. Fortunately, the number of states with access to ED data is rapidly increasing; 25 states currently have access to these data, up from 12 states in 1997.37
Unfortunately, few states currently have access to OP data; in our data, approximately 10%–20% more cases were identified annually in the OP than ED dataset alone. A potential limitation of a system based on ED data alone is that it will not identify individuals with less severe cases who may present to outpatient settings or private clinician offices, call poison control centers, or who simply do not get treated. Poison control center data may be a potential source to identify individuals with CO poisoning who do not present for treatment at a hospital-based setting; however, poison control centers are often unable to confirm the exposure status of the patient.38
In this pilot system, we chose to limit our data sources to those with clinically confirmed diagnosis.
Public health surveillance is dependent upon the establishment and adoption of a clear and reliable case definition. A case definition must include criteria for person, place, and time that should be further characterized by the degree of certainty regarding diagnosis, such as “confirmed” or “suspected.”39
We attempted to limit our case capturing to non-fire-related cases as well as unintentional cases. However, the specificity for doing so in ICD-9 CM datasets is unknown. A systematic study, probably based on chart reviews, should be undertaken to assess the validity of this approach. We also limited cases to residents of Maine. Although that is standard surveillance methodology, the impact on a nationwide system would be that residents of one state who seek care in another would not be included in any system. If multi-jurisdictional surveillance is initiated, agreements for the receipt of data for residents treated elsewhere should be established.
Because the only patient identifier in the MHDO data is a scrambled medical record number, the data could be reliably de-duplicated only within facilities. This could have resulted in counting the same case more than once if a person was transferred to another facility. We attempted to identify transfers from outpatient facilities to the one facility with a hyperbaric chamber by matching cases on date of birth, date of admission/transfer, sex, and residential ZIP code; only two possible transfers were identified based on the limited demographic information available. Because the number was relatively small and identification was not assured, these cases were retained in both the outpatient and inpatient datasets.
While few cases were identified in the death certificate files that met the case definition for CO poisoning, between one and three death certificate file records a year did meet the CSTE case definition for being a suspected case. Information from medical examiners' records may add information that would allow more specificity in case classification. Previous studies have found 12%16
of death certificate records were misclassified as not due to CO poisoning. These studies used a broader case definition than the one we applied and we found no published reports of misclassification of CO poisoning in hospital admissions data. This issue should be investigated as new data sources are identified and used for surveillance.
Our overall ED rates were higher than those from a national review of ED data from the National Electronic Injury Surveillance System All Injury Program (NEISS-AIP), 10
which reported an overall annual rate of 5.3/100,00 from 2001–2003, compared with our overall rate of 8.6/100,000. The demographic characteristics of our cases was also substantially different from those reported; the NEISS-AIP report found the highest rates among the youngest age group with a linear decline to the lowest rates in individuals aged 65 and older and reported a slightly higher rate among women than men; our highest rates were among those aged 18–34, and we observed slightly higher rates among men than women. These differences could be due to the use of different data sources (chart abstraction and billing data, respectively), regional differences in hospital utilization, or in exposure patterns. The latter includes the possibility that there is a higher rate of occupational exposure in Maine than on average nationally. Our findings were consistent with other reports of proportionally more cases in the fall and winter.16,40,41
The MHDO data do not contain information about patient race or ethnicity; non-white race and Hispanic ethnicity have been identified as important risk factors for carbon monoxide poisoning6,8,10,16,40,41
and efforts should be made to collect this valuable information so that prevention programs can be targeted appropriately.
We found that E-codes provided some information about exposure source and setting; however, while approximately two-thirds of records had one or more E-codes describing exposure, the remainder did not. Records may not be E-coded either because the clinician did not record the information in the chart (either the information was not available or an adequate exposure history was not obtained) or because the technician abstracting the chart did not code the exposure (either due to hospital coding guidelines limiting the use of E-codes or due to oversight). Efforts are needed to better understand why some records do not contain E-codes and to both increase their use and ensure consistency of coding practices among facilities.
An important component of a carbon monoxide surveillance system is characterizing sources of exposure, understanding how those sources are changing over time, and identifying new sources as they emerge. However, because of the coding limitations discussed above, data on hospital visits do not provide adequate detail for this purpose. Data from poison control centers may provide more detailed information on exposure source, but these sources are not confirmed and capture a somewhat different patient population.38
Perhaps the best approach for better characterizing exposure sources is making CO poisoning a reportable condition and conducting case identification and follow-back investigations. CO poisoning in Maine is currently reportable only as an occupational condition and rarely reported. To successfully establish CO as a reportable condition, public health agencies will need to dedicate resources to developing and maintaining a system for investigating case reports, educating providers and other reporting sources, and analyzing and disseminating the data.
CSTE indicators estimate occupationally related hospitalizations by using the principal payer field; i.e., a code for “Worker's Compensation” classifies the record as occupational. It is well documented that this approach underestimates cases.34,42
To address this issue, we combined information from the E-code, which describes place of injury, with the payer code, which gives the principal source of reimbursement. This approach substantially increased our estimate of the burden of CO poisoning that may occur in a place of work (from 13% using payer code alone to 23%). Review of newspaper accounts of occupationally-based events involving multiple people provided some validation for this approach; the vast majority of the records related to these events were not coded as Worker's Compensation but did have a place of occurrence E-code that indicated an occupational setting. Maine hospital visit data have a principal diagnosis field and up to nine additional diagnostic fields that may contain E-codes; data formats and coding practices vary considerably among states and this method may not be applicable in all locales.
The newspaper search also provided information about exposure setting and source, as well as confirmed our suspicion that deaths occurring in January 1998 were indeed excess deaths related to the ice storm. Among the cases we classified as ice-storm-related compared with non-ice-storm-related cases, we observed higher rates of CO poisoning among women and individuals aged 65 and older. The 2004 Maine Behavioral Risk Factor Surveillance System survey queried respondents about generator use and placement; female gender was identified as the sole demographic predictor of improper generator placement. While these results may reflect recall or response bias, the consistency of findings across the data sources suggests that education about using alternative fuel sources during or in the aftermath of a power outage should include women and the elderly.
Cases identified during the ice storm were more likely to occur in a residence and less likely to be occupationally related; there were more cases attributable to domestic fuel and fewer to motor vehicle exhaust. These findings are consistent with the higher rates of residential exposure subsequent to a natural disaster. We chose to classify disaster and routine cases by time, without a geographic component. Although the principal impact of the ice storm was in central Maine, power outages occurred throughout the state, and power was restored in a staggered manner to homes throughout the three-week period, during which we classified all cases as “ice storm-related.” Any such geographic and/or temporal misclassification would have resulted in non-disaster-related cases being classified as disaster-related cases; hence, any bias in our findings would be toward the null hypothesis.