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Letters to the Editor |
1 Epidemic Intelligence Service, Centers for Disease Control, and Prevention, Atlanta, GA
2 Air Pollution and, Respiratory Health Branch, Centers for Disease Control, and Prevention, Atlanta, GA
3 Bureau of Epidemiology, Florida Department of Health, Tallahassee, FL
aAddress correspondence to this author at: Air Pollution and Respiratory Health Branch, Centers for Disease Control and Prevention, 1600 Clifton Rd NE, MS E-17, Atlanta, GA 30333. E-mail dvansickle{at}cdc.gov.
To the Editor:
We recently conducted an investigation of unintentional carbon monoxide (CO) poisonings in hurricane-affected areas of Florida during the 2004 hurricane season (1). During this process, we reviewed the medical records of 167 persons treated for CO poisoning at 10 hospitals around the state, including 2 with hyperbaric oxygen chambers.
Blood carboxyhemoglobin (HbCO) concentrations are used to confirm the diagnosis of CO poisoning, although these concentrations are not always well correlated with clinical manifestation or outcome. However, a 1995 survey of 23 hospitals and laboratories in the Boston area that provide HbCO measurement found wide variation in the reference intervals among facilities; 38% used inappropriately high concentrations for nonsmokers (2).
In our investigation, we found that 2 of 10 (20%) hospitals used excessively high upper limits for the reference interval for blood HbCO concentrations. Although the majority of hospitals considered HbCO values <3% to be normal in nonsmokers, these 2 hospitals used <20% as the threshold for normal HbCO in nonsmokers. At these facilities, laboratory results with blood HbCO concentrations <20% were not indicated as increased, and the reference concentration printed beside the result noted <20% as normal. These hospitals have been contacted and are reviewing their reference intervals internally. Although all individuals included in our study had been (by definition) diagnosed with CO poisoning, a total of 27 patients at one hospital (or 50% of the total number of patients treated at that facility) and 4 patients at another (44% of the total number of patients treated at the second hospital) had increased blood HbCO concentrations (>3%) that were reported as normal in their laboratory results.
The use of appropriate reference intervals for blood HbCO is essential, given the nonspecific symptoms of CO poisoning and the potential for harm resulting from a missed diagnosis. Recognizing HbCO saturation between 3% and 20% as abnormal is particularly important for evaluating those individuals whose presentation to medical care may have been delayed, those who received supplemental oxygen before blood sampling, or those with occult sources of exposure. As Marshall et al. (2) suggested, inappropriate blood HbCO reference intervals may mislead physicians unfamiliar with background saturations, possibly causing "misdiagnoses, false reassurances, and perhaps less aggressive treatment than might be warranted."
Although no national standards for measuring blood HbCO exist, reference intervals (<2% to 3% in nonsmokers; <7% to 9% in current smokers) have been established from population-based surveys (3), cross-sectional epidemiologic studies (4)(5), and population exposure research (6). We encourage all hospitals and laboratories to review their blood HbCO reference intervals and update them as needed to ensure that concentrations currently recommended in the scientific literature are being used.
References
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