ICD-9-CM Coding of Emergency Department Visits for Food and Insect Sting Allergy
Introduction
Ascertainment of cases for epidemiologic studies performed in clinical settings often involves the use of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes. Many studies examined the accuracy of identifying disease cases by ICD-9-CM code 1, 2, 3, 4, 5 and found that the ICD-9-CM classification system has several limitations, including diagnostic codes that do not completely encompass the condition of interest and conditions being distributed across multiple codes (3). These studies found that identification of soft tissue disorders of the neck and upper limbs (2), ischemic cerebrovascular disease (3), acute ischemic stroke (1), pneumococcal pneumonia (5), and cerebrovascular malformations (4) by using ICD-9-CM codes often yielded incomplete or inaccurate case lists and concluded that ICD-9-CM codes should not be the only source used to identify cases of disease.
In 1993, the ICD-9-CM system introduced codes to identify specific food allergies. To our knowledge, no study has examined the accuracy of ICD-9-CM coding for allergic reactions or anaphylaxis. Additionally, little is known about how frequently these specific codes are used to identify specific allergic reactions, such as food allergy or insect sting allergy, in the emergency department (ED). The objective of the current study is to determine the accuracy of using ICD-9-CM codes to identify ED visits for food and insect sting allergies and determine the potential bias that might be introduced by sole reliance on ICD-9-CM codes.
Section snippets
Methods
This retrospective cohort study was performed as part of the Multicenter Airway Research Collaboration, a division of the Emergency Medicine Network (EMNet) (6). Using a standardized protocol and data abstraction forms, investigators at 10 EDs examined two cohorts, one of ED visits for food allergy and one of ED visits for insect sting allergy, each during any 1-year period between January 1999 and November 2001. Sites reviewed charts of randomly selected patients who presented with ICD-9-CM
Food Allergy
Of 775 consecutive patients with an acute allergic reaction, 216 patients (28%) were coded as having a food-related allergic reaction. Of the remaining 559 patients with unspecified allergy, 190 patients (34%) were identified as having probable or definite food allergy based on chart review. Thus, only 53% (95% confidence interval, 48%–58%) of patients with a food-related allergic reaction were identified by specific ICD-9-CM codes. The number of patients identified by each ICD-9-CM code is
Discussion
To our knowledge, we present the first study assessing the accuracy of using ICD-9-CM codes to identify ED visits for food-related and insect sting-related acute allergic reactions. The use of ICD-9-CM codes in epidemiologic studies of acute allergic reactions and anaphylaxis requires that the codes accurately and completely identify cases to ensure valid conclusions. In our cohort of patients with food and insect sting allergy, specific ICD-9-CM codes identified only 53% of patients with food
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S.C. is supported by grant no. T32 ES07069 from the National Institute of Environmental Health Services (Research Triangle Park, NC) and C.A.C. is supported by grant no. AI52338 from the National Institute of Allergy and Infectious Disease (Bethesda, MD). This project was supported by unrestricted grants from Dey Laboratories (Napa, CA) and the Food Allergy and Anaphylaxis Network (Fairfax, VA).