Validation of International Classification of Disease Ninth Revision codes for atopic dermatitis

Background: Evaluation of large-scale data sets is needed to better understand the epidemiology, cost, and burden of atopic dermatitis (AD). We sought to validate the use of ICD-9-CM codes for identifying AD. Methods: Patients from a large metropolitan quaternary care medical center with a diagnostic code of either 691.8 (AD) or 692.9 (eczema and contact dermatitis) were queried. Medical records were reviewed for demographics, Hanifin & Rajka (H&R) and United Kingdom Working Party (UKWP) criteria. Sensitivity, specificity, and positive predictive values (PPV) of the codes were calculated. Results: Of 43 278 patients identified with associated ICD-9 codes of 691.8 or 692.9, 519 and 253 with 691.8 and 692.9 were randomly selected for chart review. There was extensive overlap: 34.3% had ≥1 occurrences of 691.8 and 692.9 and 25.6% had multiple occurrences of both codes. Among patients with ≥1 occurrence of 691.8, 29.9% and 30.8% met the H&R and UKWP criteria, respectively. Similarly, among patients with ≥1 occurrence of 692.9, 33.7% and 32.2% met the H&R and UKWP criteria. Increased PPV was associated with concomitant diagnoses of asthma, hay fever, and food allergy and increased disease severity. Conclusions: In the outpatient setting, the ICD-9-CM codes 691.8 and 692.9 alone have poor PPV. Incorporation of diagnoses of asthma, hay fever, and food allergy improves PPV and specificity. In the inpatient setting, a primary discharge diagnosis of 691.8 had excellent PPV. Although ICD-10 has been adopted in Europe and more recently in the USA, the same systematic errors would likely occur unless providers standardize their coding.