Sensitivity of International Classification of Diseases Codes for Hyponatremia Among Commercially Insured Outpatients in the United States

Alisa M Shea; Lesley H Curtis; Lynda A Szczech; Kevin A Schulman

Disclosures

BMC Nephrology 

In This Article

Abstract and Background

Background: Administrative claims are a rich source of information for epidemiological and health services research; however, the ability to accurately capture specific diseases or complications using claims data has been debated. In this study, the authors examined the validity of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis codes for the identification of hyponatremia in an outpatient managed care population.
Methods: We analyzed outpatient laboratory and professional claims for patients aged 18 years and older in the National Managed Care Benchmark Database from Integrated Healthcare Information Services. We obtained all claims for outpatient serum sodium laboratory tests performed in 2004 and 2005, and all outpatient professional claims with a primary or secondary ICD-9-CM diagnosis code of hyponatremia (276.1).
Results: A total of 40,668 outpatient serum sodium laboratory results were identified as hyponatremic (serum sodium <136mmol/L). The sensitivity of ICD-9-CM codes for hyponatremia in outpatient professional claims within 15 days before or after the laboratory date was 3.5%. Even for severe cases (serum sodium 125mmol/L), sensitivity was <30%. Specificity was >99% for all cutoff points.
Conclusion: ICD-9-CM codes in administrative data are insufficient to identify hyponatremia in an outpatient population.

Hyponatremia, defined as an abnormally low level of serum sodium, is the most frequently observed electrolyte disorder in the United States and is associated with significant morbidity and mortality in patients with heart failure,[1,2] myocardial infarction,[3,4] and liver cirrhosis,[5,6] as well as in the hospitalized elderly population at large.[7] Among general acute care patients, the prevalence of hyponatremia is estimated to be approximately 1%.[8,9] However, much higher rates–ranging from 18% to 30%–have been observed among elderly nursing home residents[10] and in intensive care settings.[11] Little is known about the prevalence of hyponatremia in outpatient settings or in the general population.

Administrative claims data are a rich source of information for epidemiological and health services research. With increasing frequency, researchers are turning to administrative claims data to ascertain information about patient outcomes and hospital quality.[12,13,14,15,16,17,18,19] However, the ability to accurately capture specific diseases or complications using claims data has been a subject of considerable debate.[20,21,22,23,24]

Most validation studies of diagnosis and procedure codes have relied on retrospective chart review as the source of comparative information. Using medical record review as the gold standard, Quan et al[25] found that the validity of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes for invasive or major surgical procedures in inpatient discharge claims was high; however, codes for routine procedures were often inaccurate or incomplete. Geraci et al[26] used the same method to assess the validity of 30 ICD-9-CM codes for common in-hospital complications as observed in patient discharge records from nine hospitals. They found an overall sensitivity of 34% and a positive predictive value of 32%. A handful of studies have used laboratory information to validate administrative claims codes. Wei and Walsh[27] compared managed care claims data with laboratory results and found that less than 25% of female beneficiaries with a positive test for chlamydia were coded as such. In another study, researchers used clinical, radiological, and laboratory data to assess the validity of ICD-9 codes for the diagnosis of gout in an ambulatory managed care population and found a positive predictive value of 61%.[28]

To our knowledge, there has been only one published study of the validity of ICD-9-CM codes for the diagnosis of hyponatremia. Movig et al[29] compared inpatient hospital discharge records with inpatient laboratory data and reported a sensitivity of 30% for even the strictest definition of hyponatremia (115mmol/L). Positive predictive value for laboratory results showing serum sodium 135mmol/L was 91.7%. The study did not address the validity of coding for hyponatremia outside the inpatient setting. Therefore, we sought to examine the validity of ICD-9-CM diagnosis codes for the identification of hyponatremia in an outpatient managed care population.

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