[Reliability of health insurance claim statistical data based on the principal diagnosis method].
OBJECTIVE
The estimation of disease-specific medical expenses in Japan is based on the assumption that all medical care expenses in a given claim are spent on the principal diagnosis even though health insurance claims actually include multiple diagnoses. The purpose of this study was therefore to evaluate the validity of this methodology.


METHODS
Medical expenses and the diagnosis based on the International Classification of Diseases, which are presently used for the Japanese National Health Insurance program, were obtained from 8471 outpatient medical expense claims from the National Health Insurance for Medical services for the Aged in Okaya city, Nagano Prefecture in May 2004. We calculated the frequency of each specific disease and estimated the disease-specific medical expenses using only the principal diagnosis in a claim and 10 diagnoses (principal diagnosis and nine additional diagnoses). Disease-specific medical expenses using 10 diagnoses was estimated according to the Proportional Disease Magnitude method.


RESULTS
The proportion that the principal diagnosis method reflected the frequency based on the method using ten diagnoses differed depending on the diagnosis. The proportion for hypertension was 59.7%, whereas it was 33.3% for diabetes mellitus. Hypertension and diabetes mellitus were estimated to be 18.32% and 5.38%, respectively, of all medical care expenses, as determined using the principal diagnosis method. However, when using ten diagnoses, hypertension and diabetes mellitus were estimated to be 8.50% and 5.16%, respectively, of all medical care expenses.


CONCLUSIONS
The above described principal diagnosis method is therefore considered to overestimate the medical care expenses of diseases that are often selected as the principal diagnosis.
Tanihara
Shinichi
S
Department of Hygiene and Preventive Medicine, Faculty of Medicine, Fukuoka University, Japan. taniyan@fukuoka-u.ac.jp
Yamagata
Zentaro
Z
Une
Hiroshi
H
D017048:Health Care Costs / Q000706:statistics & numerical data*; D019458:Insurance Coverage; D007564:Japan; D009313:National Health Programs / Q000191:economics* / Q000592:standards