Objective: The International Classification of Diseases 10th revision (ICD-10) is a standard international diagnostic classification for medical diagnoses. Reliable diagnostic coding is of high medical and epidemiological importance. Coding diagnoses with ICD-10 is the basis of reimbursement in some healthcare systems.
Design: The ICD-10 coding of each case was performed by two raters to investigate the inter-rater agreement. The degree of agreement was assessed using Cohen's kappa. Kappa was divided into two groups: Kappa > or = 0.61 meaning high or satisfactory and kappa < or = 0.6 (incl. < or = 0.000 and 0.000*) meaning low or unsatisfactory.
Subjects: Cross-sectional data were collected from 8877 randomly selected patients. The 209 participating general practitioners used a standardized data collection form. The first of the reasons for encounter was taken into account on new and chronic managed problems.
Results: Kappa values were satisfactory on the chapter level with on average 0.685 (chronic managed problems) and 0.675 (new managed problems). Kappa was unsatisfactory when the three-digit level was used (0.428) and lower for terminal codes (four-digit level) at 0.199 on average (chronic managed problems). For new managed problems the kappa values were at 0.384 (three-digit level) and 0.166 (four-digit level) on average.
Conclusion: The ICD-10 is reliable for coding managed problems on the chapter level. Further refinement of ICD-10 with three- and four-digit codes leads to significant coding uncertainties. There is no reliable coding scheme that meets the demands of general practice. The use of coded data for healthcare reimbursement requires a simplification of ICD-10 to provide a realistic picture of morbidity.