Diagnostic coding and medical rehabilitation length of stay: their relationship.

Objective: To determine if diagnostic information provided in the form of International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) codes improves rehabilitation length of stay (LOS) prediction when used in combination with the Functional Independence Measure-Function Related Groups (FIM-FRGs) classification system.
Design: Various models characterizing diagnostic information using ICD-9-CM codes were created that included individual ICD-9-CM codes and groupings of those codes by organ or etiology involved. Each method was evaluated using linear regression with the natural logarithm of LOS as the dependent variable. Separate validation data sets were held back to quantify the incremental effect of diagnosis when combined with the FIM-FRG classification system.
Setting: Records from 252 rehabilitation facilities and hospital units across the nation.
Patients: Analyses were undertaken using 82,646 records from patients discharged in 1992.
Results: The addition of ICD-9-CM diagnostic information to the FIM-FRG classification system increased the variance explained by a maximum of 1.9%, from 31.5% to 33.4%.
Conclusions: Refinement of the FIM-FRGs to include ICD-9-CM diagnoses does not appear warranted on the basis of the small increase in the percentage of explained variance in LOS. We believe the lack of improved prediction with the addition of ICD-9-CM codes relates primarily to incomplete coding practices and to the effect of patients' diagnoses being absorbed in variables as already expressed by the FIM-FRG system. Although ICD-9-CM codes, overall, did not greatly improve LOS prediction, they appeared to have some impact in certain impairment categories.