The NNIS classification system was used to categorize procedures on the basis of the International Classification of Diseases, 9th Edition (ICD-9) code (20).
In selecting controls, the greatest weight was given to matching procedure type (ICD-9 code, NNIS procedure, or NNIS procedure group), then to sex, age, and duration of procedure.
The ASA score was evaluated as both a 5-level ordinal variable, with values of 1,2,3,4, and 5, and as a dichotomous variable (ASA [is greater than or equal to] 3 and ASA [is less than] 3, corresponding to the NNIS scoring system).
An NNIS risk index score was calculated for each patient by assigning one point each for a contaminated wound, an ASA score [is greater than or equal to] 3, and surgical procedures lasting longer than the NNIS-derived 75th percentile for the duration of the procedure (23).
Two variant scores were also computed; in one of these, a chronic disease score [is greater than or equal to] 5,000 was substituted for ASA score [is greater than or equal to] 3, and in the other, a point was added to the NNIS score for a chronic disease score [is greater than or equal to] 5,000.