A combination code is a single code used to classify:
Two diagnosis or
A diagnosis with an associated secondary process (manifestation)
A diagnosis with an associated complication
Combination codes are identified by referring to subterm entries in the Alphabetic Index and by reading the inclusion and exclusion notes in the Tabular list.
Assign only the combination code when that code fully identifies the diagnostic condition involved or when the Alphabetic Index so directs. Multiple coding should not be used when the classification provides a combination code that clearly identifies all of the elements documented in the diagnosis. When the combination code lacks necessary specificity in describing the manifestation or complication, an additional code should be used as a secondary code.
Here is an example: Acute cholecystitis with cholelithiasis should be coded with a signle code that fully describes the condition: 574.00