Coding Knowledge and Skills
Scope
This domain tests applied code selection across ICD-10-CM, ICD-10-PCS, CPT, and HCPCS Level II, in both inpatient and outpatient settings, plus the grouping logic (MS-DRG, APC) that follows.
ICD-10-CM conventions
Know the difference between Excludes1 (the two conditions are never reported together) and Excludes2 (the second condition is not inherent to the first and may be coded if documented as distinct). Understand code first (etiology sequenced first) versus code also (an additional condition that is not the principal diagnosis), the meaning of "with" (an inherent, assumed linkage), "in diseases classified elsewhere" (manifestation codes that never stand alone), seventh characters (initial/subsequent/sequela), laterality, and combination codes.
ICD-10-PCS
Every character must be justified: section, body system, root operation, body part, approach, device, qualifier. The root operation reflects intent and extent - Excision takes out some of a body part; Resection takes out all of it; Detachment cuts off without cutting into; Drainage and Extirpation have their own definitions.
CPT and HCPCS
Know section structure, E/M leveling, and modifiers: 25 (significant, separately identifiable E/M with a procedure), 59/X{EPSU} (distinct service at a different site or encounter), and the NCCI bundling logic behind them.
Sequencing
Principal diagnosis = the condition established after study as chiefly responsible for admission. Outpatient first-listed = the reason for the encounter.
Trap and rule of thumb
A common trap is assigning a root operation from the procedure name instead of the definition. Read the definition: a partial nephrectomy removes only part of the kidney, so it is Excision; a total nephrectomy removes the whole body part, so it is Resection.