Core Coding Knowledge: ICD-10-CM, CPT, and HCPCS Level II
Two code sets, two jobs
ICD-10-CM answers "what is wrong with the patient"; CPT and HCPCS Level II answer "what did the provider do." Candidates lose points by mixing the missions - assigning a procedure code to describe a symptom, or an ICD-10-CM code to describe a service.
ICD-10-CM conventions you must be able to recite
Read the tabular notes, not just the alphabetic index. Includes notes define what a category covers. Excludes1 means the codes cannot be reported together because they are mutually exclusive (a true "not coded here"). Excludes2 means the condition is not part of the code's definition but may legitimately coexist - both may be coded. Code first tells you sequencing for an underlying etiology; use additional code tells you a second code is required but does not dictate order. The word "with" creates a presumed causal link unless documentation states otherwise. Combination codes replace multiple codes when they exist; separate codes are used only when no combination applies.
Sequencing
Outpatient facility and physician claims use the first-listed diagnosis, which is often the reason for the encounter and may be a sign or symptom when no diagnosis is confirmed. Inpatient principal diagnosis is the condition established after study as chiefly responsible for admission. Never code a "rule out," "probable," or "suspected" condition on an outpatient claim - code the symptom (or a Z03.- encounter code) until the provider documents a definitive diagnosis.
CPT and HCPCS structure
CPT is hierarchical: section, subsection, subheading, category, subcategory, code, with indentation carrying meaning. Add-on codes (+) are never reported alone and never with modifier 51. HCPCS Level II covers drugs (J-codes), supplies, DMEPOS, and payer-defined modifiers.
Worked example: Type 2 diabetic with stage 3b CKD → E11.22 first, then N18.35 (the combination code's note requires the stage code). Do not report diabetes and CKD as unrelated conditions.
Rule of thumb: if one code can fully describe a condition, no second code earns payment.