Which coding guideline is primarily used to assign ICD-10-CM codes in outpatient settings?

ICD-10-CM diagnosis code assignment in the outpatient setting is governed primarily by the ICD-10-CM Official Guidelines for Coding and Reporting sections applicable to outpatient services. Outpatient rules differ from inpatient because there is no “principal diagnosis” established “after study” for an admission; instead, outpatient coding generally relies on the reason for the encounter and the conditions evaluated/managed that day, including documented chronic conditions that meet reporting criteria (often framed operationally as MEAT: monitor, evaluate, assess/address, treat). UHDDS is an inpatient discharge dataset concept used to define principal diagnosis and other inpatient reporting constructs, not the outpatient foundation. CPT guidelines govern procedure coding, not diagnosis coding; while CPT and ICD-10-CM must be consistent, CPT guidance does not replace ICD-10-CM outpatient diagnostic rules. From an outpatient CDI perspective, this is why documentation must clearly support encounter diagnoses, their status (active vs history), specificity (type, acuity, manifestations), and medical necessity for services rendered—so the outpatient ICD-10-CM guidelines can be applied correctly and consistently.