Code First Underlying Condition
In tabular list, this designation most commonly represents what is known as the "Manifestation/Etiology" sequencing rule. When a particular condition is due to another underlying causal condition, the underlying condition (etiology) code is sequenced first, followed by the code for the manifestation.
In the alphabetic index, the underlying code is listed first, followed by the manifestation code, which is italicized in brackets. A manifestation code can never be sequenced as the first-listed or principal diagnosis.