Inpatient coding uses the principal diagnosis, which is the condition responsible for admitting the patient to the hospital. Outpatient coding uses the first-listed diagnosis, which is the sign or symptom that is the reason for the visit. While uncertain diagnoses like "probable" or "rule out" can be coded as if the condition existed for inpatients, these uncertain diagnoses should never be coded for outpatients.

1. Differences between inpatient and outpatient coding
Inpatient Outpatient
Principal diagnosis First-listed diagnosis
A condition after study that is A sign or symptom that is the
responsible for admitting a reason for the visit.
patient to the hospital. .
Uncertain Diagnoses Uncertain Diagnoses
Diagnoses that says “probable” NEVER code the diagnosis
or “rule out” can be coded as if that says “probable” or “rule
the condition existed. out.”
Puckett, Craig D, 2008 Annual Hospital Version ICD-9-CM, Reno, Nevada: Channel, 2007