Short answer: Under Medicare rules, an inpatient admission is generally appropriate when the admitting doctor expects the patient to need medically necessary hospital care that crosses two midnights; shorter expected stays are usually treated as outpatient or observation care.
The rule is in 42 CFR 412.3. The doctor’s expectation must be based on medical factors (history, severity of symptoms, current needs, risk of an adverse event) documented in the record. If the patient dies or is transferred before the second midnight, the stay can still count as inpatient.
There are two exceptions: surgeries on Medicare’s inpatient-only list are inpatient regardless of length, and a doctor may admit for a shorter expected stay based on clinical judgment that the medical record supports.
The rule is about the doctor’s expectation, not the clock: a patient can cross two midnights in observation and still be an outpatient if no admission order was written. Medicare Advantage plans must follow the same inpatient admission criteria (42 CFR 422.101(b)(2)), though they may still require prior authorization, so check the plan.