An inpatient hospital stay is not paid code by code. The diagnosis and procedure codes on the claim are inputs to a grouping process, and the output - a single MS-DRG - is what determines payment. This is why a single additional documented condition can change an entire stay's reimbursement.
From codes to a group
MS-DRG stands for Medicare Severity Diagnosis Related Group. The grouping logic takes the codes on the claim and works through them roughly in this order:
- The principal diagnosis - the condition established after study to be chiefly responsible for the admission - determines the major diagnostic category.
- Whether a qualifying operating room procedure was performed splits surgical from medical groups.
- Secondary diagnoses are examined for severity, which usually determines the final group.
There are 766 groups in the current set. Each carries a relative weight, and that weight, applied to the hospital's base rate, is the payment.
CC and MCC: where the money is
Most DRGs come in families of two or three that describe the same clinical situation at different severity levels:
| Tier | Meaning |
|---|---|
| MCC | With major complication or comorbidity |
| CC | With complication or comorbidity |
| - | Without CC/MCC |
A secondary diagnosis that qualifies as a CC or an MCC moves the stay to a higher-weighted group. This is the single largest lever in inpatient coding, and it is why complete secondary-diagnosis capture matters so much - a condition that was present, treated and documented but not coded is revenue the hospital has earned and not claimed.
It cuts the other way too. A condition coded as a CC that the documentation does not actually support is exactly what audits look for.
Principal diagnosis is not “the worst thing”
The principal diagnosis is the condition chiefly responsible for the admission, established after study. It is not necessarily the most severe condition present, the most expensive to treat, or the one the patient found most alarming.
“After study” matters: the principal diagnosis reflects what the workup concluded, not what was suspected at presentation. A patient admitted for chest pain who is found to have a different cause is coded to what was found.
Because the principal diagnosis selects the diagnostic category, getting it wrong does not shift the payment slightly - it can route the stay into an entirely different family of groups.
Present on admission
Inpatient claims flag whether each diagnosis was present on admission. Certain conditions that arise during the stay - hospital-acquired ones - are excluded from acting as a CC or MCC. The reasoning is straightforward: a hospital should not be paid more for a condition the stay itself caused.
This makes POA reporting a payment-relevant field, not an administrative one.
What this means for coding
- Secondary diagnoses are not filler. Each one is a potential severity input.
- Principal diagnosis selection deserves genuine care; it sets the category.
- Documentation drives all of it - a condition that was managed but not documented cannot legitimately be coded.
- Queries are the tool for closing gaps between what was treated and what was written down.
You can browse the full validity list on this site, and each group's page shows the ICD-10-CM diagnosis codes that map into it.