What is a casemix index?

The average cost weight across a hospital’s episodes over a period. An index above the national average means the hospital treats a more complex mix than average, and should be paid more per episode as a result.

Is casemix the same thing as DRG?

They are closely related but not the same. A DRG is the class one episode falls into. Casemix is the profile of all your episodes across those classes. DRG is the unit; casemix is the aggregate.

Can a hospital raise its casemix index?

By treating more complex patients, or by documenting accurately the complexity it already treats. The second is where most of the realistic movement is, and it is legitimate — recording a comorbidity that was treated is not gaming, it is completing the record. Recording one that was not is fraud.

What happens to long-stay or unusually expensive patients?

Most casemix systems carry some form of outlier arrangement, because any classification averages and a system with no escape valve would penalise hospitals for cases they cannot avoid. The exact mechanism is set by the payer rather than by the classification.

Does casemix funding reduce quality?

It changes the incentives rather than the quality directly. Paying per episode removes the reward for unnecessary activity but introduces a reward for discharging early, which is why casemix systems are normally paired with readmission and outcome monitoring. The mechanism needs the monitoring to work as intended.

How long does a transition take?

The software is rarely the constraint. Coding capacity and documentation practice are, because both are people, and both take longer to build than a system takes to configure. Running in shadow is how most organisations find out which of the two is their real bottleneck.

What does this have to do with value-based healthcare?

Casemix is the step that makes it arithmetically possible. Paying for value requires being able to express what was done in comparable units and what it cost to do — which is casemix plus patient-level costing. Without both, "value" has no denominator.

Do we need patient-level costing from day one?

No, and most organisations do not start there. Casemix funding works without it; you will simply be able to see what you are paid and not what you spent. Costing is what turns the payment data into a management view, and it is usually the second phase rather than the first.

Casemix funding, explained

Casemix is the mix of episode types a hospital actually treats, expressed through the DRG classes those episodes fall into and the cost weight of each. Casemix funding pays a hospital for the work its casemix represents rather than for the number of items on its bills — which is why two hospitals with identical admission counts can be worth very different amounts.

By NANO Health Suite Clinical & Coding Team · Last updated 2026-09-20

What casemix actually measures

A hospital that performed 1,000 admissions has told you almost nothing. A hospital whose 1,000 admissions were mostly high-complexity surgical episodes has told you a great deal, and should not be funded like one whose 1,000 admissions were mostly short medical stays.

Casemix is the way to say that precisely. Each episode carries the cost weight of the DRG it grouped to, and the average of those weights — the casemix index — is a single number describing how demanding the work was. Multiply casemix by volume and you have a defensible basis for funding.

Why payers move to it

It makes hospitals comparable, because activity is expressed in a common unit rather than in each provider’s own billing habits.
It removes the incentive to add services with no clinical need, because additional line items no longer add revenue.
It transfers efficiency risk to the provider, who keeps the difference on an episode delivered efficiently and absorbs it on one that is not.
It gives the payer a predictable per-episode cost, which is what makes budgeting and premium-setting possible.
It creates a dataset that can be used for planning and quality comparison, not just for payment.

What changes for the hospital

The revenue line stops being a function of what was billed and becomes a function of what was documented and coded. That is a genuine organisational change rather than a systems one: the people who determine revenue are now the clinicians writing notes and the coders reading them.

Three functions that could previously operate independently now have to work as one. Clinical documentation decides what the coder can see. Coding decides what the grouper receives. Grouping decides what is paid. A weakness at any point is invisible at the other two and shows up only in the money.

Knowing what the switch is worth, before it happens

The single most useful thing to do before a casemix transition is to run it in parallel. Shadow billing groups and prices your episodes under the new rules while you continue to bill under the old ones, so you can see your real casemix index, where your documentation is costing you, and what the change is worth in either direction.

Doing this after the payment rules change is possible but expensive: the gap is then a revenue shortfall being discovered rather than a project being planned.

Casemix, costing and value

Casemix tells you what an episode is classified as and therefore what it pays. It does not tell you what that episode cost you to deliver. Those are different numbers, and the gap between them is the actual margin.

Patient-level costing is what closes it: allocating real costs to real episodes, so you can see which parts of your casemix are profitable and which are subsidised by the rest. That is also the point at which casemix funding stops being a payment mechanism and starts being a management tool — and it is the foundation the value-based care conversation rests on, because an outcome you cannot cost is one you cannot pay for differently.

Frequently asked questions

What is a casemix index?: The average cost weight across a hospital’s episodes over a period. An index above the national average means the hospital treats a more complex mix than average, and should be paid more per episode as a result.
Is casemix the same thing as DRG?: They are closely related but not the same. A DRG is the class one episode falls into. Casemix is the profile of all your episodes across those classes. DRG is the unit; casemix is the aggregate.
Can a hospital raise its casemix index?: By treating more complex patients, or by documenting accurately the complexity it already treats. The second is where most of the realistic movement is, and it is legitimate — recording a comorbidity that was treated is not gaming, it is completing the record. Recording one that was not is fraud.
What happens to long-stay or unusually expensive patients?: Most casemix systems carry some form of outlier arrangement, because any classification averages and a system with no escape valve would penalise hospitals for cases they cannot avoid. The exact mechanism is set by the payer rather than by the classification.
Does casemix funding reduce quality?: It changes the incentives rather than the quality directly. Paying per episode removes the reward for unnecessary activity but introduces a reward for discharging early, which is why casemix systems are normally paired with readmission and outcome monitoring. The mechanism needs the monitoring to work as intended.
How long does a transition take?: The software is rarely the constraint. Coding capacity and documentation practice are, because both are people, and both take longer to build than a system takes to configure. Running in shadow is how most organisations find out which of the two is their real bottleneck.
What does this have to do with value-based healthcare?: Casemix is the step that makes it arithmetically possible. Paying for value requires being able to express what was done in comparable units and what it cost to do — which is casemix plus patient-level costing. Without both, "value" has no denominator.
Do we need patient-level costing from day one?: No, and most organisations do not start there. Casemix funding works without it; you will simply be able to see what you are paid and not what you spent. Costing is what turns the payment data into a management view, and it is usually the second phase rather than the first.

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