Few numbers in a skilled nursing building get as much attention as the case-mix index, or CMI. It is a single value that summarizes the relative resource needs of residents, and it is often used in conversations about reimbursement and resident acuity. Because it is a summary, it is easy to read too much into it.
This post offers a practical way to read CMI: what can move it, what to compare it with, and what to ask when it changes.
What the CMI represents
In general terms, a case-mix index is an average of case-mix weights across a group of residents or stays. Under PDPM, there are separate components, such as therapy, nursing and non-therapy ancillary, and each has its own index. A higher value generally reflects a population with greater expected resource needs.
A CMI is not a measure of quality, and it is not a measure of how well a building is run. It describes the mix.
What can move the number
Several things influence CMI, and only some of them are within your control:
- Admission mix. Different referral sources send different kinds of residents. A change in hospital partners or service lines can change the mix.
- Length of stay. The longer residents stay, the more the distribution can shift.
- Seasonality. Some times of year bring more of certain kinds of admissions.
- Assessment practices. Timeliness, completeness and consistency of MDS documentation affect classification.
- Small numbers. In a small building, a few residents can swing the average.
Because these factors combine, a single month's change rarely tells you much.
How to read it more carefully
Look at trends, not points
Chart CMI over several months, and note the range of normal variation. A move within the usual range is probably noise.
Look at components
A total hides which part moved. Examine each PDPM component separately, as described in earlier discussions of case-mix tracking.
Look at volume alongside
CMI is an average. Pair it with admission and census counts so you know how many residents are behind the number.
Look at admission sources
If CMI shifted, did the sources of your admissions shift too?
Compare with your own history first
Comparisons with other buildings can help, but differences in market, services and size make them imperfect. Your own history is usually the cleanest baseline.
Questions to ask when CMI changes
- Did our admission mix change?
- Did volume change enough to matter statistically?
- Did assessment timing or completion change?
- Did the staff who complete assessments change?
- Does the change match what clinical leaders see on the floor?
If the answer to the last question is no, that deserves a gentle investigation, not an assumption.
Avoid these traps
- Treating a high CMI as success or a low one as failure. The number describes residents, not performance.
- Setting targets for CMI. A target can create pressure to document in ways that do not reflect reality. Focus on accuracy instead.
- Ignoring cost. A higher-acuity mix may require more staffing and supplies, so margin matters as much as the index.
- Forgetting residents. Behind each value is a person with needs.
Connecting CMI to margin
A useful complement is looking at revenue and cost per resident day alongside CMI. If acuity rises but staffing and supply costs rise as well, the net effect may be different than expected. Combining clinical, payroll and billing data shows the full picture.
A hypothetical example
Imagine a hypothetical building whose CMI dips for two months. A closer look shows that a hospital partner temporarily sent fewer complex post-surgical patients because of changes in its own programs. Assessment timeliness had not changed. With that context, leadership focuses on referral relationships and not on documentation. Without the look beneath the number, they might have chased the wrong problem.
A simple monthly view
Consider a one-page summary with:
- CMI trend by component
- Admissions by source
- Average length of stay
- Assessment timeliness
- Revenue and cost per resident day
Reviewing it monthly with the MDS coordinator, DON and administrator gives context at a glance.
Where CarePulse fits
CarePulse brings together MDS, census, billing and payroll information so CMI can be read alongside the factors that move it. If you would like to see how that looks for your building, a demo is a good place to begin.