Healthcare Delivery Systems and Reimbursement Methodologies (Including the Inpatient Prospective Payment System)
Know which payment system governs the stay
Medicare Part A covers inpatient hospital care, SNF, home health, hospice, and some inpatient rehabilitation; Part B covers physician services, outpatient care, durable medical equipment, and clinical laboratory tests. A CDI specialist must know which benefit category a service falls under, because that determines the payment system and the documentation rules that attach to it.
Hospitals are paid under the Inpatient Prospective Payment System (IPPS). Payment is a product: the MS-DRG relative weight (reflecting principal diagnosis, secondary diagnoses qualifying as CC or MCC, procedures, discharge disposition, and age/sex) multiplied by the hospital's adjusted base rate. The base rate starts with a standardized amount and is adjusted for geographic wage index, indirect medical education (IME), disproportionate share hospital (DSH) uncompensated care, and outlier payments. Other settings run their own systems: OPPS for hospital outpatient, ASC groups, SNF PPS, IRF PPS, home health PPS, and the physician fee schedule built on relative value units.
Traps: confusing the DRG weight with the base rate; assuming every secondary diagnosis is a CC; forgetting that the principal diagnosis is the condition established after study as chiefly responsible for admission; overlooking how transfers, discharge against medical advice, and the two-midnight rule affect payment. Observation is a Part B service, not an inpatient Part A stay.
Rule of thumb: documentation moves the weight, never the base rate.
Worked example: a newly documented MCC moves the weight from 1.10 to 1.62. With a base rate of $6,400, payment shifts from $7,040 to $10,368 - roughly $3,300, which is why validation matters more than volume.