A 71-year-old patient with stage 3 CKD arrives for nephrology follow-up on January 5. Her contrast-enhanced renal imaging is scheduled for January 9. But the prior authorization attached to the study disappeared on January 1, when she disenrolled from Medicare Advantage and returned to Traditional Medicare.

Her MA authorization no longer governs the service. The 90-day continuity-of-care window CMS created in the 2024 Medicare Advantage final rule does not apply, because the protection runs from one MA plan to the next, not from MA to Traditional Medicare. The care team now has days, not weeks, to ensure the workup is documented under Traditional Medicare’s medical-necessity framework before downstream audits, denials, or scheduling delays emerge.

This is the operational shape of a quiet trend. KFF, working with 2021–2022 Medicare beneficiary data, found that people who disenrolled from MA had Medicare spending 27% higher on average than similar people who were continuously in Traditional Medicare: about $2,585 more per person, after adjusting for differences in health risk factors. Among the line items driving that differential, skilled nursing facility spending dominated at 34%. Outpatient hospital ran another 23%; inpatient hospital, 20%. The pattern is not evenly distributed. Among Black beneficiaries the differential ran 55% ($5,203 per person); among Hispanic beneficiaries, 54% ($4,434); for White beneficiaries, 25% ($2,464). Dual-eligibles saw the widest gap of all: 61% above continuously-enrolled peers, or $9,435 per person.

For physician advisors, the analytic question — why are higher-utilizing beneficiaries disproportionately leaving MA? — is interesting. The operational question is harder. What happens on the floor when a patient who used to be inside an MA plan is suddenly inside Traditional Medicare?

Three things change inside the first 48 hours, and a fourth changes over the first quarter.

First, the authorization framework changes. An MA plan’s prior auth is a plan-level construct that does not carry to Traditional Medicare. Traditional Medicare itself runs little prospective prior auth on most outpatient services; medical-necessity review happens on the back end against National Coverage Determinations (NCDs) and Local Coverage Determinations (LCDs). That is not an absence of risk — it is a different shape of risk. The 90-day transition protection CMS finalized in CMS-4201-F, which requires that a new MA plan accept an enrollee’s active course of treatment without re-authorizing for 90 days, applies when an enrollee switches between MA plans. The protection runs MA → MA, not MA → Traditional Medicare. A patient returning to Traditional Medicare loses the MA prior auth and does not gain the 90-day protection. The documentation burden shifts forward.

Second, network constructs disappear. This is mostly upside for the patient — she can see any Medicare-participating provider — but it changes the workflow. The carefully-managed network of specialists the MA plan had negotiated, and that the discharge planner had been routing into, is no longer a constraint. The physician advisor’s eye should be on whether the documentation that justified the MA plan’s specialist referral pattern still supports medical necessity in a non-network context.

Third, the supplemental-benefits scaffolding is gone. Transportation, dental, vision, OTC, in-home support — all are MA plan add-ons. They do not exist in Traditional Medicare. For a patient discharged to home and dependent on her MA plan’s transportation benefit to get to dialysis, the absence is clinical, not administrative.

Fourth, the three-day inpatient stay rule applies again going forward. Many MA plans waive Traditional Medicare’s three-day inpatient hospital stay requirement for SNF coverage under CMS’s plan-design flexibility. Traditional Medicare itself does not waive it for most beneficiaries outside specific value-based program contexts. A patient returning to Traditional Medicare may newly become subject to that three-day rule for any future SNF eligibility episode. For a patient previously admitted directly to SNF from observation under her MA plan’s waiver, that pathway is no longer available unless and until she has a documented three-day inpatient stay. The physician advisor’s role here is the documentation move: when an inpatient admission is clinically warranted, the chart should reflect the medical reasoning that would survive a Traditional-Medicare medical-necessity review under the two-midnight benchmark.

Consider what this looks like as a denied claim sixty days later. The renal imaging from the opening scenario was performed January 9 on the assumption that the patient’s prior MA authorization carried weight. The Medicare Administrative Contractor reviews the resulting claim against the applicable Local Coverage Determination and finds the documentation lean: no progress note linking the imaging to a specific clinical question that meets the LCD’s coverage indications, no order language reflecting the medical necessity the LCD expects to see. The claim is denied — not because the imaging was inappropriate, but because the chart did not say so in the language Traditional Medicare reads. The physician advisor’s January-5 documentation move is what would have prevented this trajectory.

The harder pattern sits underneath. JAMA Network Open published a cohort study in July 2025 finding that among 5.9 million long-stay nursing home residents observed between 2010 and 2022, conventional MA plan enrollees disenrolled to Traditional Medicare at a rate of 11.6% in 2021–2022, down from 22.0% a decade earlier, but still substantially higher than I-SNP rates (3.4–5.2%). D-SNP disenrollment ranged 11.5–16.0%. The residents who switched payers were younger, less likely to be White (66.9% vs. 77.3%), and more likely to live in larger, for-profit, lower-rated facilities (78.4% for-profit vs. 70.2%). The authors note that high disenrollment outside of I-SNPs “may signal unmet needs” rather than preference shopping.

For physician advisors managing hospital throughput in markets with high MA penetration, the practical question is when a patient’s payer changes versus when their care plan should change. The answer is rarely the same. The documentation move is to date-stamp the change, re-establish the medical-necessity narrative under the new program’s rules, and flag the gaps — the lost prior auths, the absent supplemental benefits, the three-day-stay reset — before they become denied claims sixty days from now.

The patient who walked into clinic on January 5 needs her nephrology workup documented under Traditional Medicare’s medical-necessity standard. The physician advisor’s task is to make sure that documentation is in the chart this week, not this quarter.