Allogeneic Processed Thymus Tissue–agdc (Rethymic) — CPB 1055 | AETNA Coverage Update

Aetna modified CPB 1055 for allogeneic processed thymus tissue–agdc (Rethymic), effective January 23, 2026. Here's what billing teams need to know.

Aetna, a CVS Health company, updated its coverage policy for Rethymic under CPB 1055 Aetna system, covering immune reconstitution treatment for pediatric members with congenital athymia. This policy sets strict medical necessity criteria, hard exclusions, and dosing limits that directly shape your prior authorization documentation and claim submission. The diagnostic workup codes—including CPT 86359, 86360, and 86361 for T-cell counts, plus CPT 86828 through 86835 for HLA antibody testing—are central to building a compliant prior auth package.

Quick-Reference Table

Aetna Rethymic Coverage Criteria and Medical Necessity Requirements 2026

Aetna's Rethymic coverage policy is narrow by design. This is a single-administration biologic for a rare pediatric condition—congenital athymia—and Aetna treats it that way. Medical necessity approval requires meeting every criterion simultaneously. Miss one, and the claim fails.

The primary diagnosis must be confirmed using flow cytometry showing fewer than 50 naïve T cells/mm³ in peripheral blood, or naïve T cells making up less than 5% of total T cells. CPT 86359 (T cells, total count) and CPT 86361 (absolute CD4 count) are your documentation anchors here. ICD-10 Q89.2 is the correct diagnosis code for congenital athymia.

Before authorization, the member must be screened for anti-HLA antibodies. This is a hard requirement, not a suggested step. Use CPT 86828 through 86835 to document that screening. If the member tests positive for anti-HLA antibodies, the treating team must source Rethymic from a donor who does not express the matching HLA alleles. Your prior auth submission must reflect which scenario applies.

The treating clinician must also document that infection control measures—including immunoprophylaxis—can be maintained until thymic function is established. This isn't a checkbox. Aetna wants evidence that the clinical team has a concrete infection control plan in place.

The prescriber must be a pediatric immunologist, or the prescription must come in consultation with one. This is a Aetna Rethymic billing requirement that trips up teams routing through general pediatrics or surgery. Confirm the prescriber specialty before you submit.

Dosing carries its own hard cap. Aetna approves a single, one-time dose only. That dose cannot exceed 22,000 mm² of Rethymic surface area per m² of recipient body surface area (BSA). It also cannot exceed 42 slices as calculated and supplied by the manufacturer. Document the BSA calculation and the planned slice count in your prior auth package. Aetna will check this.

Aetna Rethymic Exclusions and Non-Covered Indications

Three absolute exclusions knock a member out of eligibility entirely. Aetna will not approve Rethymic when any of the following are present:

The SCID exclusion is worth flagging to your clinical team. SCID and congenital athymia can look similar early in a diagnostic workup. The flow cytometry confirmation requirement exists partly to separate these diagnoses. A claim submitted with any D81.x code as the primary or secondary diagnosis will deny.

The lifetime exclusion is absolute. One administration per lifetime. No appeals pathway exists for a second administration. If your records show any prior Rethymic use, stop before submitting.

Aetna also lists any comorbidity that the treating clinician believes creates a high risk of severe complications—including pre-existing renal impairment, active CMV, or active Epstein-Barr virus infection—as grounds to deny on clinical judgment grounds. Document the absence of these factors explicitly. Don't assume Aetna will infer it from a clean lab panel.

All indications outside congenital athymia are classified as experimental, investigational, or unproven. There is no off-label coverage pathway under this coverage policy.

Coverage Indications at a Glance

Aetna Rethymic Billing Guidelines and Action Items 2026

The effective date of January 23, 2026 is already live. If your team has any cases in the pipeline, these steps apply now.

If your team handles these cases infrequently—or if this is your first Rethymic authorization—talk to your compliance officer before the January 23, 2026 effective date criteria apply to your submission. The dosing documentation and HLA matching requirements are specific enough that a one-time review with someone who knows Aetna's specialty pharmacy process is worth the time.

CPT, HCPCS, and ICD-10 Codes for Rethymic Under CPB 1055

CPT Codes Related to Rethymic Coverage

The HLA antibody codes (86828–86835) represent different methodologies and test specificities. Work with your lab team to confirm which specific test is ordered—Aetna may audit the method billed against the method documented. Don't default-bill a single code across all HLA panels without confirming the lab's approach.

Key ICD-10-CM Diagnosis Codes

The exclusion codes are as important to your charge capture and prior auth workflow as the covered diagnosis. Train your coding team to flag any of the D81.x, B20, B25.9, P35.1, or Z21 codes in the problem list before a Rethymic authorization goes out.