Modifier 33 was created by the American Medical Association in response to the Affordable Care Act to identify services provided for preventive intent. It is appended to CPT/HCPCS codes that could be either preventive or diagnostic/treatment (for example, a colonoscopy or a lipid panel) when the service aligns with USPSTF A or B recommendations or other mandated preventive services. The modifier signals to payers that patient cost‑sharing (copayments, deductibles, coinsurance) should be waived under ACA requirements when the payer recognizes modifier 33.

Do not append modifier 33 when the code descriptor already denotes a preventive service (for example, screening mammography codes or the preventive E/M codes) or when billing Medicare. Medicare generally does not recognize or accept modifier 33 and may deny claims that include it; Medicare instead uses separate preventive G‑codes or inherently preventive code descriptors. Because payer acceptance varies, practices should check payer-specific guidance and document the preventive intent in the medical record so the claim can be supported if audited.

In practical billing scenarios, append modifier 33 to a service code when the original intent was preventive even if a therapeutic intervention occurs during that encounter (for example, a screening colonoscopy where a polyp is removed). When accepted by private payers, modifier 33 does not change the allowed amount for the service but affects patient responsibility by triggering waiver of cost‑sharing. Always confirm whether each payer recognizes modifier 33 before relying on it to eliminate patient cost‑share.