GPO, MSO, IDN, ACO: what each one actually does

GPO, MSO, IDN, ACO: what each one actually does

These four acronyms get used as though they were interchangeable, or as though they sat on a ladder from small to large. They are not the same kind of thing at all. Each one answers a different question, and the confusion is expensive, because a practice that thinks it already has one of these covered often has something else entirely.

Here is what each does, in one line, before the detail.

A GPO negotiates what you pay for things. An MSO runs the business functions of your practice. An IDN owns the pieces of the care delivery system. An ACO carries financial accountability for a population's cost and quality.

Purchasing. Administration. Ownership. Risk. Four different questions.

GPO: what you pay

A group purchasing organization is a purchasing agent. It aggregates the volume of many buyers and negotiates supply and service contracts on their behalf. Members buy at the negotiated price without changing distributors or ordering process.

The GPO does not own you, employ you, manage you, or take on your clinical or financial risk. Its only job is price and terms.

It is also usually free to the member, because suppliers pay the GPO an administrative fee on member purchases rather than the member paying dues. Federal law does not cap that fee. It requires a written member agreement that either states the fee is 3 percent or less of the purchase price, or, if it is more, states the actual or maximum amount each vendor will pay (42 CFR 1001.952(j)).

Adoption is nearly universal above a certain size. Virtually all of the roughly 7,000 hospitals in the United States belong to at least one GPO, and hospitals belong to two to four on average (HSCA). Belonging to more than one is normal.

MSO: how the business runs

A management services organization provides the non clinical side of running a practice. The Milbank Memorial Fund defines it as an entity that provides non clinical services such as billing, human resources, payer contracting, and information technology support to physician practices (Milbank Memorial Fund).

In practice MSOs fall along a spectrum, and the spectrum is the important part. Milbank groups their functions into three: administrative service provision, network aggregation, and capital investment and control.

At the light end, an MSO is a vendor. It handles revenue cycle or credentialing and leaves your governance alone.

At the heavy end, an MSO aggregates practices into networks for payer contracting, supplies capital, acquires practice assets, and takes over operations. Milbank notes that MSOs at this end may assume increasing control over a practice's governance and decision making, and that the resurgence of risk based reimbursement has driven significant corporate investment into aggregation focused MSOs (Milbank Memorial Fund).

Two organizations can both be called an MSO and be almost nothing alike. If someone tells you they work with an MSO, you have learned very little until you ask which end of the spectrum.

IDN: who owns what

An integrated delivery network is an ownership structure. A single organization owns or directly controls multiple points of care delivery, typically hospitals plus employed physician groups plus ancillary services, and sometimes an insurance arm.

The defining characteristic is common ownership. That is what separates an IDN from a network where independent parties agree to cooperate. Ownership brings a single balance sheet, which is why purchasing inside an IDN was historically simple and why purchasing programs designed for IDNs translate poorly to networks built on affiliation instead of ownership.

ACO: who carries the risk

An accountable care organization is a risk structure. CMS describes Medicare Shared Savings Program ACOs as groups of doctors, hospitals, and other health care providers who collaborate to give coordinated high quality care to people with Medicare, focusing on delivering the right care at the right time while avoiding unnecessary services and medical errors. When an ACO both delivers high quality care and spends more wisely, it may share in the savings it achieves (CMS).

Note the word collaborate. An ACO generally does not own its participants. It is accountable for their collective cost and quality performance without controlling them, which is a genuinely odd position and the source of most of the operational difficulty in value based care.

The scale is substantial. For 2026, CMS reported 511 Shared Savings Program ACOs serving 12.6 million people in Traditional Medicare with more than 700,000 participating providers and organizations, 74 ACO REACH ACOs serving an estimated 1.7 million people, and an overall estimate of 14.3 million beneficiaries receiving ACO coordinated care (CMS).

Where they overlap, and why that trips people up

The categories are distinct but real organizations wear several hats.

A heavy MSO frequently aggregates its practices into an ACO, so the same corporate parent is doing administration and carrying risk. Milbank identifies Aledade and Pearl Health as investor backed MSOs that primarily aggregate practices into ACOs while supplying technology and operational guidance (Milbank Memorial Fund).

An IDN usually operates an ACO. An MSO often offers purchasing as one of its services, which is where practices conclude they already have GPO coverage when what they have is a purchasing function bundled into a management contract, sitting on a contract portfolio someone else negotiated.

And clinically integrated networks belong in this picture too. CINs and ACOs are the legal structures that let otherwise independent providers contract jointly without violating antitrust rules against price fixing (Milbank Memorial Fund). They are the mechanism that makes independence and collective action compatible.

The practical test

If you are trying to work out what you actually have, ask four questions.

Who negotiates the price of what I buy, and can I see the contract? That is the GPO question.

Who runs my billing, credentialing, and payer contracts, and how much of my governance did that cost me? That is the MSO question.

Who owns the entities in this network? That is the IDN question.

Whose financial result moves when my patients cost more than expected? That is the ACO question.

Most practices can answer the last three and go quiet on the first. That silence is usually worth 10 to 18 percent on supply spend, which is the range the Leibowitz analysis found for providers buying through GPOs (HSCA).