Ambulatory Health Care Services (U.S.) — NAICS 621 Subsector Primer

A Histometrics rollup primer for a general investing audience — relevant to both public-market and private investors. NAICS = North American Industry Classification System, the U.S. government's standard scheme for grouping businesses by activity. This page covers the three-digit subsector 621, which sits inside sector 62 (Health Care and Social Assistance) alongside hospitals (622), nursing and residential care (623), and social assistance (624). It synthesizes seven already-written child primers and adds our ground-truth federal statistics for the combined level.

1. Overview

"Ambulatory" is the medical word for care delivered without an overnight hospital admission — walk-in, same-day, outpatient, or at-home. NAICS 621 is the federal drawer for that entire tier of American medicine: the doctor's office, the dentist, the physical therapist, the dialysis chair, the freestanding lab, the visiting nurse, and the ambulance. It is the front line of the health system — where most people touch health care most often — and it is enormous: roughly $1.41 trillion of receipts (revenue), 8.7 million paid workers across about 702,000 establishments, and $621 billion of annual payroll [1]. On employment alone it is one of the largest subsectors in the entire U.S. economy.

But 621 is not a single market you can buy — it is a bracket holding seven very different industries (its four-digit "industry groups") that share only a delivery model. They run on unlike economic engines, answer to different regulators, are owned by wildly different mixes of people (public companies, insurers, private equity, nonprofits, governments, and hundreds of thousands of solo clinicians), and offer investors radically uneven ways in. Physician offices are 45% of the money; ambulances and blood banks together are under 5%. The cleanest listed pure-plays in the whole subsector sit in two small corners (dialysis and surgery centers, and freestanding labs), while its single largest child — physician offices — has almost no clean public vehicle at all.

The distinctive value of a subsector view, then, is the contrast across the seven children (Section 2): which are big, which are growing, who owns them, how concentrated they are, and how (if at all) an outsider can get exposure. The one thread that ties them together is a durable structural tailwind — the decades-long site-of-care shift, in which payers and patients keep moving treatment out of expensive hospitals into these cheaper ambulatory settings. Sections 3–10 then treat the level as a whole.

2. What's inside — the seven children, and how they differ

NAICS 621 splits into seven four-digit industry groups. The table below is the heart of this primer; it is ordered by revenue, largest first. (Acronyms are defined in the note beneath the table and again on first substantive use in later sections.)

Notes. Shares are of the level's 2022 receipts and 2023 employer establishments (Section 3). MD = Doctor of Medicine; DO = Doctor of Osteopathic Medicine; HMO = health maintenance organization; EMS = emergency medical services; PE = private equity; DSO = Dental Support Organization; "payvider" = a health insurer (payer) that also owns care providers. The whale is physician offices (6211) at nearly half of all revenue; the smallest by revenue is "other ambulatory" (6219). Tickers are identifiers, not recommendations, and are defined in Sections 4 and 10.

How to read the contrast. Four cuts matter most to an investor:

• Size is lopsided. One child — physician offices (6211) — is 45% of the subsector's revenue. The top three (physicians, outpatient centers, dentists) are 72%. So sweeping statements about "ambulatory care" are mostly statements about physician offices, with dialysis/surgery and dentistry next.
• Public investability runs opposite to size. The biggest child (physician offices) has the weakest direct public exposure, while the cleanest listed pure-plays sit in small, capital-heavy, concentrated corners — dialysis and surgery (inside 6214) and freestanding labs (6215). The rule that recurs across the subsector: the more concentrated and capital-heavy a business, the cleaner the public exposure; the more it is a cottage industry of solo clinicians, the more it is private or un-investable.
• "Who owns it" spans the full ownership spectrum — from tax-funded government (municipal ambulance, county mental-health clinics), through non-tradable nonprofits (Kaiser, blood centers, Planned Parenthood, community health centers), to a fragmented base of physician- and clinician-owners, up to PE roll-ups and, at the very top, a handful of vertically integrated insurers. No other subsector of health care mixes so many owner types under one code.
• Growth is not shared. Behavioral health, ambulatory surgery, home-based care, and value-based senior care are the clear structural growth stories; dialysis and blood banking are flat annuities; the family-planning brick-and-mortar footprint is shrinking. A single "ambulatory" forecast would be wrong for at least one child.

3. How big it is — the level as a whole

Our ground-truth federal figures for NAICS 621. Note the reference-year split: firms, receipts, and concentration are from the 2022 Economic Census (EC); establishments, employment, and payroll are from 2023 County Business Patterns (CBP). These are two different federal programs and two different years — do not read 2022 receipts against 2023 payroll as a margin.

How the children add up — a clean check on the data:

Establishments and employment sum to the level totals exactly, and receipts sum to within a rounding whisker ($1,409.88B vs. $1,409.92B) — strong confirmation the pieces are measured consistently. Annual payroll sums to $621.2B against the level's $621.08B (rounding on the approximate child figures). Firm counts sum to ~509,500 against the level's 505,862 — about 3,600 higher, exactly as expected, because a company operating in two children is counted in each child but only once at the subsector level [1].

A few shape statistics for the level:

• ~$2.79 million of receipts per firm and ~12.4 employees per establishment — small-business economics on average, stretched across seven very different shapes.
• ~$71,400 average annual pay and payroll at ~44% of receipts (across the vintage mismatch) — the fingerprint of a labor-heavy professional-services subsector where the scarce input is licensed clinicians, not hard assets or brands.
• Revenue per worker diverges sharply by child — highest in labs (~$244,000, capital-heavy diagnostics) and physician offices (~$231,000), lowest in home health (~$71,000), which is the purest labor-for-time service. Home health is only 8% of revenue but 19% of employment — the most people per dollar; physician offices are 45% of revenue on 32% of employment. That gap is the difference between capital-and-billing-leveraged medicine and hands-on caregiving.

The aggregation trap — the single most important caveat at this level. Read literally, 621 looks like near-perfect competition: an HHI (the sum of squared market shares antitrust regulators use, where anything under 1,500 is "unconcentrated") of just 10.3, and the four largest firms holding only 4.1% of revenue [1]. That number is real but deeply deceptive, in two layers. First, it is lower than every child's own HHI except dentistry — because blending seven industries whose market leaders don't overlap (the biggest dialysis company, the biggest lab, the biggest home-health platform, and the biggest physician group are different firms) mechanically dilutes every company's share. Second, even the children's low numbers hide pockets of genuine local and sub-industry dominance: a near-monopoly HMO (Kaiser), a two-company dialysis market (~74% of facilities), and two national labs that hold roughly a third of medical-lab revenue — all invisible in the blended 10.3 [5][6]. Never read the subsector HHI as evidence the market is competitive. Judge concentration child by child, and inside the biggest children, sub-industry by sub-industry.

Undercount caveats — the level understates its true footprint. These are employer-only counts: CBP and the Economic Census exclude the self-employed and no-payroll ("nonemployer") businesses, and most government-run operations [1]. That gap is unusually large here because every child skews toward small, individually owned, nonprofit, or government operators:

• Solo clinicians are largely invisible. Cash-only psychiatrists, solo therapists, single-doctor chiropractic and optometry offices, and one-owner dental and physician practices often file as nonemployers and never appear.
• Government and nonprofit care spills into other codes or off the books. Municipal, fire-based, and volunteer ambulance services (the plurality of 911 response), county mental-health clinics, VA/military outpatient sites, community health centers, and self-directed Medicaid home-care workers are mostly excluded [7][8].
• Physician-delivered care leaks upward. As hospitals buy practices, office revenue reclassifies into hospitals (622); the federal health accounts attribute $978 billion to "physician and clinical services" nationally in 2023, far above this subsector's $640 billion of physician-office receipts, precisely because so much physician work now sits inside hospital and integrated-care codes [10].

Our ground-truth file provides no subsector-wide figure for patient volume, payer mix, utilization, margin, or nonemployer receipts, and no growth series — so none is asserted here. Treat the $1.41 trillion and ~702,000 establishments as a solid floor for the employer-based ambulatory economy, never as a full census of the activity, and never as evidence the field is competitively structured.

4. The investable universe — where value concentrates across the children

Public-market value is not spread across 621 in proportion to revenue. It clusters in a few corporations inside a few children and thins to almost nothing elsewhere. There is no exchange-traded fund (ETF) and no clean public proxy for the subsector as a whole. Every listed name below mixes these ambulatory lines with hospitals, insurance, pharmacy, devices, or physician practices, and none reports a clean NAICS-621 revenue line — tickers signal exposure, not pure-play status, and are confined to this section and Section 10.

Where the listed pure-plays and near-pure-plays actually are:

• Dialysis and surgery (inside 6214) — the cleanest exposure in the subsector. Dialysis: DaVita (DVA) and Fresenius Medical Care (FMS), a near-duopoly. Ambulatory surgery: Surgery Partners (SGRY) as the pure-play and Tenet Healthcare (THC) via its surgery-center platform [5].
• Freestanding labs (6215) — genuine public depth on the lab half. The two national majors, Quest Diagnostics (DGX) and Labcorp (LH), plus a real cohort of molecular/genomic testers; imaging is thinner, led by RadNet (RDNT) [6].
• Rehab and mental health (inside 6213) — genuine operators in two of five. Outpatient rehab: U.S. Physical Therapy (USPH) and Select Medical (SEM). Outpatient mental health: LifeStance Health (LFST) [4].
• Home health (6216) — thin and shrinking. A handful of listed operators — Addus (ADUS), Aveanna (AVAH), Pennant (PNTG), Option Care (OPCH), BrightSpring (BTSG) — most now blended with hospice, pharmacy, or personal care [7].
• Ambulance (inside 6219) — one newly opened window. Global Medical Response (GMRS), the largest U.S. EMS operator, listed in 2026; the rest of the child is indirect only (plasma makers, organ-preservation tech) [8].
• The whale (6211), dentistry (6212), and the rest — indirect or private. Physician offices have no clean pure-play; dentistry has no public office owner at all (only suppliers and distributors); optometry, family planning, and blood/organ banks offer only diluted adjacencies (eyewear retailers, contraceptive makers, plasma-medicine firms) [2][3].

The one security that spans the most of 621 is the diluted "payvider." UnitedHealth's Optum, Humana's CenterWell, and CVS Health now reach across multiple children — employing tens of thousands of physicians (6211), running surgery and value-based clinics (6214), and owning the largest home-health platforms (6216, after Optum absorbed LHC Group and Amedisys). They are the closest thing to a "buy the subsector" vehicle, and their expansion is itself the dominant consolidation story here — but each is buried inside a giant insurer, so you are buying health insurance with an ambulatory-care overlay, not the overlay by itself [5][7].

The cross-cutting pattern: the great majority of the assets in 621 — independent and PE-owned practices, physician-owned surgery centers, nonprofit health systems and blood/family-planning centers, county and municipal providers, small home-health agencies, and the medical real estate under all of them — are private or non-tradable, and no complete cap table exists. For the full company-by-company map in any child, read that child's primer.

5. How the money works

Despite their differences, all seven children run on one master identity:

Revenue ≈ volume × net reimbursement per unit — visits, treatments, procedures, tests, scans, transports, or covered members — earned across a payer mix (commercial insurance, Medicare, Medicaid, self-pay, government grants) against a cost base dominated by clinician labor.

Three levers recur across the whole subsector:

• Utilization is the profit lever. These are labor-heavy businesses with a large fixed base, so the operating number that matters is how full the schedules, chairs, rooms, scanners, therapy panels, or member books are kept. A skilled human hour is the input, and an empty appointment slot cannot be inventoried and sold later. Clinic count is a weak measure — same-center (organic) volume, payer mix, and cost per unit are what matter in every child.
• Payer mix is destiny. Commercial (employer) insurance pays a multiple of what government payers pay for the same service, so the commercially insured minority generates a disproportionate share of profit and cross-subsidizes the Medicare/Medicaid book. Government payers (which set or anchor most prices) leave operators with minimal pricing power, so margins live or die on volume and mix, not list price.
• Labor is the product and the largest controllable cost — commonly 50–70% of revenue. Because capital intensity is generally low and licensure creates local, not national, moats, barriers to entry are low and competition is local.

On top of that shared base sit distinct engines worth flagging because they change the math: a fee-for-service (FFS) billing model priced off Medicare fee schedules across most offices and labs; a fast-growing value-based-care / capitation overlay in physician offices and HMO/integrated care, where a group takes a fixed per-member-per-month (PMPM) payment and profits on the spread between premium and the cost of care (gauged by the medical loss ratio); policy-funded, cost-recovery economics in the safety-net and blood/organ segments (grants, 340B drug discounts, sliding-scale fees); and a cost-of-readiness model in ambulance, where owners earn the spread between collections per transport and the cost of standing ready 24/7. The one economic sentence that spans the subsector: almost everything here is priced by someone other than a walk-in customer — a government fee schedule, an insurer contract, an employer, or a downstream drug maker — so revenue quality turns on payer mix and contract terms, not retail demand. Our federal file publishes no subsector-wide margin, utilization, or payer-mix figure, so any such numbers in the child primers come from trade sources and are flagged there as estimates.

6. What drives demand

Demand across 621 is largely demographic and clinical, which makes it steadier than the broad economy — with a discretionary overlay only on the cash-pay pieces (elective dentistry, cosmetic and wellness care). The shared drivers:

• Aging population — the universal tailwind. The U.S. population aged 65 and older reached 61.2 million (18.0% of all Americans) in 2024, and older adults now outnumber children in a growing number of states [9]. Seniors use far more of every service here — more physician visits, more joint and cataract surgery, more dialysis, more home care, more tests and scans — so rising Medicare enrollment lifts the entire subsector.
• Chronic disease. Diabetes, cardiovascular disease, and cancer generate recurring care — repeat labs, imaging, dialysis, foot and eye care, and chronic-care management — not one-offs.
• The site-of-care shift — the master structural story. Better minimally invasive techniques, anesthesia, remote monitoring, and deliberate payer steering keep moving care out of costlier hospitals into these ambulatory settings. This is the single biggest reason the whole subsector grows faster than health spending overall [5].
• Behavioral-health demand outrunning supply. Falling stigma, insurance parity, and telehealth normalization have pushed mental-health demand far past the supply of clinicians — the strongest structural demand story in the subsector, across both physician psychiatry (6211) and non-physician therapy (6213, 6214) [4][5].
• Preference to age in place, and coverage expansion. An overwhelming preference to receive care at home underwrites home health (6216), while Medicare Advantage growth, mental-health parity, and scope-of-practice expansion (letting optometrists and nurse practitioners do and bill for more) all convert latent need into billable volume [7].

The binding constraint everywhere is clinician supply. Demand is rarely the problem; staffing is. Strong projected job growth across these professions coexists with chronic shortages, which means demand can convert into higher wages rather than higher owner profit — the recurring tension of the whole subsector.

7. Regulation

Ambulatory care is one of the most heavily regulated revenue models in the economy, and the regulation runs on two axes: a federal payment-and-fraud overlay common to almost every child, and profession-by-profession, state-by-state licensure that varies enormously beneath it.

The common federal spine:

• The Centers for Medicare & Medicaid Services (CMS) sets or anchors the prices. The Medicare Physician Fee Schedule (physician and other-practitioner offices, imaging), the Clinical Laboratory Fee Schedule (labs), the dialysis bundle and ambulatory-surgical-center rates (outpatient centers), the Home Health Prospective Payment System, and the Ambulance Fee Schedule together set the effective price floor for most of the subsector — usually with no automatic inflation update, so a skinny annual rate notice hits margins directly [2][5][6][7][8].
• Fraud-and-abuse law governs money that touches referrals — the Anti-Kickback Statute, the physician self-referral Stark Law, and the False Claims Act — across the whole level, alongside HIPAA (the Health Insurance Portability and Accountability Act) protecting the vast patient data these operators hold [2][6].
• Corporate Practice of Medicine (CPOM) doctrines in roughly two-thirds of states bar non-clinicians from owning a clinical practice, which is why outside capital enters every child through a management-services organization (MSO) paired with a clinician-owned professional entity — investors own the management company, licensed clinicians own the practice. This is a fast-rising area of state legislative and antitrust attention [2][4].

The state-and-profession layer is where the subsector fragments: dental and other-practitioner scope of practice, Certificate-of-Need laws that gate new dialysis and imaging facilities, Drug Enforcement Administration rules on telehealth prescribing of controlled substances (binding on psychiatry and addiction care), FDA biologics licensure for plasma, Title X and post-Dobbs state law for family planning, and the No Surprises Act's ban on balance billing (which covers air but not ground ambulances). The investor takeaway: treat licensure and ownership structure as transaction-level diligence, not boilerplate — the legal structure varies by profession and state and is often the difference between a compliant deal and an unwind.

8. Consolidation

By the federal numbers, 621 is one of the least-concentrated subsectors in the economy — CR4 of 4.1% and HHI of 10.3 [1]. It is, at the national level, a cottage industry of small practices, and that fragmentation is the entire thesis for consolidators, who apply one playbook (professionalize billing, purchasing, staffing, and back office across many small offices) that already reshaped dentistry, dermatology, and gastroenterology. But the average hides very different innings by child, and the direction of travel is unmistakably toward consolidation:

• Physician offices (6211) are consolidating fastest by dollars — by 2024 roughly 78% of U.S. physicians were employed by hospitals, insurers, or corporate/PE-backed entities rather than independent, up from a slim majority a decade earlier [5][11].
• The payviders are the apex consolidators — insurers buying physician groups, surgery centers, and the two largest home-health platforms, integrating payment and delivery under one roof [5][7].
• Dentistry, rehab, mental health, and the specialty offices are PE roll-up stories at different stages — rehab the veteran (and a cautionary leverage bust), mental health the current hotspot, dentistry and optometry mid-cycle, chiropractic and podiatry early [3][4].
• Labs and dialysis are consolidating from already-concentrated tops, while blood centers and the family-planning safety net are consolidating defensively — merging to survive against flat volumes and funding pressure [5][6][8].

Two things are true subsector-wide. Competition is local even when ownership is national — patients dialyze, get surgery, and see a therapist near home — so a firm with a small national share can dominate a county, and the "unconcentrated" statistics coexist with real local pricing power. And regulatory scrutiny is rising: a cross-government inquiry by the Federal Trade Commission, Department of Justice, and Department of Health and Human Services into corporate and PE ownership of healthcare providers, plus forced divestitures in insurer deals (the ~164 divestitures the DOJ required in the UnitedHealth–Amedisys home-health merger), have put roll-ups squarely on regulators' radar [7][12].

9. Key risks

The subsector shares a common risk profile, with a different emphasis in each child:

• Reimbursement dependence (whole level). Most revenue is a government-set or government-anchored price with little pricing power behind it; a skinny annual update to any fee schedule — physician, lab, dialysis, home-health, ambulance — flows straight to margin [2][5][6][7].
• Labor is the binding constraint (whole level). Clinician scarcity, wage inflation, vacancy time, and burnout cap both capacity and margin; strong demand can turn into higher pay rather than higher profit [9].
• Commercial-mix erosion. The profit engine is the commercially insured minority; insurer network narrowing, denials, and site-neutral payment reform all threaten the cross-subsidy that funds the government-paid book [5].
• Value-based / capitation execution risk. Full-risk operators eat cost overruns — a surge in senior utilization pushed several capitated providers into bankruptcy in 2023–2024, a warning that runs through the physician-office and HMO corners [5].
• CPOM / ownership and antitrust risk. State corporate-practice rules and rising scrutiny can limit or invalidate PE ownership, MSO control, or fee arrangements in any child; roll-ups relying on debt face distress when rates rise and reimbursement falls together (rehab's leverage bust is the standing scar) [4][12].
• Policy and funding whiplash. Medicaid cuts (the 2025 reconciliation law is estimated to trim federal Medicaid spending materially over a decade), Title X freezes, and defunding riders make the safety-net and home-care segments policy-cyclical rather than economy-cyclical [7].
• Discretionary / cyclical exposure on the cash-pay pieces — elective dentistry, cosmetic and wellness care, premium eyewear — deferrable in a downturn.
• Data, disclosure, and measurement risk. The federal statistics exclude nonemployer and government activity and are not a facility, visit, or procedure census; and listed operators blend these lines with hospitals, insurance, pharmacy, and devices, so a ticker can look like ambulatory exposure while most earnings come from elsewhere. Never read the level HHI as evidence of a competitive market (Section 3).

10. How to invest, and the outlook

There is no single ticker for NAICS 621, and its headline "unconcentrated" statistics should never anchor a thesis. Invest child by child — and, inside the biggest children, sub-industry by sub-industry — matching the route to how each business is actually owned.

Public routes — narrow and clustered. The cleanest listed exposure sits in a few concentrated corners: dialysis (DVA, FMS) and ambulatory surgery (SGRY, THC) inside outpatient centers; the lab majors (DGX, LH) plus molecular testers, with imaging thinner (RDNT); outpatient rehab (USPH, SEM) and mental health (LFST); a thin, shrinking home-health set (ADUS, AVAH, PNTG, OPCH, BTSG); and one near-pure-play ambulance operator (GMRS). The broadest single-name proxies are the diluted payviders — UnitedHealth/Optum (UNH), Humana/CenterWell (HUM), CVS Health (CVS) — which span physician offices, outpatient clinics, and home health, but are dominated by their insurance earnings. Dentistry, physician offices, optometry, and family planning offer no clean public office owner — only adjacencies (dental and eyewear suppliers, contraceptive makers, telehealth). For every listed name, isolate the actual ambulatory earnings from the surrounding hospital, insurance, pharmacy, device, or physician-practice businesses before comparing valuation multiples, EBITDA (earnings before interest, taxes, depreciation, and amortization), or dividend yield — and judge it against the medical-cost cycle and government-payment risk, not against clinic-count growth. There is no ETF dedicated to the subsector.

Private routes — where most of the economics actually sit. Owning 621 mostly means owning practices and facilities: direct practice ownership and buy-ins (physician, dental, and specialty offices); PE-backed MSO/DSO platforms across every child; physician-ownership stakes in surgery centers; value-based-care platforms; PE-backed and franchised roll-ups in rehab, mental health, chiropractic, podiatry, home health, and occupational health; and the medical real estate under all of them, accessible through medical-office landlords and healthcare real estate investment trusts (REITs). In every child, underwrite the local unit economics first — normalized owner cash flow (not headline revenue), same-clinic collections and visit trends, payer contracts and denial rates, clinician retention and succession, and the MSO/management-agreement terms and physician-control provisions that make a deal compliant. The large nonprofit and government segments — Kaiser and community health centers, county mental-health providers, blood and organ banks, the family-planning safety net, and municipal EMS — are not investable; the only adjacent security is tax-exempt financing.

Outlook (editorial judgment). The structural direction — cheaper, more convenient, more coordinated care moving out of hospitals and into offices, clinics, and the home — is durable and favorable across all of 621, and it is powerfully reinforced by an aging, chronically ill population. But the seven children will not move together: behavioral health, ambulatory surgery, home-based care, value-based senior care, and precision diagnostics are the clearest growth stories; dialysis and blood banking are resilient flat annuities; dentistry and the cash-pay wellness edges carry a discretionary tilt; and the family-planning footprint is contracting. This is a low-margin, labor-bound, reimbursement-driven, deeply fragmented subsector that will stay fragmented for years, where winners are decided by unglamorous capabilities — recruiting and keeping clinicians, negotiating payer economics, and running compliant billing at scale — not by financial engineering. The most investable structural story is less "healthcare demand" (steady but not explosive) than the professionalization and roll-up of a still-fragmented cottage industry, advancing against a rising wall of state regulation and antitrust scrutiny. Our ground-truth federal file for NAICS 621 contains no revenue-growth forecast, margin benchmark, or complete ownership census, and none is asserted here. The recurring mistake is treating this three-digit code as one industry, or its "competitive" HHI as real. It is seven unlike businesses in one drawer — read the child primers (6211, 6212, 6213, 6214, 6215, 6216, 6219) before acting on any of them.

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Sources

Section 3 figures for NAICS 621 are our ingested ground-truth federal statistics (stats-621.md), drawn from the U.S. Census Bureau's 2023 County Business Patterns (establishments, employment, payroll) and 2022 Economic Census (firms, receipts, concentration, HHI). Child-level figures, tickers, and all external facts are synthesized from the seven child primers, which carry the full source lists.

1. U.S. Census Bureau, County Business Patterns 2023 (NAICS 621: establishments 701,862; employment 8,697,480; annual payroll $621.08B; Q1 payroll $148.04B) and 2022 Economic Census — Concentration of Largest Firms (NAICS 621: receipts $1,409.92B; firms 505,862; CR4 4.1% / CR8 7.2% / CR20 12.4% / CR50 18.4%; HHI 10.3). Histometrics ground-truth extract. https://www.census.gov/programs-surveys/cbp.html; https://data.census.gov/table/ECNSIZE2022.EC2200SIZECONCEN
2. Histometrics child primer, NAICS 6211 — Offices of Physicians (receipts $640.4B; CR4 6.1% / HHI 18), synthesizing CMS Physician Fee Schedule rules, AMA practice-arrangement data, and value-based-care/CPOM sources cited therein.
3. Histometrics child primer, NAICS 6212 — Offices of Dentists (receipts $160.3B; CR4 3.9% / HHI 5.5), synthesizing DSO/PE consolidation, PPO-reimbursement, and dental-supplier sources cited therein.
4. Histometrics child primer, NAICS 6213 — Offices of Other Health Practitioners (receipts $125.8B; CR4 5.2% / HHI 12.4), synthesizing BLS occupational projections, USPH/SEM/LFST/JYNT filings, and PE-roll-up sources cited therein.
5. Histometrics child primer, NAICS 6214 — Outpatient Care Centers (receipts $222.5B; CR4 22.6% / HHI 163.4; dialysis duopoly, Kaiser HMO near-monopoly), synthesizing DaVita/Fresenius/Surgery Partners/Tenet and payvider filings, MedPAC, and CMS rate rules cited therein.
6. Histometrics child primer, NAICS 6215 — Medical and Diagnostic Laboratories (receipts $81.1B; CR4 24.9% / HHI suppressed; lab majors ~36% of medical-lab revenue), synthesizing Quest/Labcorp/RadNet filings, CLFS/PAMA, and molecular-tester sources cited therein.
7. Histometrics child primer, NAICS 6216 — Home Health Care Services (receipts $114.2B; CR4 9.1% / HHI 42.2), synthesizing Addus/Aveanna/Pennant/Option Care/BrightSpring and payvider filings, CMS Home Health PPS, MedPAC, PHI workforce data, and the DOJ–Amedisys divestiture cited therein.
8. Histometrics child primer, NAICS 6219 — Other Ambulatory Health Care Services (receipts $65.6B; CR4 19.3% / HHI 143.3; ambulance + blood/organ + miscellaneous), synthesizing GMR/DocGo/ModivCare and plasma/organ-tech sources, CMS Ambulance Fee Schedule, and FDA biologics rules cited therein.
9. U.S. Census Bureau, Older Adults Outnumber Children in 11 States (65+ population 61.2 million, 18.0% in 2024), 2025. https://www.census.gov/newsroom/press-releases/2025/older-adults-outnumber-children.html
10. Centers for Medicare & Medicaid Services, National Health Expenditure Fact Sheet (2023 data; physician and clinical services ~$978B). https://www.cms.gov/data-research/statistics-trends-and-reports/national-health-expenditure-data/nhe-fact-sheet
11. Physicians Advocacy Institute / Avalere and American Medical Association, physician-employment and practice-arrangement data (~78% of physicians employed by hospitals, insurers, or corporate/PE entities by 2024). https://www.physiciansadvocacyinstitute.org/
12. U.S. Federal Trade Commission, Department of Justice, and Department of Health and Human Services, Cross-Government Inquiry into Corporate and Private-Equity Ownership of Health Care Providers (2024), and DOJ, UnitedHealth–Amedisys divestiture (2025). https://www.ftc.gov/; https://www.justice.gov/opa/pr/justice-department-requires-broad-divestitures-resolve-challenge-unitedhealths-acquisition