Opinion for the Court filed by Circuit Judge TATEL.
Under Medicaid’s “buy-in” program, states must use Medicaid funds to enroll certain needy, Medicare-eligible individuals in Medicare’s Part B supplemental insurance program. In this case, we must determine whether the buy-in program requires states to reimburse Medicare providers the entire twenty percent co-payment that patients normally pay for a particular service under Part B, or whether, as the United States Department of Health аnd Human Services has long permitted, states may limit reimbursement to the almost always lower Medicaid rate for the same service. Relying on HHS policy, the District of Columbia began capping copayment reimbursement at Medicaid rates in 1990. Appellants, a group of District of Columbia doctors, challenge the District’s policy, arguing that until Congress amended the buy-in statutes in 1997, the law required the District to reimburse them at Medicare rates. Finding the pre-1997 statutes ambiguous as to state copayment reimbursement obligations, and finding HHS’s interpretation reasonable, we affirm the district court’s grant of summary judgment for the District.
I
Enacted in 1965, Medicare finances medical procedures for people over 65 and people with disabilities. See 42 U.S.C- §§ 1395-1395ccc (1994). Medicare has two *78 parts, Part A and Part B. Part A provides reimbursement for inpatient hospital care and related post-hospital, home health, and hospicе care. See id. §§ 1395c to 1395Í-4. Enrollment in Part A is automatic. Part B is voluntary. It provides supplemental insurance for hospital out-patient services, physician services, and other medical services not covered under Part A. See id. §§ 1395j to 1395w-4. Part B imposes cost-sharing obligations on people who choose to participate. These include an annual deductible, monthly premiums, and — of particular relevance to this case— copayments. Copayments consist of twenty percent of the “reasonable charge” for the service rendered, an amount determined annually by HHS. See id. § 1395Z(a). Medicare directly reimburses Part B providers for the remaining eighty percent. See id.
Also enacted in 1965, Medicaid, a cooperative federal-state program, finances medical care for the poor, regardless of age. See 42 U.S.C. §§ 1396-1396v (1994). Participating states must establish financiаl eligibility criteria, identify covered medical services, develop rate schedules, and submit their plans to HHS for approval. See id. §§ 1396a(a), 1396a(b). HHS approval entitles a state to substantial federal funding, ranging from fifty percent to eighty-three percent of the cost of medical services provided under the plan. See id. § 1396d(b). Doctors and other health care providers are not required to service Medicaid patients, but if they do they must accept reimbursement from the state at its Medicaid rate as payment in full; they may not demand additional payment from patients. See id. §§ 1320a-7b(d), 1396o. State Medicaid rates for any given service are almost always lower than the “reasonable charge” for the same service under Medicare Part B. Indeed, Medicaid rates are often even lower than the eighty percent of the reasonable charge that the fеderal government reimburses Medicare providers.
Medicare and Medicaid intersect with respect to the elderly poor — so-called “dual eligibles.” While these people are eligible to purchase supplemental medical insurance through Medicare Part B, many cannot afford Part B’s premiums, deductibles, and copayments. Medicaid has therefore long allowed states to use Medicaid dollars to enroll dual eligibles in Medicare Part B by paying their cost-sharing obligations. See Pub.L. No. 89-97, § 121(a), 79 Stat. 286, 346 (1965) (codified at 42 U.S.C. § 1396a(a)(15)) (repealed 1988). Because the federal government heavily subsidizes Medicaid, this “buy-in” program enables states to shift a large portion of the cost of caring for the elderly poor to the federal treasury.
In 1986, Congress expanded the buy-in program beyond dual eligibles to include a newly created category of “qualified medicare beneficiaries” (“QMBs”): elderly people not quite poor enough to qualify for Medicaid but who nonetheless met certain neediness criteria. See Pub.L. No. 99-509, § 9403, 100 Stat. 1874, 2053-55 (1986) (codified at 42 U.S.C. §§ 1396a(a)(10)(E), 1396d(p)(l) (1994)). Initially optional, the QMB buy-in program became mandatory in 1988. See Pub.L. No. 100-360, § 301, 102 Stat. 683, 748 (1988) (deleting “at the option of a State” from 42 U.S.C. § 1396a(a)(10)(E)). Also in 1988, Congress redefined the term “QMB” to include dual eligibles. See Pub.L. No. 100-485, § 608(d), 102 Stat. 2343, 2416 (1988).
This appeal presents the following issue: Are Medicare providers рerforming Part B services to QMBs entitled to state reimbursement for the entire twenty percent copayment that a non-QMB Medicare patient would normally pay, or may states limit reimbursement such that providers receive no more than the state’s Medicaid rate for the same service? For example, suppose that the reasonable charge for a given Part B service is $100, but a state’s Medicaid rate for the same sеrvice is only $90. If a Medicare doctor performs that service, the federal government reim
*79
burses the doctor $80, whether or hot the patient is a QMB. If the patient is a QMB, does the buy-in scheme require the state to reimburse the doctor for the patient’s entire $20 Medicare Part B copayment? Or may the state give the doctor only $10 so that total reimburs'ement, including the federal government’s $80, equals $90, the Medicaid rate? If the Medicaid rate for the particular service is $70, may the state refuse to reimburse the doctor at all because the $80 provided by the federal government already exceeds the Medicaid rate?
See Paramount Health Sys., Inc. v. Wright,
Four statutory provisions added to the buy-in scheme in 1986 frame this issue. Read alone, two suggest that states must use Medicaid funds to reimburse Medicare providers performing Part B services to QMBs for the full twenty percent copayment ($20 in the above example) for which non-QMB Medicare patients would be responsible. Under 42 U.S.C. § 1396a(a)(10)(E)(i), a state Medicaid plan “must” provide for “making medical assistance available for medicare cost-sharing ... for qualified medicare beneficiaries.” Section 1396d(p)(3)(D) in turn defines “medicare cost-sharing” to include Medicare premiums, deductibles, and “[t]he difference between the [80 percent of the reasonable charge that the federal government reimburses providers under Part B] and the amount that would be paid ... if any reference to ‘80 percent’ ... were deemed a reference to 100 percent.’ ” Section 1396a(a)(10)(E)(i)’s mandatory language coupled with section 1396d(p)(3)(D)’s reference to specific percentages suggests that states must use buy-in funds to reimburse providers for the entire twenty percent Part B copayment.
The other two provisions enacted in 1986 suggest a different interpretation. Seсtion 1396a(a)(VIII) provides that “medical assistance made available to [QMBs] ... shall be limited to medical assistance for medicare cost-sharing. ..., subject to the provisions of [section 1396a(n) ].” Before its amendment in 1997, section 1396a(n), entitled “Payment amounts,” in turn provided:
In the case of [Medicaid funds provided] for medicare cost-sharing respecting the furnishing of a service or item to a qualified medicare beneficiary, the State рlan may provide payment in an amount ... that results in a sum of such payment amount and any amount of payment made [by the federal government under Medicare Part B for] the service or item exceeding the amount that is otherwise payable under the State [Medicaid] plan for the item or service for eligible individuals who are not qualified medicare beneficiaries.
Id. § 1396a(n) (amended 1997) (emphasis added). Section 1396a(n)’s use of the word “mаy” rather than “shall” suggests that states are permitted, not obligated, to reimburse Part B providers above the Medicaid rate — $10 if as in the above example the Medicaid rate were $90, or zero if the Medicaid rate were $80 or less.
Even before the 1986 enactment of these four QMB provisions, HHS had long taken the position that the buy-in scheme required states to reimburse providers for Part B copayments only in an amount equal to the difference, if any, between the Medicaid payment and the eighty percent of the Medicare Part B charge that the federal government pays. See Policy Information Memorandum from Director, Bureau of Program Policy, Department of Health and Human Services, to Associate Regional Administrators (Sept. 29, 1981) (“California’s payment of amounts only up to its standard maximum allowable rate under its [Medicaid] program is acceptаble.”); Policy Information Memorandum No. 6 from Associate Commissioner for Program Coordination, Department of Health, Education and Welfare, to Health Services Administration Regional Staff (Mar. 4, 1971) <“[T]he [state] agency is not necessarily obligated to pay the full *80 amount of the deductibles and co-insurance costs according to the rates established under [Medicare], but only that amount which will satisfy the requirement for payment in full acсording to the [Medicaid] method of payment.”)- HHS reiterated this policy following the 1986 amendments to the buy-in scheme. See Dep’t of Health & Human SvCS., State Medicaid Manual § 3490.14 (1991).
In 1990, the District of Columbia (a state for Medicaid purposes) amended its Medicaid program to limit reimbursement for QMB Part B copayments to the Medicaid rate. See 37 D.C.Reg. 5593 (1990). HHS approved the District’s plan in 1991. The District implemented its plan for more than six years without challenge.
In 1997, a coalition of D.C. doctors and the Medical Soсiety of the District of Columbia sued the city in the Superior Court for the District of Columbia, claiming that the buy-in statutes required states to pay QMB Part B copayments in full. Alleging breach of contract, unjust enrichment, and promissory estoppel, the doctors sought retroactive reimbursement. The doctors also sued the District for injunctive relief in the United States District Court for the District of Columbia. The city removed the first suit to federal court, where the two cases were consolidated.
One month later, Congress enacted the Balanced Budget Act of 1997, Pub.L. No. 105-33, 111 Stat. 251 (“Budget Act”). Section 4714(a) of the Budget Act, entitled “Clarification Regarding State Liability for Medicare Cost-Sharing,” expressly authorized states to limit Medicare cost-sharing payments for QMBs based on Medicaid rates.
See id.
§ 4714(a),
The district court upheld section 4714(c), concluding:
[0]ne thing is clear: the law regarding state liability to pay for the health services provided to QMBs has never been crystal clear. Section 4714 has certainly provided clarification where it was needed. For this reason, the Court concludes that applying section 4714 retroactively, as Congress directed, is not impermissible under the Constitution.
McCreary v. Offner,
II
According to the doctors, pre-1997 law clearly required states to reimburse them for all Part B cost-sharing obligations incurred by QMBs. The Budget Act, they argue, could not constitutionally change that requirement retroactively. The United States (supported by thе District) responds that: (1) pre-1997 law was ambiguous regarding state cost-sharing obligations, and under
Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc.,
The doctors insist — as they must to avoid Chevron deference — that before 1997 sections 1396a(a)(10)(E) and 1396d(p)(3)(D) unambiguously required states to reimburse providers .in full for coрayments for Part B services performed on QMBs. They argue that the permissive “may” language in section 1396a(n) comports with this reading, interpreting that section simply to authorize states to deviate from their otherwise rigid Medicaid payment schedules. According to the doctors, section 1396a(n) did nothing more than provide an exception to the general rule that states must never reimburse Medicaid providers in excess of HHS-apprоved schedules. Section 1396a(n) used the permissive “may” instead of the mandatory “shall,” the doctors contend, because state Medicaid rates occasionally exceed the Medicare rates for the same service.
The doctors’ interpretation of the buy-in statutes is certainly plausible. But as we read the pre-1997 statutes and their legislative history, we think Congress has not so “unambiguously expressed” its intent as to make the doctors’ interpretation mandatory.
Chevron,
To begin with, if the buy-in statutes really spoke as clearly as the doctors contend, section 1396a(n) would have had no need to provide separately that states could deviate from their otherwise mandatory Medicaid schedules. Addressing the same issue, the Seventh Circuit put it this way: “[I]f ... [sections 1396a(a)(10)(E) and 1396d(p)(3)(D) of] the statute
clearhj
entitle[ ] [providers] to rеimbursement at Medicare rates (if it is not clear,
Chevron
is back in play), the state could hardly be penalized for such reimbursement. That would be penalizing it for complying with the statute.”
Paramount,
The government also points out that the very provision from which the doctors derive a state obligation to pay cost-sharing in full — section 1396a(a)(10)(E)(i) — requires that state plans make cost-sharing available for QMBs. Because states must detail their QMB cost-sharing policies in their Medicaid regulations before submitting those regulations to HHS for approval, the argument goes, states’ cost-sharing obligations could never cause them to run afoul of their own regulations. This argument makes sense. Again, the doctors nowhere respond.
The doctors’ interpretation of section 1396a(n) suffers from another problem. During the almost twenty years prior to its enactment, states often reimbursed providers for Medicare cost-sharing in excess of Medicaid rates. Why then did Congress
*82
need to enact section 1396a(n) to authorize such reimbursement?
See Paramount,
The doctors claim to find support for their position in the House Report accompanying the 1986 enactment of the QMB program, which stated that “the Medicaid program would pay the Part B deductible and the beneficiary’s 20 percent coinsurance.” H.R.Rep. No. 99-727, at 106 (1986), re-printed in 1986 U.S.C. 3607, 3696. But because this language did not speak to whether states must make the entire co-payment even in excess of Medicaid rates, it helps the doctors little. Moreover, subsequent legislative history squarely conflicts with the doctors’ interpretation of the buy-in program. The House Report accompanying the 1988 amendments said:
It is the understanding of the Committee that, with respect to dual Medicaid-Medicare eligibles, some States pay the coinsurance even if the amount that Medicare pays for the service is higher than the State Medicaid payment rate, while others do not. Under the Committee bill, States would not be required to pay the Medicаre coinsurance in the case of a bill where the amount reimbursed by Medicare — i.e., 80 percent of the reasonable charge — exceeds the amount Medicaid would pay for the same item or service.
H.R.Rep. No. 100-105(11), at 61 (1987),
reprinted in
1988 U.S.C.C.A.N. 857, 884;
see also
H.R.Rep. No. 101-247, at 364 (1989),
reprinted in
1989 U.S.C.C.A.N. 1906, 2090 (“The Medicaid programs typically pay the Medicare coinsurance only to the extent that their payment, plus the Medicare payment, does not exceed what the Medicaid program would pay for the service in question .... The Committee bill ... does not change the current policy regarding the amount which a Medicaid program must reimburse on such claims.”). Although post-enactment legislative history may or may not be a valid tool for ascertaining congressional intent,
see United States v. Carlton,
We have a similar reaction to four pre-Budget Act circuit court decisions that found the buy-in scheme unambiguous.
See Haynes Ambulance Serv., Inc. v. Alabama,
Ill
Proceeding to the second step of the Chevron inquiry, we ask whether HHS has reasonably interpreted the buy-in statutes. The United Státes’s position is simple: Because the word “may” in section 1396a(n) is permissive, not mandatory, states are allowed to but need not exceed their Medicaid rates. To us, this seems eminently reasonable — “may” means may.
The doctors make only one argument challenging the reasonableness of HHS’s interpretation. Relying on
INS v. Cardoza-Fonseca,
Since 1971, HHS policy has been to require State agencies that have a “buy-in” agreement to pay, in addition to the Part B premium, the Part B coinsurance and deductible amount for services provided to beneficiaries under Part B, even if the services are not routinely provided under the Medicaid State Plan.
48 Fed.Reg. 10,378,10,379 (1983) (notice of proposed rulemaking). Thаt rulemaking has no relevance to the question presented here, however, because there HHS merely concluded that the buy-in program does not require states to pay Part B cost-sharing for services not covered by their Medicaid plans; the rulemaking did not address whether a state must pay QMBs’ full copayments for services that are covered under its Medicaid, plan. The most relevant commentary in the rulemaking, moreover, аctually comports with the position HHS takes in this case: “[I]f a State limits the amount, duration or scope of Medicaid services covered in the State plan, then the State may similarly limit payment of Medicare Part B cost sharing amounts on those same services in accordance with its Medicaid service limitations.” 52 Fed.Reg. 47,926, 47,928 (1987) (final rule). Not only does this rulemaking suggest no agency inconsistency, but the doctors have failed to cite аny other instances of alleged agency inconsistency in the twenty-eight years since HHS first articulated its copayment reimbursement policy. Indeed, HHS appears to have approved the Medicaid plans of every state that has chosen to limit total copayment reimbursement to Medicaid rates.
Because we conclude that HHS’s interpretation of the buy-in statutes is reasonable, we have no need to reach the doctors’ constitutional challenge to the Budget Act. The district court’s grant of summary judgment for the District is affirmed.
So ordered.