Comparing Medicare Fee-for-Service Beneficiaries with ESKD Who Switched to Medicare Advantage versus Remained in Traditional Medicare

Abstract

Patients choosing Medicare Advantage versus Medicare fee-for-service (FFS) differ with respect to race, socioeconomic status, and burden of disease. However, it is unclear whether these differences also occur among patients with kidney failure, who were newly allowed to switch to Medicare Advantage after the 21st Century Cares Act. We used data from the United States Renal Data System to examine differences in characteristics of dialysis patients and kidney transplant recipients who switched from FFS to Medicare Advantage compared with those who stayed with FFS in 2021, the first year such switching was allowed. We used unadjusted and adjusted logistic regression to compare odds of switching among demographic and geographic subgroups. Among 411,513 patients with FFS coverage in 2020, 10.1% switched to Medicare Advantage in 2021. Switchers constituted 12% of the dialysis population and 5% of the kidney transplant population. In the dialysis population, patients of Black race and Hispanic ethnicity were more likely to switch than patients of White race (adjusted odds ratio [OR], 1.69; 95% confidence interval [CI], 1.64 to 1.73 and OR, 1.42; 95% CI, 1.40 to 1.47; respectively), as were patients with dual eligibility for Medicaid (adjusted OR, 1.12; 95% CI, 1.09 to 1.15). Patients living in the South were also more likely to switch to Medicare Advantage than those living in the West (adjusted OR, 1.48; 95% CI, 1.43 to 1.52). Similar differences were observed among kidney transplant recipients. Patients who switched from FFS to Medicare Advantage were disproportionately from historically marginalized groups, including Black, Hispanic, and low-income individuals. They were also more likely to live in the South. These differences may threaten the generalizability of United States Renal Data System data that relies on FFS insurance claims and suggest that comparisons of outcomes between FFS and medicare advantage beneficiaries with kidney failure should be adjusted for key patient characteristics.

Introduction

The Medicare Advantage program gives beneficiaries the option of receiving benefits from private plans rather than from the traditional fee-for-service (FFS) Medicare program.¹ The share of eligible Medicare beneficiaries enrolled in Medicare Advantage has been increasing steadily over the past 2 decades since the Medicare Modernization Act was enacted in 2003.² In 2023, a majority of the Medicare population was enrolled in a Medicare Advantage program for the first time—30.8 million, or 51% of eligible beneficiaries.² This increase in Medicare Advantage enrollment has occurred because more new beneficiaries have been enrolling and because more beneficiaries are switching from FFS to Medicare Advantage than from Medicare Advantage to FFS.² Recent reports have highlighted sizable differences in demographic characteristics of Medicare Advantage and FFS beneficiaries.^3,4 Specifically, Medicare Advantage beneficiaries are more than twice as likely to be of non-White race, are more likely to be lower income, and are more likely to live in the Southern region of the United States.^5,6

Since 1972, individuals with kidney failure have been eligible for Medicare coverage.^7,8 Before 2021, this coverage was through the Medicare FFS program by law: patients newly eligible for coverage based on a diagnosis of ESKD were automatically enrolled in FFS Medicare. Except for a small number enrolled in ESKD-specific Medicare Advantage Special Needs Programs, available in limited geographic areas,⁹ patients with FFS Medicare who developed ESKD were precluded from later switching to Medicare Advantage programs.^7,8 However, Medicare Advantage beneficiaries who subsequently developed ESKD were allowed to continue in Medicare Advantage plans.⁸ Thus, the percentage of patients with ESKD with Medicare Advantage coverage has been increasing but at a slower rate than that of the general Medicare population.¹⁰ For example, 9.8% of all patients with ESKD and 13.3% of Medicare beneficiaries with ESKD were enrolled in Medicare Advantage programs in 2011; by 2019, 18.3% of patients with ESKD and 25.5% of Medicare beneficiaries were in Medicare Advantage programs.¹⁰ Beginning in the open enrollment period for coverage in 2021, with the implementation of the 21st Century Cures Act, all Medicare beneficiaries with ESKD became eligible to enroll in Medicare Advantage plans,¹¹ and a sizeable percentage did so.^10,12

Increasing medicare advantage (MA) enrollment among patients with ESKD poses potential threats to the United States Renal Data System's (USRDS) ability to track health care utilization and outcomes in the population with ESKD. Specifically, analyses on the basis of FFS beneficiaries will represent an ever-shrinking proportion of the overall population. Furthermore, any systematic differences in demographic and clinical characteristics of patients switching to Medicare Advantage have the potential to threaten the generalizability of data generated from FFS beneficiaries and to lead to bias in assessing trends. A recent study found that a higher percentage of patients with ESKD who switched to Medicare Advantage in 2021 were of Black race, Hispanic ethnicity, or low income on the basis of dual eligibility for Medicaid coverage.¹² We sought to further examine differences among those who changed to Medicare Advantage and those who remained in FFS. We performed multivariable analyses to examine the extent to which race and ethnicity, low income, and other factors were independently associated with switching to Medicare Advantage. We also focused on geographic variations by Census region and by state.

Methods

Study Population and Inclusion Criteria

For this analysis, we included all prevalent patients who were receiving maintenance dialysis or had a functioning kidney transplant, were aged ≥18 years, had FFS Medicare as the primary payer on December 31, 2020, and were alive on January 1, 2021. We examined insurance coverage on January 1, 2021, and divided patients into those who remained with FFS coverage and those who switched to Medicare Advantage.

Data Source and Patient Characteristics

We used the USRDS database for this study. USRDS records the insurance status of patients at dialysis initiation and tracks any subsequent change in coverage. We obtained patient characteristics from the Patients file. ESKD treatment modality (dialysis or transplant) was determined using the Modality Sequence file.

Patient characteristics included demographic characteristics, primary cause of ESKD, and state and census region of residence. Patient demographics consist of age (continuous age and age groups: 18–44, 45–54, 55–64, 65–74, 75–84, 85+ years), sex, race, and ethnicity. Age was determined as of December 31, 2020. All patients with Hispanic ethnicity were categorized as being of Hispanic ethnicity and others were divided according to race categories as Asian, Black, White, or other race. We used dual eligibility for Medicare and Medicaid as an indicator of low socioeconomic status. We categorized cause of ESKD as diabetes, hypertension, GN, or other.

Exposure

The exposure of interest was switching from Medicare FFS coverage to Medicare Advantage for 2021. We designated individuals who had FFS as the primary payer on December 31, 2020, and switched to Medicare Advantage in 2021 as switchers and those who stayed on Medicare FFS in 2021 as nonswitchers.

Statistical Analysis

Given their many differences in characteristics, we performed all analyses separately for patients receiving dialysis and kidney transplant recipients. We examined the percentage switching to Medicare Advantage coverage across subgroups and the resulting differences in demographic characteristics among switchers and nonswitchers using Chi square tests. We then used logistic regression modeling to examine the unadjusted and fully adjusted odds ratios (ORs) for switching to Medicare Advantage. All analyses were performed using SAS software, version 9.4 (SAS Institute Inc., Cary, NC).

Results

Overall Study Population Characteristics and Percentage Switching to MA

In total, 411,513 patients with kidney failure were included in this study: 314,417 were treated with maintenance dialysis, and 97,096 had a functioning kidney transplant. As expected, patients on dialysis were slightly older, more likely to be White, and less likely to be dually eligible for Medicaid than transplant recipients (Table 1). Overall, 41,619 patients (10.1%) switched from FFS Medicare to Medicare Advantage in 2021. More than one of every nine patients on dialysis switched (11.6%) from Medicare FFS to Medicare Advantage in 2021, compared with 5.4% of the kidney transplant population.

Table 1.

Characteristics of patients who did and did not switch to Medicare advantage in 2021^a

Characteristic	Patients on Dialysis			Kidney Transplant Recipients
	Overall (314,417)	Switched to Medicare Advantage (36,358)	Stayed in FFS (278,059)	Overall (97,096)	Switched to Medicare Advantage (5,261)	Stayed in FFS (91,835)
Age, median (25th–75th percentile), yr	65 (55–74)	61 (53–68)	66 (55–74)	64 (52–71)	62 (53–69)	64 (52–71)
Age group, yr
18–44	11	10	11	15	11	15
45–54	15	20	14	16	19	16
55–64	24	33	23	23	29	22
65–74	29	26	29	35	33	35
75–84	17	9	17	11	7	11
≥85	5	2	5	0.9	0.6	0.9
Female sex, %	42	42	42	40	40	40
Race and ethnicity, %
Asian	5	3	5	5	4	6
Black	35	48	33	23	38	22
Hispanic	16	18	15	14	16	14
Other	3	2	3	2	2	2
White	42	30	44	56	41	56
Eligible for Medicaid, %	45	52	44	30	36	29
Cause of kidney failure, %
Diabetes	45	47	45	26	31	26
Hypertension	31	34	30	22	26	22
GN	10	9	10	23	20	23
Other	14	11	15	30	24	30
Census region, %
Northeast	15	9	15	18	13	18
Midwest	18	14	19	23	21	23
South	43	57	41	37	48	36
West	24	20	25	23	19	23

FFS, fee-for-service.

^aP values comparing characteristics of those who switched with those who remained in FFS were <0.001, except for sex (P = 0.99 for dialysis and P = 0.30 for transplant recipients).

Dialysis Population

Among patients receiving dialysis, a higher percentage of patients aged 45–64 years of age switched to Medicare Advantage than the percentage of younger and, especially, older individuals. Sixteen percent of Black individuals switched to Medicare Advantage compared with 13% of Hispanic individuals and only 8% of White and 7% of Asian individuals (Table 2). A higher percentage of dual eligible beneficiaries switched to Medicare Advantage (13%) than non-Medicaid eligible beneficiaries (10%). The percentage of beneficiaries switching to Medicare Advantage was more than twice as high (15%) in the South as in the Midwest (7%), and Southern states generally had the highest percentage switching to Medicare Advantage and those in the Midwest the lowest (Figure 1A). In Georgia and South Carolina, 20% of patients on dialysis with FFS coverage switched to Medicare Advantage, and the percentage was 0.4% in Alaska and 1% in North Dakota.

Table 2.

Adjusted and unadjusted odds ratios comparing patients who switched to Medicare advantage from Medicare fee-for-service versus with those who did not, by treatment modality

Variable	Dialysis				Kidney Transplant
	% Switching	Unadjusted OR (95% CI)		Adjusted OR (95% CI)	% Switching	Unadjusted OR (95% CI)	Adjusted OR (95% CI)
Age, yr
18–44	11		Reference	Reference	4	Reference	Reference
45–54	16		1.51 (1.45 to 1.58)	1.50 (1.44 to 1.57)	7	1.69 (1.52 to 1.88)	1.65 (1.49 to 1.84)
55–64	16		1.59 (1.53 to 1.65)	1.65 (1.59 to 1.72)	7	1.85 (1.67 to 2.04)	1.93 (1.75 to 2.14)
65–74	10		0.95 (0.91 to 0.99)	1.05 (1.00 to 1.10)	5	1.34 (1.22 to 1.48)	1.60 (1.45 to 1.77)
75–84	6		0.56 (0.53 to 0.59)	0.66 (0.63 to 0.70)	4	0.89 (0.78 to 1.02)	1.13 (0.99 to 1.30)
≥85	4		0.36 (0.33 to 0.39)	0.45 (0.42 to 0.50)	3	0.86 (0.59 to 1.26)	1.13 (0.77 to 1.66)
Sex
Female	12		1.00 (0.98 to 1.02)	1.01 (0.99 to 1.04)	6	0.97 (0.92 to 1.03)	0.98 (0.92 to 1.04)
Male	12		Reference	Reference	5	Reference	Reference
Race and ethnicity
Asian	7		0.79 (0.73 to 0.84)	0.83 (0.78 to 0.89)	4	0.90 (0.77 to 1.05)	0.92 (0.79 to 1.07)
Black	16		2.17 (2.12 to 2.23)	1.69 (1.64 to 1.73)	9	2.36 (2.22 to 2.51)	2.01 (1.88 to 2.16)
Hispanic	13		1.73 (1.68 to 1.79)	1.42 (1.40 to 1.47)	6	1.53 (1.40 to 1.66)	1.39 (1.27 to 1.52)
Other	8		0.99 (0.92 to 1.07)	0.88 (0.81 to 0.95)	4	1.08 (0.86 to 1.35)	1.06 (0.84 to 1.33)
White	8		Reference	Reference	4	Reference	Reference
Dual eligible
No	10		Reference	Reference	5	Reference	Reference
Yes	13		1.37 (1.34 to 1.40)	1.12 (1.09 to 1.15)	7	1.35 (1.27 to 1.43)	1.24 (1.17 to 1.32)
Primary cause  of ESKD
Diabetes	12		Reference	Reference	6	Reference	Reference
Hypertension	13		1.06 (1.04 to 1.09)	1.00 (0.98 to 1.03)	7	1.02 (0.95 to 1.10)	0.92 (0.86 to 1.00)
GN	10		0.83 (0.80 to 0.92)	0.83 (0.80 to 0.87)	5	0.68 (0.63 to 0.72)	0.78 (0.72 to 0.85)
Other	9		0.73 (0.70 to 0.75)	0.82 (0.79 to 0.85)	4	1.08 (0.86 to 1.35)	0.83 (0.77 to 0.90)
Census region
Midwest	7		0.89 (0.86 to 0.92)	0.90 (0.86 to 0.93)	4	1.15 (1.05 to 1.25)	1.16 (1.06 to 1.28)
Northeast	9		0.72 (0.69 to 0.75)	0.74 (0.70 to 0.77)	5	0.89 (0.81 to 0.98)	0.89 (0.80 to 0.98)
South	15		1.71 (1.66 to 1.76)	1.48 (1.43 to 1.52)	7	1.63 (1.51 to 1.76)	1.44 (1.33 to 1.56)
West	10		Reference	Reference	4	Reference	Reference

CI, confidence interval; OR, odds ratio.

Figure 1.

Percent switching to Medicare Advantage by state of residence. (A) Patients receiving dialysis and (B) Kidney transplant recipients.

In univariate logistic regression analysis, younger patients were generally more likely to switch to Medicare Advantage than older ones (Table 2). Black individuals were more than twice as likely (OR, 2.17; 95% confidence interval [CI], 2.12 to 2.23) and Hispanic individuals 73% (OR, 1.73; 95% CI, 1.68 to 1.79) more likely to switch than White individuals. Beneficiaries whose incomes were low enough to qualify for Medicaid coverage were 37% (OR, 1.37; 95% CI, 1.34 to 1.40) more likely than those whose incomes were higher. There were large regional differences in the odds of switching to Medicare Advantage, with those in the South more likely and those in the Midwest and Northeast less likely to switch than those in the West.

In adjusted analyses, associations with race and ethnicity and Medicaid dual eligible status persisted but were somewhat attenuated. Black (OR, 1.67; 95% CI, 1.64 to 1.73) and Hispanic individuals (OR, 1.42; 95% CI, 1.37 to 1.47) remained more likely to switch to Medicare Advantage, and Asian patients (OR, 0.83; 95% CI, 0.78 to 0.89) and those of other race (OR, 0.88; 95% CI, 0.81 to 0.95) were less likely to switch than White patients. Patients who were dually eligible for Medicaid were more likely to switch to Medicare Advantage from FFS than those who were not (OR, 1.12; 95% CI, 1.09 to 1.15). There was less attenuation of observed regional differences. Patients in the South Census region remained more likely (OR, 1.48; 95% CI, 1.43 to 1.52) and those in the Northeast (OR, 0.74; 95% CI, 0.70 to 0.77) and Midwest (OR, 0.90; 95% CI, 0.86 to 0.93) remained less likely to switch to Medicare Advantage than those in the West (Figure 1A and Table 2).

As expected, these differences in odds of switching to Medicare Advantage led to considerable differences in characteristics of switchers and nonswitchers (Table 1). The median age among patients on dialysis who switched to Medicare Advantage was 61 years (interquartile range: 53–68 years), compared with 66 years (interquartile range: 55–74) for individuals who did not switch. The switcher (Medicare Advantage) population had higher percentages of Black and Hispanic patients, Medicaid dual eligible patients, and patients with diabetes compared with the nonswitching (FFS) population. More than half of those who switched to Medicare Advantage resided in the South Census region (57%) compared with only 41% of those who remained in FFS.

Transplant Population

Like dialysis patients, transplant recipients aged 45–64 years were most likely to switch to Medicare Advantage (Table 2). However, the percentage of the youngest transplant recipients (aged 18–44 years) that switched to Medicare Advantage (4%) was similar to that of much older recipients, aged ≥75 years (3.4%–3.5%). Black individuals were more than twice as likely to switch to Medicare Advantage as White individuals even after adjustment for dual eligible status and Census region (OR, 2.01; 95% CI, 1.88 to 2.16). Hispanic individuals were about 40% more likely to switch (OR, 1.39; 95% CI, 1.27 to 1.52). Patients who were eligible for Medicaid coverage were 24% (OR, 1.24; 95% CI, 1.17 to 1.32) more likely to switch to Medicare Advantage even after adjustment. Similar to the dialysis population, those living in the South Census region were most likely to switch to Medicare Advantage (adjusted OR, 1.44; 95% CI, 1.33 to 1.56), and those in the Midwest were also more likely (OR, 1.16; 95% CI, 1.06 to 1.28) than those in the West (Figure 1B and Table 2).

When we analyzed results on a state-by-state basis, we found that rates of switching were the highest among transplant recipients in Southern states (Figure 1B). In six states, more than 8% of transplant recipients with FFS coverage switched to Medicare Advantage: Louisiana, 9%; Alabama, 9%; South Carolina, 8%; Georgia, 8%; Mississippi, 8%; and Texas, 8%. In two states, the percentage switching was <2%: Alaska, 0% and South Dakota, 1%.

The median age of patients who switched to Medicare Advantage was 1.2 years younger than the median age of those who did not (62 [interquartile range: 53–69] versus 64 [52–71] years) (Table 1). Similar to dialysis patients, percentages of Black and Hispanic, Medicaid dually eligible patients, and patients with diabetes were higher in the switching (Medicare Advantage) population than in the nonswitching (FFS) population.

Discussion

We found that patients aged 45–64 years, Black and Hispanic patients, patients with dual eligibility for Medicaid, and patients living in the South were more likely to switch to Medicare Advantage; this was true for patients on dialysis and for kidney transplant recipients. These differences were observed even after multivariable adjustment. In other words, the higher odds of switching to Medicare Advantage among Black and Hispanic individuals persisted after adjusting for Medicaid eligibility as an indicator of poverty. Striking regional differences were also not explained by differences in patients' race and ethnicity or poverty. As a result of these differences in odds of switching to Medicare Advantage across patient characteristics, characteristics of the populations who switched to Medicare Advantage and remained in FFS were quite different.

Our study shows that in 2021, a substantial number of individuals with ESKD switched to Medicare Advantage in the first year of eligibility, and the characteristics of those who switched to Medicare Advantage differed considerably from those who remained in FFS. These findings mirror trends observed in the general Medicare population. The share of the Medicare population enrolled in Medicare Advantage grew from 24% in 2013 to 33% in 2019 and 51% in 2023,^2,3 reflecting a growing shift toward Medicare Advantage preference over the past decade. Choosing Medicare Advantage is enticing to many individuals because of the availability of zero-premium plans, lower out-of-pocket spending limits, Special Needs Plans, and other supplemental benefits not available through Medicare FFS, including prescription drug coverage and dental and vision benefits.^12–15 Cost is an especially large factor for low-income patients making decisions about enrollment plans,¹⁵ and many Medicare Advantage plans limit patients' out-of-pocket spending, often by restricting provider networks, using prior-authorization techniques for prescribed medicines, and steering patients toward specific facilities.^14,16 The importance of cost is reflected in patient decisions over the past decade: the number of dual eligible enrollees choosing Medicare Advantage dramatically increased from 2013 to 2019, going from 25% in 2013 to 44% in 2019.^1–3

Aggressive television and radio marketing campaigns likely also contributed to patients with ESKD switching to Medicare Advantage in 2021 and to regional differences in enrollment among patients with ESKD. A recent study found that 85% of all television advertisements for Medicare were for Medicare Advantage during the 2022 open enrollment period.¹⁷ Most advertising aired locally rather than nationally, and half of the 20 cities with the highest number of local television advertisements for Medicare were in the Southeast. Disproportionately heavy advertising in this part of the country corresponds with our findings that people living in the Southeastern United States were more likely to switch to Medicare Advantage. These advertisements may also drive higher rates of switching to Medicare Advantage among lower-income (Medicaid eligible) patients. More than 90% of advertisements for Medicare Advantage promoted extra benefits such as dental, vision, and hearing, and 85% promoted low-cost features such as $0 copays and low premiums.¹⁷ Compounding the effects of heavy advertising, insurance brokers and agents also have an incentive to steer patients toward Medicare Advantage plans because they are paid more to do so in the form of commissions and other fees.¹⁸

Higher rates of switching to Medicare Advantage among Black, Hispanic, and lower-income patients and those living in the South Census region with ESKD could exacerbate barriers to receipt of high-quality medical care in these already marginalized populations. Specifically, limited provider networks and prior authorization requirements may make it harder for patients to access care. Medicare Advantage plans often limit beneficiaries to specific networks of providers, and patients have higher cost sharing for care received outside from out-of-network providers.¹ Although Centers for Medicare & Medicaid Services (CMS) requirements specify that providers must maintain a network of appropriate providers that is sufficient to provide adequate access to covered services to meet the needs of the population served,¹⁹ provider networks may still limit patients' access to dialysis facilities and transplant centers and thereby increase distances they need to travel to receive care for ESKD. A recent study examined access to dialysis facilities in over 500 Medicare Advantage plans and found that only about half of ESKD facilities were in-network on average across plans covering 87% of patients with ESKD.²⁰ In almost one-quarter of plans, ≤25% of dialysis facilities were in-network.

Another widely cited barrier is the widespread requirement for and use of prior authorizations in Medicare Advantage plans,^21,22 which may be particularly difficult for lower-income individuals and those with complex medical conditions such as ESKD to navigate. The burden of prior authorization requirements and denial of a substantial proportion have led to patient and provider frustration²³ and to questions about whether patients are receiving necessary care.²⁴ A recent study found that rates of switching from Medicare Advantage to traditional Medicare among patients with incident ESKD were much higher than among all Medicare Advantage beneficiaries.²⁵ In addition, numerous hospitals and physician groups have begun refusing to accept patients covered by Medicare Advantage plans.²³ These actions may further erode accessibility and quality of care for Medicare Advantage beneficiaries.

In terms of quality of care, studies have yielded mixed results comparing quality of care delivered through Medicare Advantage plans and FFS Medicare overall.¹⁶ However, a recent study found that the quality of care delivered by Medicare Advantage plans varied considerably by region.²⁶ Indeed, variation across regions was much greater than variability among groups of enrollees defined by other sociodemographic characteristics. Quality of care was worse in the Southeastern region of the United States,²⁶ where the largest percentage of ESKD beneficiaries switched to Medicare Advantage coverage in 2021. Taken together, these studies suggest that switching to Medicare Advantage may not benefit patients with ESKD.

Our findings raise questions about whether USRDS characterizations that rely on data from the FFS population will be sufficiently representative of the overall Medicare population, especially if the high rates of Medicare Advantage enrollment continue. For some outcomes, such as mortality, data from Medicare Advantage beneficiaries can and should be examined alongside data for FFS beneficiaries. However, it is more challenging to examine other outcomes using Medicare Advantage data. Given that Medicare Advantage payments are capitated, individual claims are not submitted to CMS for many services. Furthermore, even for outcomes that can be ascertained in Medicare Advantage data, there is additional delay in processing that leads to lag in the data such that outcomes typically cannot be observed until almost 3 years after the service is provided. Nevertheless, it is becoming essential that outcomes in this growing population be tracked to the extent possible, and the USRDS is expanding its inclusion of Medicare Advantage data to this end.

There are limitations to our study. Data on switchers are limited because of the recent change in policy. Therefore, ongoing monitoring of the size and characteristics of the Medicare FFS and Medicare Advantage populations going forward will be necessary. Our data are based on CMS administrative data and thus we are unable to ascertain why patients decided to switch to Medicare Advantage or remain on FFS Medicare. Future studies could examine patients' reasons for switching, their satisfaction with care, and whether outcomes differ among Medicare Advantage and FFS beneficiaries.

In summary, we found that Black, Hispanic, and lower-income (dual eligible for Medicaid) Medicare beneficiaries and those living in the South were more likely to switch to MA when switching became an option in 2021. The USRDS, policymakers, and other stakeholders will need to devise ways to continue to track outcomes among Medicare beneficiaries with ESKD in a reliable fashion going forward. Such approaches will need to carefully consider differences in patient characteristics among Medicare Advantage and FFS beneficiaries.

Disclosures

Disclosure forms, as provided by each author, are available with the online version of the article at http://links.lww.com/CJN/B965.

Funding

This work was supported through contracts with the National Institute for Diabetes and Digestive and Kidney Disease (75N94019C00006 and 75N94024C00001). K.L. Johansen reports participation in Advisory Boards for Akebia and Vifor.

Author Contributions

Conceptualization: Kirsten L. Johansen, Christopher D. Knapp, Jiannong Liu.

Formal analysis: Jiannong Liu.

Funding acquisition: Kirsten L. Johansen.

Methodology: Jiannong Liu.

Resources: Kirsten L. Johansen.

Supervision: Kirsten L. Johansen, Christopher D. Knapp.

Writing – original draft: Allan Y. Gao.

Writing – review & editing: Allan Y. Gao, Kirsten L. Johansen, Christopher D. Knapp, Jiannong Liu.