Dialyzing Acute Kidney Injury Patients after Hospital Discharge

Dialysis-requiring AKI represents a sudden, unexpected, and profound change in a patient’s life, with important implications for morbidity, mortality, and quality of life. Although the vast majority of AKI research focuses on the inpatient setting, less is known about AKI after hospital discharge. Historically, Medicare reimbursement policy affecting the dialysis-requiring AKI population has hindered research as it has been challenging to accurately identify this vulnerable subgroup of patients using administrative or registry data. Prior to 2017, patients with AKI receiving dialysis services at free-standing maintenance hemodialysis facilities were not separately categorized in the United States Renal Data System (USRDS) registry. This policy was reversed in 2017, which allowed—for the first time—researchers to specifically study patients who were discharged to outpatient dialysis units but not certified as requiring maintenance dialysis by the treating nephrologists.

In this issue of CJASN, Dahlerus et al. (¹) report the results of a retrospective cohort study using 2017 outpatient dialysis Medicare claims to examine patient characteristics and mortality in patients with dialysis-requiring AKI discharged to receive outpatient dialysis compared with the non-AKI incident dialysis population. The authors found that patients with dialysis-requiring AKI had more comorbidities documented. Consistent with prior research (^2–4), they also found that patients with dialysis-requiring AKI suffer significantly higher mortality compared with patients on non-AKI incident dialysis. This was particularly evident during the first several months on dialysis. The mortality risk converged between the two groups by month 7. The higher mortality in their analysis persisted after adjustment for comorbidities and (in sensitivity analysis) for the type of dialysis access.

The study by Dahlerus et al. (¹) distinguishes itself from similar prior studies by being national in scope (whereas most prior outpatient dialysis-requiring AKI studies were on the basis of single-center or single–health care system data only). Another strength is the nonreliance on the Centers for Medicare & Medicaid Services (CMS) Medical Evidence 2728 Form to identify AKI. The authors’ approach to identifying AKI on the basis of dialysis billing codes rather than on the 2728 Form allowed them to determine outcomes earlier in the clinical trajectory. To our knowledge, only a couple of abstracts presented at Kidney Week 2019 have leveraged data from after the 2017 change in Medicare policy permitting the treatment of dialysis-requiring AKI in outpatient dialysis facilities.

The results from this important study contain a number of interesting observations, which suggest potential future research directions.

First, in this nationwide study of Medicare patients, 17% (10,821 of 63,447) of new patients in dialysis units were considered by their physicians to have had dialysis-requiring AKI and may or may not have permanent kidney failure. Because patients with AKI had to have an outpatient dialysis claim to be included in the cohort and outpatient claims were not available for Medicare Advantage patients, we use here as the denominator 10,821 + 52,626=63,447 to exclude those with Medicare Advantage from both groups. By contrast, a prior report that relied on CMS Form 2728 estimated that only 3% of patients with incident dialysis had dialysis-requiring AKI (²). We suspect that many researchers and clinicians will be surprised that such a high proportion of new patients was admitted to outpatient dialysis units because of AKI.

Second, this 17% figure may actually be an underestimation as it is likely that quite a number of patients with dialysis-requiring AKI were misclassified as having permanent kidney failure. Dahlerus et al. (¹) noted that 1927 patients whose physicians classified them as having permanent kidney failure recovered kidney function. Because presumably only patients with AKI were able to recover kidney function to come off dialysis, these 1927 patients had to be patients with dialysis-requiring AKI misclassified as permanent kidney failure. If one assumes that the recovery rate among these misclassified patients with AKI was similar to the recovery rate among those classified as dialysis-requiring AKI (2012 of 10,821), then potentially approximately 10,000 of the patients classified here as permanent kidney failure actually had dialysis-requiring AKI. In other words, roughly as many patients with dialysis-requiring AKI were (mis-)classified as permanent kidney failure as were classified as dialysis-requiring AKI in 2017. Although the method for ascertaining recovery differs between the two groups, the data are likely reliable, and this comparison is valid (in terms of order of magnitude) because death and absence of billing for dialysis should be fairly reliably tracked. Because this study only captures data from the first year after the policy change, it will be interesting to see if over time, given more familiarity with the new reimbursement system, nephrologists end up classifying fewer patients with dialysis-requiring AKI as having permanent kidney failure.

Third, among the 7108 patients with dialysis-requiring AKI who eventually were classified as permanent kidney failure, only 9% had “acute kidney failure” listed as the cause of permanent kidney failure on CMS Form 2728. In contrast, 35% listed diabetes mellitus, and 24% listed hypertension. We believe this reflects a shortcoming of CMS Form 2728, whereby only one cause of permanent kidney failure is allowed. In reality, almost all patients who have nonrecovery from AKI resulting in permanent dialysis dependency have some degree of preexisting CKD—because conditional upon surviving, almost all patients with simple acute tubular necrosis and preserved baseline kidney function recover enough to come off of dialysis (⁵). Thus, our traditional reliance on USRDS CMS Form 2728 substantially underestimates the contribution of dialysis-requiring AKI as a pathway to permanent kidney failure. For many patients with preexisting stage 3 or 4 CKD (from diabetes or hypertension), they would not be patients with permanent kidney failure without the superimposed episode of AKI.

Fourth, by drawing attention to the clinical and public health importance of dialysis-requiring AKI as a proximate cause of new patients requiring care in the outpatient dialysis unit, this study highlights major gaps in knowledge about how these patients should be cared for. In this national study, the chance of outpatient recovery from dialysis-requiring AKI was 19% (2012 of 10,821), a rate similar to that reported in a 2006–2009 study from the Mayo Clinic Health System (⁶) but lower than the 35% seen in a recent study using 2005–2014 data from USRDS (²). A number of recent estimates of outpatient recovery have been closer to 40% (e.g., 43% [⁷], and 42% [⁸]). Although some of this variability is undoubtedly due to differences in patient characteristics, it is also possible that the chances of kidney function recovery are affected by practice variation.

We believe these results should motivate future observational studies examining how processes of care are related to chances of recovery of kidney function (and other outcomes) among patients with dialysis-requiring AKI. For example, are more frequent laboratory evaluations (which can be assessed by Medicare charges for blood work and 24-hour urine collections) or more frequent physician rounding visits (as assessed by professional fee billings) associated with higher rates of recovery or shorter time to recovery? Do patients who did not meet their new outpatient nephrologists at the outpatient dialysis unit for 3 or 4 weeks after discharge from the initial hospitalization have lower rates of recovery (and higher rates of readmission or mortality)?

In summary, Dahlerus et al. (¹) provide evidence that in the contemporary era, patients with dialysis-requiring AKI make up a sizable fraction of new patients requiring care in the outpatient dialysis unit. These patients experience high rates of mortality but simultaneously have the potential for much better outcomes than other patients initiating treatment in the outpatient dialysis unit because of possible kidney function recovery to come off dialysis completely. Beyond observational studies, clinical trials are urgently needed in this vulnerable population to test different strategies to foster better outcomes, including higher rates of recovery or earlier recovery of kidney function from dialysis-requiring AKI.

Disclosures

C.-y. Hsu reports that he has consulted for legal cases involving acute kidney disease or CKD. He has also consulted on an ad hoc basis for companies regarding kidney disease. C.-y. Hsu reports receiving research funding from Satellite Healthcare, receiving royalties from UpToDate, and receiving honoraria from Satellite Healthcare. The remaining author has nothing to disclose.

Funding

C.-y. Hsu is supported by National Institute of Diabetes and Digestive and Kidney Diseases grants K24DK92291, R01DK114014, and R01DK122797.