Guest post: Querying for AKI after a kidney transplant

by Howard Rodenberg, MD, MPH, CCDS

Charles Dickens’ novel Great Expectations is a coming-of-age story, full of lively characters and an insightful exploration of themes such as wealth and poverty, love and rejection, and the triumph of good over evil. At least that’s what Wikipedia says, because I’ve never read it. Nor have I read Oliver Twist, A Tale of Two Cities, or David Copperfield. (I do, however, watch A Muppet Christmas Carol at least twice each holiday season.) 

Expectations are at the heart of a recent issue raised by my friend Lori Drodge, RHIT, CCS. Like my last correspondent from Minnesota, Lori is also from the Frozen North, but this time it’s Portland, Maine. While I spent some time in Maine years ago as a locums emergency physician, about all I recall of the local culture is that everything was “wicked,” people in Fort Kent speak French (maybe), Stephen King’s house in Bangor is the one with bats on the wrought-iron fence, and I have no idea how anything can be “Down East” when on the map, Maine is clearly up.

Lori shared a conundrum with me about renal function following nephrectomy. In this case, an otherwise healthy patient donated a kidney for transplantation. Post-operatively the patient’s creatinine rose from 0.89 mg/dl before the surgery to 1.51 mg/dl after the procedure. Should the CDI specialist query for acute kidney injury (AKI)?

The reflexive answer might be yes. Many of us use the Kidney Disease Improving Global Outcomes (KDIGO) criteria to validate the presence of AKI. With an acute change in creatinine of 0.52 mg/dl from baseline values, the patient clearly exceeds the KDIGO threshold value of an acute rise in creatinine of 0.3 mg/dl required to validate the diagnosis.

But wait (as they say on the infomercial), there’s more! In this case, the transplant surgeon removed an entire kidney, so now half of the patient’s functional nephrons are gone. A change in creatinine would certainly be an expected outcome of this surgery, and no diagnosis of AKI should be made. So, what’s the answer, and how should the CDI specialist proceed?

At first glance, the literature provides somewhat of a mixed message on this account. Part of the problem in determining an answer is that when you look at the literature evaluating renal function after nephrectomy, some of them make liberal use of the term AKI while others do not. Once you get past that verbiage, however, the picture begins to clear.

(One note before we proceed. Most of us are familiar with the use of serum creatinine, a breakdown product of muscle and protein metabolism, as a marker of renal function. A less familiar marker is the glomerular filtration rate [GFR], an estimated value of the volume of blood passing through the glomerular mass each minute. GFR calculations are intimately associated with age, sex, and creatinine values but additionally reflect renal blood flow. Measures of creatinine and GFR are complementary.

Studies of renal function following radical nephrectomy, or total removal of the kidney, have been conducted mostly in patients with renal carcinoma. A twelve-year study evaluating post-operative AKI following radical nephrectomy for cancer noted that 5.5% of this population experienced post-operative AKI. Interestingly, the incidence of AKI rose from 2.2% in 1998 to 10.4 % in 2010, likely reflecting the broader parameters accepted for the diagnosis of acute kidney injury over time.

This latter conclusion—that the incidence of AKI is dependent upon the definition AKI in play at the time of diagnosis—is supported by a 2011 study using Risk, Injury, Failure, Loss of kidney function, and End-stage renal disease (RIFLE) criteria for diagnosing post-operative AKI that showed a 34% incidence of acute kidney injury. Renal function improved towards baseline over time, but never fully returned to preoperative values. Patients without AKI in this work still noted a decrease in post-operative renal function; in neither group did GFR fully return to baseline at 3- and 12-month follow-up. An even more recent (2017) work conducted after the KDIGO definitions were in common use showed a mean rise in serum creatinine within the first week following surgery of 55%; 59% of the patients reviewed experienced AKI.

Fewer works have specifically looked at kidney donors, but those that do tend to show similar results. One work identified a chronic post-operative rise in creatinine of 20% over baseline, with gradual improvement over time due to functional adaptation and hypertrophy of the remaining organ.

Another body of work has looked at risk factors for developing AKI following nephrectomy for malignancy. These risk factors include male gender, laparoscopic procedure, black race, preoperative chronic kidney disease (CKD), increased preoperative comorbidities, and performance of radical (total) nephrectomy. Postoperative AKI was also associated with increases in morbidity, mortality, inpatient length of stay, and hospital costs. It may be reasonable to presume that these same groups might exhibit a higher risk of AKI following nephrectomy for kidney donation.

While these works clearly demonstrate that rises in serum creatinine and decreases in kidney function are common following total nephrectomy, we’re still left with the question of if these changes truly represent AKI or are simply an expected outcome of the procedure. The fact that some degree of change is seen in virtually all patients following total nephrectomy, even if the changes do not reach a threshold value for the diagnosis of AKI, leads me to consider that most changes in renal function following nephrectomy are likely inherent to the procedure.   

Another characteristic of post-nephrectomy changes in renal function we might use as sorting criteria is the time frame of the event. Research has noted that compensatory hypertrophy of the remaining kidney will restore GFR to 70% of pre-operative baseline within 10-14 days, and to 75-85% of prior values with long-term follow-up. This differs for the clinical course of AKI, where resolution of renal impairment usually occurs in less than a week.

At what point does the loss of renal function cross the line from an expected outcome of surgical nephrectomy to something that exerts an unanticipated impact upon the patient’s needs for care? To some degree, that’s going to be a judgement call based on the contents of the medical record. But certainly any kind of invasive procedure related to decreased renal function (urgent or emergent dialysis), the use of medications to promote renal blood flow or function, or a new diagnosis of CKD would suggest that the line has been crossed from an expected outcome of surgery to a pathologic event.

Of course, the best source of information is always the renal physician or urologist caring for the patient. Involving your medical staff in determining the proper thresholds for querying AKI at your institution is always the right move.

Editor’s note: Rodenberg is the adult physician advisor for CDI at Baptist Health in Jacksonville, Florida. Contact him at howard.rodenberg@bmcjax.com or follow his personal blog at writingwithscissors.blogspot.com. Opinions expressed are that of the author and do not necessarily represent those of ACDIS, HCPro, or any of its subsidiaries.