Is Acute Kidney Injury Really a Single Disease?

The search for a better biomarker than creatine for acute kidney injury (AKI) has been “long and elusive.” However, could researchers be on the right path now?

“The thinking is moving away from trying to find one biomarker that can be used for different types of kidney injury to a recognition that AKI is not just a single disease that a patient has or doesn’t have,” Rob D. Nerenz, PhD, an associate professor in the Department of Pathology and Laboratory Medicine at the Medical College of Wisconsin, Milwaukee, told Medscape Medical News. “It’s lots of different diseases that all affect the kidney in different ways.”

AKI is actually a “loose collection” of hepatorenal, cardiorenal, nephrotoxic, and sepsis-associated syndromes, as well as acute interstitial nephritis (AIN), he said. “So the question is not, ‘Is AKI present — yes or no?’ It’s, ‘What kind of AKI is present, and how do I treat it?’”

‘Mediocre Markers’

AKI affects about 10%-30% of hospitalized patients, according to Nerenz. It’s associated with an increased risk for adverse outcomes, including post-AKI chronic kidney disease and a mortality rate of approximately 24%.

Currently, AKI is defined by a rapid increase in serum creatinine, a decrease in urine output, or both.

“Those are mediocre markers,” Nerenz said, as serum creatinine is not very sensitive to acute change, and the increase is often detected after the therapeutic window of intervention has passed. In addition, “it only tells us that the kidneys are unhappy; it doesn’t say anything about the cause.”

Urine output is limited as a marker because many conditions affect it. “If you’re dehydrated, urine output is going to decrease,” he said. “And in some forms of AKI, urine output actually goes up.”

What’s needed, he said, is a more sensitive biomarker that’s detectable within a shorter timeframe of 2-6 hours following injury.

“Right now, we’re looking at 48 hours before a change becomes apparent, and that’s just too long. Plus, it should be kidney specific. One of the major limitations of the biomarkers that have been evaluated to this point is that, yes, they’re released by the kidney, but they’re also released by other tissue types within the body, and that hinders their effectiveness as a marker.”

Neutrophil Gelatinase-Associated Lipocalin (NGAL)

Although research on better biomarkers is ongoing, “there’s also a recognition that some of the protein markers that have been around for a while, if used appropriately, can provide value,” Nerenz said. These include, among others, NGAL.

NGAL works well in pediatric patients without other comorbidities, but it has been less useful in adult patients because it is also released by other cell types. However, recent research suggests it shows promise in patients with both cirrhosis and AKI.

There are three main causes of AKI in cirrhosis, Nerenz explained. The first is prerenal and can be primarily addressed through rehydration.

“When these patients come in, clinicians won’t do anything right away other than provide fluids. If creatinine improves over the 48-hour period of fluid replenishment, then the patient is sent home because there really isn’t extensive damage to the kidneys.”

If improvement isn’t seen after those 48 hours, then it could be one of two things: Hepatorenal syndrome or acute tubular necrosis. Patients with hepatorenal syndrome are candidates for terlipressin, which the US Food and Drug Administration (FDA) approved for this indication in 2022 after it displayed notable efficacy in a double-blind study.

“You don’t want to give terlipressin to just anybody because if the issue is not a diminished blood supply to the kidney, it’s not going to help, and comes with some serious side effects, such as respiratory failure,” Nerenz explained. “Having a biomarker that can distinguish between hepatorenal syndrome and acute tubular necrosis really helps clinicians confidently identify which patients are good candidates for this drug. Right now, we’re flying blind to a certain extent, basically using clinical intuition.”

Currently, the determination of NGAL is FDA-cleared only for pediatric use. One way hospitals have dealt with that is by making the test in their own labs, using appropriate reagents, validation, and so forth. These tests are then safe for use in adults but haven’t gone through the FDA approval process.

However, the FDA’s recent announcement stating that the agency should oversee lab-developed tests has made this situation unclear, Nerenz said.

“At this point, we don’t know if there’s still an opportunity to take the NGAL test (or any other cleared biomarker) and validate it for use in a different patient population. Many hospital labs simply don’t have the resources to take these tests through the whole FDA approval process.”

A New Biomarker for AIN?

Meanwhile, research is also moving forward on a better biomarker for AIN, which is also under the AKI umbrella.

“It’s important to diagnose AIN because it has a very specific treatment,” Dennis G. Moledina, MD, PhD, Yale School of Medicine in New Haven, Connecticut, told Medscape Medical News.

“AIN is caused by a bunch of different medications, such as proton pump inhibitors, cancer drugs, nonsteroidal anti-inflammatory drugs, and antibiotics, so when someone has this condition, you have to stop potentially life-saving medications and give unnecessary and potentially toxic immunosuppressive drugs, like prednisone,” he said. “If you get the diagnosis wrong, you’re stopping vital drugs and giving immunosuppression for no reason. And if you miss the diagnosis, AIN can lead to permanent chronic kidney disease.”

“Right now, the only way to diagnose AIN is to do a kidney biopsy, which is risky because it can often lead to significant bleeding,” he said. “Some people can’t undergo a biopsy because they’re on medications that increase the risk of bleeding, and they can’t be stopped.”

Furthermore, he noted, “the longer a patient takes a drug that’s causing AIN without getting a diagnosis, the less the chances of recovery because the longer you let this kidney inflammation go on, the more fibrosis and permanent damage develops. So it is important to diagnose it as early as possible, and that’s again why we have a real need for a noninvasive biomarker that can be tested rapidly.”

Moledina and colleagues have been working on identifying a suitable biomarker for close to 10 years, the latest example of which is their 2023 study validating urinary CXCL9 as just such a marker.

“We’re most excited about CXCL9 because it’s already used to diagnose some other diseases in plasma,” Moledina said. “We think that we can convince labs to test it in urine.”

In an accompanying editorial, Mark Canney, PhD, and colleagues at the University of Ottawa and The Ottawa Hospital in Ottawa, Ontario, Canada, wrote that the CXCL9 study findings “are exciting because they provide a road map of where diagnostics can get to for this common, yet poorly identified and treated, cause of kidney damage. The need for a different approach can be readily identified from the fact that clinicians’ gestalt for diagnosing AIN was almost tantamount to tossing a coin (AUC of 0.57). CXCL9 alone outperformed not only the clinician’s prebiopsy suspicion but also an existing diagnostic model and other candidate biomarkers both in the discovery and external validation cohorts.”

Like NGAL, CXCL9 will have to go through the FDA approval process before it can be used for AIN. Therefore, it may be a few years before it can become routinely available, Moledina said.

Nevertheless, Nerenz added, “I think the next steps for AKI are probably continuing on this path of context-dependent, selective biomarker use. I anticipate that we’ll see ongoing development in this space, just expanding to a wider variety of clinical scenarios.”

Nerenz declared receiving research funding from Abbott Labs for evaluation of an AKI biomarker. Moledina is a co-inventor on a pending patent, “Methods and Systems for Diagnosis of Acute Interstitial Nephritis”; a co-founder of the diagnostics company Predict AIN LLC; and a consultant for Biohaven, Inc. 

Marilynn Larkin, MA, is an award-winning medical writer and editor whose work has appeared in numerous publications, including Medscape Medical News and its sister publication MDedge, The Lancet (where she was a contributing editor), and Reuters Health.