NephJC 20 was conducted on February 17th at 9PM EST and 18th 8PM GMT. We discussed AKI biomarkers using these two papers from the critical care literature.

Discovery and validation of cell cycle arrest biomarkers in human acute kidney injury (PDF)

and

Validation of cell-cycle arrest biomarkers for acute kidney injury using clinical adjudication.

PMID: 24559465

The introduction was written by Sarah Faubel of the University of Colorado.

​Both of these studies underlie the NephroCheck diagnostic test:

Which is now humbly being marketed as the "first real advance in AKI testing in 60 years."

These two papers are a combination of three separate studies. The first paper contains two studies, Discovery and Sapphire, while the second paper is the Topaz study. For ease – we will discuss the three studies separately.

Discovery study.

Objective: Discover the best blood or urine biomarker that will predict acute kidney injury (AKI) in ICU patients.

Method: Test a whopping 340 biomarkers (identified by Medline search) in 522 adults in three different ICUs who don’t have AKI.  Then, figure out which biomarker best predicts AKI 12 hours later. AKI is defined as KDIGO stage 2 or 3 (translation: serum creatinine ≥2 times baseline or serum creatinine ≥ 4 mg/dL or initiation of renal replacement therapy). Research assays were used to measure biomarkers.

Results: Urine insulin-like growth factor binding protein (IGFBP) 7 and urine tissue inhibitor of metalloproteinase (TIMP) 2 best predicted AKI in 12 hours with AUCs of 0.77 and 0.75, respectively. (An AUC of 0.5 is a coin flip, an AUC of 1.0 is a perfect test, an AUC of 0.7 or better is pretty good and will probably get you FDA approval for your test).

Strengths

1. An amazing number of AKI biomarkers, 340, were tested
2. A heterogeneous population (ICU patients) was tested
3. Definition of AKI is hefty AKI (serum creatinine double or more from baseline)
4. Good result – AUCs for the two biomarkers was reasonable to predict AKI 12 hours after measurement

Sapphire study

Objective: Really prove (validate) that the two markers identified in the Discovery study predict AKI.

Method: Biomarkers were tested in 728 adults admitted to the ICU within 24 hours, without AKI, and with at least one risk factor for AKI. Patients were admitted to ICUs in 20 sites in North America and 15 sites in Europe. Urine and blood were collected within 18 hours of enrollment, biomarkers were measured and correlated with the development of AKI (KDIGO 2 or 3) 12 hours later. Urine IGFBP7 and urine TIMP-2 were measured simultaneously using the NephroCheck Test™ – a method developed after the Discovery study using a platform with clinical utility (in the Discovery study, two was better than one – so they looked at IGFBP7 x TIMP-2). Other famous AKI biomarkers were also measured (i.e., urine NGAL, plasma Cystatin C, urine Kim-1, plasma NGAL, urine IL-18, urine pi-GST, and urine L-FABP).

Results: Urine TIMP-2 x IGFBP7 as measured by the NephroCheck™ machine did a good job of predicting AKI 12 hours later. Specifically, the AUC for urine TIMP-2 x IGFBP7 was 0.80 for the development of AKI 12 hours after measurement and was significantly better (P<0.002)  than the AUCs for the other biomarkers. Highest tertile of TIMP-2 x IGFBP7 levels had a 10-fold relative risk of developing AKI versus the lowest tertile. Higher levels of TIMP-2 x IGFBP7 were associated with a higher level of developing AKI. Subgroup analysis indicated that these makers performed well in sepsis (AUC 0.82) and post-surgery (AUC 0.85).

What are these things? TIMP-2 and IGFBP7 are G1 cell cycle arrest proteins. Basically, when there is an injury, cells enter cell cycle arrest and stop dividing – basically the cells are waiting for the injury storm to conclude so the cell can repair itself versus rather than end-up in some sort of cell death pathway.

Strengths:

1. TIMP-2 x IGFBP7 was measured using a clinically applicable platform
2. Lots of patients, N=728, were studied
3. The test worked well in various causes of AKI – sepsis, post-surgery, etc.

Topaz study

Objective:  Do a study that is so well done that the FDA will approve NephroCheck™ as a test to predict AKI in patients.

Methods: Urine TIMP-2 x IGFBP7 was measured 420 patients admitted to the ICU within 24 hours who had no evidence of AKI. The risk of AKI for patients with a urine TIMP-2 x IGFBP7 > 0.3 (a pre-specified cutoff) 12 hours after measurement was assessed. AKI was defined as KDIGO 2 or 3 (≥ 2x serum creatinine). Clinical adjudication of AKI was performed (i.e., three nephrologists independently decided if the patients had AKI or not). TIMP-2 x IGFBP7 was measured using the NephroCheck TM platform at three separate sites (thus, measurements were in triplicate).

Primary analysis was for a sensitivity plus specificity greater than 1 for TIMP2 x IGFBP7 cutoff of 0.3. Secondary analysis was for a cutoff of 2.0. Post-hoc subgroup analysis was planned for AKI from sepsis.

Results: The AUC for a single measurement of TIMP-2 x IGFBP7 to predict AKI in 12 hours was 0.82.

“Only 1 in 27 patients (3.7% absolute risk) with a test result of 0.3 or less would manifest AKI within 12 hours, whereas more than one in four (27% absolute risk) above 0.3 would do so.”

The higher the value, the higher the relative risk of developing AKI (Figure 4 of the study).

Strengths

1. A pre-specified cut point was used for analysis
2. An adjudicating committee was used to diagnose AKI (gold standard method for biomarker and drug development, in general)
3. The test is applicable to a heterogeneous population of ICU patients to predict AKI within 12 hours of measurement

Now what?

Overall, the AKI biomarker movement started because it was thought that clinical trials in AKI failed because AKI was diagnosed too late. If we diagnose AKI sooner – the thinking went - then we might have a chance to do something and improve outcomes. The search was for a “troponin” for AKI – and because troponins have resulted in earlier diagnosis and improved outcomes in MI, the thought was that identifying a troponin for AKI would also improve outcomes.

Now we have a FDA approved early test – what clinical trial will we do? Troponin has improved outcomes in MI because there is a universally accepted treatment for acute MI – open the vessel. We don’t have a universally accepted treatment for early AKI – so I am anxious to see what clinical trials will emerge.

Put simply, now that we have a test that predicts and diagnoses AKI early, what can we do with it? I think about AKI all the time, and I am still trying to decide what I will do with this test.

There are a lot of remaining questions about the test.

• Does it work well in patients with CKD? (Few patients with CKD were studied.)
• Does it work in infants and children? (No kiddos were studied – but a recent study suggests that it does. 
• Does it work in specific AKI causes? (Like cisplatin-induced AKI, post-CABG, etc.)?
• Does it predict severity of AKI in patients who already have AKI or the need for RRT? (Seems like it does in combination with the furosemide stress test)
• Should I make sure my hospital buys this – or should I wait for more research to come out?
• And, at hospitals where it is available, how should it be used?
• Should patients in the E.R. be screened?
• What is the clinician to do with a test that is high?
• What should the nephrology consultation look like?
• Will we give fluids or Lasix?
• Will we just wait and see what the creatinine is tomorrow?

At the very least, we know that nephrotoxins and hypotension are bad – can we prevent further injury and reduce AKI severity.

Concluding thoughts

AKI is the most common reason for inpatient nephrology consultation and an important contributor to morbidity and mortality in hospitalized patients. There is now a new tool in the AKI world that nephrologists need to take the lead in its use and interpretation.

We should embrace consultations for this test and be actively involved in hospital planning for its use, interpretation, and patient management decisions based on the results – even if we can’t think of what we might do with a positive result right now.

Disclaimer: Sarah Faubel is Professor of Medicine at the University of Colorado Denver and VA Medical center. She is Chair of the ASN AKI Advisory Group and has been funded by the NIH, VA, ASN, and AHA for basic research studies in AKI. She is an avid rock climber. The views represented above are her individual opinion and do not represent her employer, the VA, the ASN, the NIH, the AHA, or other rock climbers. There is no financial or other conflict of interest except a hope that the care of patients with AKI may be better and the mortality of this disease reduced.