Acute Kidney Injury (AKI) is a common condition estimated to affect up to 18% of hospitalized patients (1). The pathogenesis of AKI is poorly understood; however, certain risk factors including age, obesity, acute and chronic co-morbidities, and exposures to nephrotoxic medications have been associated with progression to AKI (1,2). In one multinational study, septic shock was found to be the most common contributing factor accounting for 50% of cases in critically ill patients followed by major surgery, cardiogenic shock, hypovolemia and medication-induced AKI (3). AKI is strongly associated with hospital mortality and with short-term and long-term functional sequelae to the patient (1,2).
Early detection of AKI may be reversible in some cases with relatively simple remedial actions such as volume repletion, discontinuation or avoidance of nephrotoxic agents, and timely recognition of the factors leading to AKI (2). To date, AKI is difficult to identify and diagnose before functional damage starts to occur as patients are often asymptomatic. Serum creatinine and urine output as functional markers of kidney function are currently utilized to diagnose AKI but these parameters are either non-specific or do not change quickly enough to allow for early detection of AKI. The KDIGO guidelines define AKI if any of the following criteria are met: 1) an increase in serum creatinine (SCr) by 0.3 mg/dL within 48 hours; or 2) increase in SCr to >/= 1.5x baseline, which is known/presumed to have occurred within the prior 7 days; or 3) urine volume <0.5 mL/kg/h for 6 hours. These guidelines define that a change in functional markers, subsequent to kidney injury may take up to 48 hours.
In the past several years, there have been multiple early detection biomarkers of AKI identified including two cell cycle arrest proteins, tissue inhibitor of metalloproteinases-2 (TIMP2) and insulin-like growth factor-binding protein 7 (IGFBP7) (4). These biomarker proteins were found to have the best performance in the Sapphire discovery trial. These proteins are secreted by renal tubular cells in response to cellular stress or injury and are produced before kidney function begins to decline, potentially acting as a “cellular alarm” prior to the onset of kidney function damage. The Sapphire study demonstrated that combining urinary [TIMP2]x[IGFBP7] (referred to as the AKI score), at a cut-off of >0.3 (ng/mL)2/1000 achieved an area under the curve (AUC) of 0.80 for developing stage 2 or 3 AKI within 12 h. These biomarkers were further validated in the Opal and Topaz trials with subsequent characterization of two cut-offs: 0.3 (ng/mL)2/1000 and 2 (ng/mL)2/1000. The lower cut-off was defined to achieve maximal sensitivity and negative predictive value while the higher cut-off was defined to allow higher specificity and positive predictive value (Table 1) (5). In 2014, the US FDA approved the use of the urinary [TIMP2] x [IGFBP7] as a risk assessment tool for AKI marketed as Nephrocheck for patients 21 and older (6). The test is intended to be used in ICU patients with respiratory and/or acute cardiovascular compromise within the past 24 hours in predicting the risk of developing AKI (stage 2 or 3) within 12 hours.
| 0.3 (ng/mL)2/1000
The Nephrocheck assay is a tool available in the clinician’s armamentarium where previously there was none. It is considered to be a good “rule-out” test allowing ~40% of patients evaluated to be categorized as low risk and less likely to develop AKI in the next 12 h (6). Furthermore, the risk score generated from the assay can be used to tailor patient management as shown in Table 2 (7).
| AKI Risk Score
| <0.3 (ng/mL)2/1000
|| Monitor; administer AKI prophylaxis
| 0.3 – 2 (ng/mL)2/1000
|| Monitor, avoid nephrotoxic agents and optimize hemodynamic status
| >2 (ng/mL)2/1000
|| Discontinue nephrotoxic agents, contrast media, monitor hemodynamic stability, volume, serum creatinine and urine output
It should be noted that the assay has a low positive predictive value with a false positive rate of nearly 50% at the cut-off of 0.3 (ng/mL)2/1000 (4). The reason for this is partially related to a significant overlap between the AKI score cut-off of 0.3 (ng/mL)2/1000 and values observed in nominally healthy patients (0.04 – 2.25 (ng/mL)2/1000). Clinical implementation of Nephrocheck is increasing and with a high false positive rate appropriate patient selection is essential. As with any laboratory test, utilization in high prevalence settings affords a better predictive value. The assay is not recommended in all hospitalized patients; patients at high risk of developing AKI (septic patients, patient’s post cardiac bypass or undergoing other high-risk surgery) are considered to be appropriate candidates who could benefit from the assay (6). Studies also indicate a significant decline in performance if measured beyond 12 h after an AKI event (4). Serial measurement is not recommended unless if there is a change in clinical status of a stable patient (6, 8).
A false positive result has the potential for negative clinical impact and for increasing the cost of care by increased monitoring of serum creatinine, urine output, optimization of hemodynamics and volume, and temporary removal of nephrotoxic medication that could aggravate the primary disease condition. However, AKI is known to have a negative impact on the patient in addition to significant contribution to the cost of care (8). Currently, with the exception of Enhanced Recovery after Surgery (ERAS) society, the American Association of Clinical Chemistry (AACC) and National Institute for Health and Care Excellence (NICE) do not recommend routine use of this test citing insufficient evidence demonstrating positive benefits such as reducing hospital stay or the likelihood of needing additional intervention and a high false positivity rate. Prior to clinical implementation, clinicians and laboratorians should carefully weigh in the pros and cons of utilizing this test, understand the limitations of this assay as a predictive tool, and the importance of properly utilizing and applying the results in the correct patient population.
1. Lewington AJ, Cerdá J, Mehta RL. Raising awareness of acute kidney injury: a global perspective of a silent killer. Kidney Int. 2013;84(3):457-467. doi:10.1038/ki.2013.153
2. Khwaja A. KDIGO clinical practice guidelines for acute kidney injury. Nephron Clin Pract. 2012;120(4):c179-c184. doi:10.1159/000339789
3. Uchino S, Kellum JA, Bellomo R, et al. Acute renal failure in critically ill patients: a multinational, multicenter study. JAMA. 2005;294(7):813-818. doi:10.1001/jama.294.7.813
4. El-Khoury JM, Hoenig MP, Jones GRD, et al. AACC Guidance Document on Laboratory Investigation of Acute Kidney Injury. J Appl Lab Med. 2021;6(5):1316-1337. doi:10.1093/jalm/jfab020
5. Hoste EA, McCullough PA, Kashani K, et al. Derivation and validation of cutoffs for clinical use of cell cycle arrest biomarkers. Nephrol Dial Transplant. 2014;29(11):2054-2061. doi:10.1093/ndt/gfu292
6. Nephrocheck Package insert
7. Ronco C, Rizo-Topete L, Serrano-Soto M, Kashani K. Pro: Prevention of acute kidney injury: time for teamwork and new biomarkers. Nephrol Dial Transplant. 2017;32(3):408-413.
8. Vijayan A, Faubel S, Askenazi DJ, et al. Clinical Use of the Urine Biomarker [TIMP-2] × [IGFBP7] for Acute Kidney Injury Risk Assessment. Am J Kidney Dis. 2016;68(1):19-28.