Acute kidney injury (AKI) is a serious concern in hospitalized patients. In the United States, it is present in 1% of patients on admission, is diagnosed in approximately 2-5% of patients during hospitalization, and is found in as many as 67% of patients in the intensive care unit (ICU).
To properly identify and manage this population, an awareness of the stages of acute kidney injury stages is critical. Various classification systems exist to identify severity and outcomes of AKI.
The RIFLE criteria (Risk, Injury, Failure, Loss of kidney function, and End-stage kidney disease), have been commonly used since 2004. The first three criteria (risk, injury, and failure) define grades by increasing severity of AKI, and the last two (loss and end-stage kidney disease) represent the outcome classes. The three stages of AKI development (Risk, Injury, and Failure) and corresponding severity will be examined here in detail, along with clinical management.
Stage 1: Risk of Kidney Injury
The earliest warning signs of AKI emerge in the first stage – Risk – and represent an opportunity for early intervention for improved outcomes. One literature review illustrates that the risk of mortality from AKI progression increases from 18.9% to 36.1% and finally to 46.5% as a patient moves from risk to injury and then to failure classifications in RIFLE.
RIFLE considers negative alterations in both serum creatinine (SCr) or, alternatively, glomerular filtration rate (GFR) and decreased urine output (UO) separately as criteria for identifying AKI. In the first stage, an increase of SCr by 1.5 times or a decrease of glomerular filtration rate (GFR ) by more than 25% andor a UO less than 0.5 mL/kg/h × 6 hours indicates risk of kidney injury.
Potential causes of AKI can be divided into three categories. The first is prerenal (decreased perfusion, e.g. due to volume depletion, such as with vomiting, diarrhea, sepsis, and certain medications). The second is intrinsic renal injury, involving direct damage to renal structures (often due to illness, infection, inflammatory processes, or medications). Finally, postrenal causes such as urinary obstructions may be identified. These causes all contribute to increasing the patient’s overall risk.
Stage 2: Injury to the Kidney
While in Stage 1 patients may not demonstrate symptoms of AKI, as kidney injury stages progress to Stage 2, clinical signs are more likely to appear. A crucial indicator is urine output; in Stage 2, UO drops to <0.5 mL/kg/h × 12 hours. Additionally, SCr doubles and GFR decreases more than 50%. Other symptoms may include peripheral edema, fatigue, hypertension, nausea, vomiting, anorexia, shortness of breath, and mental status changes. However, it is also possible for a patient to present without symptoms or with only subclinical signs at this AKI stage. This further highlights the importance of early identification via indicators like decreased UO. Because Stage 2 AKI indicates moderate kidney damage, ICU admission may be necessary. Studies show that AKI-related ICU mortality reaches 28.4%, underscoring the urgency of early detection and intervention. This clearly illustrates that earlier medical interventions can have a significant impact on patient outcomes. Stage 3: Failure of Kidney Function The final stage of renal injury presents severe and life-threatening complications. In Stage 3, SCr triples or increases by >0.5 mg/dL to >4 mg/dL, GFR decreases more than 75% from baseline, UO decreases to <0.3 mL/kg/h × 24 hours, or anuria × 12 hours is present.
At this stage of kidney injury, continuous renal replacement therapy (CRRT) is often initiated. Mortality rates are highest in this group of patients, and studies suggest that patients with Stage 3 AKI are at higher risk for developing chronic kidney disease (CKD). This includes a significant number of patients who go on to develop end-stage renal disease (ESRD) and become dialysis-dependent.
Diagnosis and Monitoring of AKI Stages
The critical importance of AKI diagnosis and early intervention cannot be understated. Even Stage 1 AKI represents an opportunity to reverse kidney injury and prevent serious complications.
Along with SCr, GFR , and UO results indicating stages of acute kidney injury, a careful review of patient history can identify possible causes of AKI such as illness, infection, or nephrotoxic medications. Other laboratory testing may include electrolytes, glucose, blood urea nitrogen (BUN), complete blood count (CBC), serum calcium, and urinalysis including urinary protein excretion.
Where postrenal or obstructive causes of AKI are suspected, radiographic imaging such as kidney ultrasound may be performed. Older men are particularly at risk of obstructive AKI due to benign prostatic hyperplasia (enlarged prostate).
It is important to consider that monitoring SCr and GFR has limitations. SCr rises slowly in response to damage and is not sensitive as an early indicator of AKI. GFR calculation also presents challenges in early detection, as accuracy is affected by variability in SCr changes in response to clinical status.
Additionally, manual urine output tracking can be time-consuming and prone to error . Automated monitoring solutions provide real-time, accurate measurements, allowing for earlier intervention and improved patient safety. Continuous, careful monitoring of urine output and other laboratory changes can help identify subtle clinical changes (often present without overt symptoms) suggesting a patient is progressing to a higher stage of AKI.
Since AKI can appear during a hospitalization, and occurs in high rates in ICU patients, increasing healthcare providers’ awareness of new technologies to assist in early trend recognition is crucial to identifying AKI before it progresses to more serious stages. Timely and accurate urine output monitoring is a proven life-saving strategy. It has been shown to improve the overall ability to identify AKI and to facilitate earlier recognition of the diagnosis. Research also shows that diligent monitoring significantly reduces 30-day mortality, making it an essential tool for improving patient outcomes.
Management and Treatment Across Acute Kidney Injury Stages
When AKI has been identified, interventions should be tailored to the patient’s underlying causes, and continuously adjusted according to results of ongoing, careful monitoring of laboratory values and UO.
Stage 1 Treatment
AKI should ideally be recognized in this stage; fluid management to correct hyper or hypovolemia should be initiated, nephrotoxic medications discontinued or dose-corrected if needed, and obstructive causes addressed.
Stage 2 Treatment
Any derogatory changes to laboratory values or UO should be addressed immediately. Electrolyte and acid-base imbalances should be corrected. CRRT may be considered in this stage if supportive measures are not adequate to manage concerns such as hyperkalemia, volume overload, acidosis, or uremic pericarditis, pleuritis, or encephalopathy.
Stage 3 Treatment
Patients in Stage 3 AKI typically require CRRT and may go on to require intermittent hemodialysis (IHD). Continuous monitoring of UO and management of electrolyte imbalances and volume status is critical.
Preventive Measures and Patient Education
There are known risk factors for AKI; these include older age, diabetes, heart failure, liver failure, and underlying kidney disease. In addition, sepsis, cardiac surgery, contrast agents, and antimicrobials are known to contribute to some cases. To help prevent AKI, primary care physicians can identify those at high risk, avoid prescribing nephrotoxic medications, and encourage proper management of contributing health conditions like diabetes. Patients should be educated on early symptoms of AKI and instructed to seek prompt medical attention for any concerns.
Hospitalized patients who are in a high-risk category should be closely monitored for developing signs of AKI including decreased UO.
Final Thoughts
Clinical management of AKI represents a critical challenge for healthcare providers. Understanding each stage of kidney injury and taking proactive measures for early detection can help reduce the risk of patients deteriorating. Even progression from Stage 1 to Stage 2 should be avoided; beyond moving from Stage 3, or kidney failure, patients move into the outcome classifications of RIFLE — L: sustained loss of kidney function and E: end-stage renal disease.
Because patients can be asymptomatic in early AKI and progression of kidney injury is associated with worsening outcomes, close monitoring is crucial for high-risk patients. Alongside laboratory testing, urine output can be instrumental in early identification of AKI and support timely interventions that mitigate impact; routine use of automated UO monitoring is an important addition to accurate, efficient, and rapid care.
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