Acute kidney injury (AKI) is diagnosed using a combination of blood tests, urine output tracking, and urine analysis. The standard definition requires meeting at least one of three criteria: a rise in serum creatinine of 0.3 mg/dL or more within 48 hours, a creatinine increase of at least 1.5 times baseline within seven days, or urine output dropping below 0.5 mL/kg/hour for six hours. These thresholds come from the KDIGO guidelines, which have been the global standard since 2012 and are currently undergoing their first major update.
The Three Diagnostic Criteria
Creatinine is a waste product your muscles produce at a fairly steady rate. Healthy kidneys filter it out, so when kidney function drops suddenly, creatinine builds up in the blood. Doctors compare your current creatinine level against either a recent value (within the past 48 hours) or a known baseline from the past week. Even a small absolute jump of 0.3 mg/dL in two days qualifies as AKI, because it signals a meaningful drop in filtering capacity.
The urine output criterion catches cases where creatinine hasn’t risen yet. Producing less than half a milliliter per kilogram of body weight per hour, sustained over six hours, meets the threshold. In practice, this criterion is most useful in hospitals where urine is being measured continuously through a catheter. Outside that setting, many people simply notice they’re urinating far less than normal.
Physical Signs That Point to AKI
A physical exam focuses heavily on your fluid status. Doctors check blood pressure while you’re lying down and again while standing, because a significant drop when you stand up suggests dehydration, one of the most common triggers. They’ll look at the veins in your neck (which bulge when fluid is backing up), listen to your lungs for fluid crackling, press on your ankles and shins for swelling, and check your skin and mouth for dryness.
Prerenal AKI, the type caused by not enough blood reaching the kidneys, typically shows up with thirst, dizziness, low urine output, and that postural blood pressure drop. If the cause is inflammation within the kidneys themselves, you may see blood-tinged urine, swelling, and elevated blood pressure instead. AKI can also produce symptoms that overlap with chronic kidney disease, like fatigue, nausea, loss of appetite, and itching, but these develop over days rather than months.
Blood and Urine Tests
Serum creatinine is the cornerstone lab value, but it doesn’t tell you why the kidneys are struggling. A basic metabolic panel reveals electrolyte imbalances (high potassium is especially dangerous) and acid-base disturbances that accompany AKI. Blood urea nitrogen (BUN) rises alongside creatinine, and the ratio between the two offers a rough clue: a BUN-to-creatinine ratio above 20:1 leans toward prerenal causes like dehydration or heart failure.
A standard urinalysis checks for protein, blood, and white blood cells. But the most informative test is urine microscopy, where a sample is examined under a microscope for specific structures called casts. These tiny cylindrical formations take shape inside the kidney’s tubules and get flushed out in urine, acting almost like a biopsy you can collect non-invasively.
What Urine Casts Reveal
Different cast types map to different causes of AKI:
- Hyaline casts with bland sediment: typical of prerenal AKI (dehydration, blood loss, heart failure). The kidney tissue itself is still intact.
- Muddy brown casts: considered a hallmark of acute tubular necrosis (ATN), meaning the kidney’s filtering tubes are directly damaged. Large numbers of these casts in a hospitalized patient are essentially diagnostic.
- Red blood cell casts: point to inflammation of the kidney’s filtering units (glomerulonephritis). Finding misshapen red blood cells alongside these casts strengthens the diagnosis.
- White blood cell casts: suggest acute interstitial nephritis, often a drug reaction affecting the tissue between the kidney’s tubules.
Fractional Excretion of Sodium
When doctors need to distinguish prerenal AKI from ATN, they calculate a value called fractional excretion of sodium (FENa). This compares how much sodium your kidneys are holding onto versus letting go. In prerenal AKI, the kidneys are still functioning properly but are starved of blood flow, so they aggressively reabsorb sodium. The FENa comes back below 1%. In ATN, the damaged tubules can’t reabsorb sodium effectively, pushing the FENa above 1% and usually above 3%.
FENa below 1% also shows up in acute glomerulonephritis and hepatorenal syndrome, so context matters. And if you’ve recently taken diuretics, the test becomes unreliable because those drugs force sodium excretion regardless of kidney health. In that situation, doctors may use fractional excretion of urea instead, which isn’t affected by diuretics.
Imaging
Ultrasound is the first-line imaging study. It checks for obstruction (a blocked ureter from a kidney stone or enlarged prostate, for example) by looking for swelling of the kidney’s drainage system, called hydronephrosis. Ultrasound also measures kidney size: normal-sized kidneys suggest an acute process, while small, shrunken kidneys point toward chronic disease that may have been missed. A CT scan without contrast is sometimes used when ultrasound is inconclusive, particularly for stones. Contrast dyes are avoided because they can worsen kidney injury.
Staging Severity
Once AKI is confirmed, it’s classified into three stages that guide how aggressively it needs to be managed:
- Stage 1: creatinine 1.5 to 1.9 times baseline, or a rise of 0.3 mg/dL or more, or urine output below 0.5 mL/kg/hour for 6 to 12 hours.
- Stage 2: creatinine 2.0 to 2.9 times baseline, or urine output below 0.5 mL/kg/hour for 12 hours or more.
- Stage 3: creatinine 3.0 times baseline or higher, or a rise to 4.0 mg/dL or above, or urine output below 0.3 mL/kg/hour for 24 hours, or no urine output for 12 hours, or the need for dialysis.
Higher stages carry significantly greater risk of lasting kidney damage and death, so early detection at stage 1 matters.
Newer Biomarkers for Earlier Detection
One limitation of creatinine is that it’s slow. Kidney damage can be underway for hours before creatinine starts climbing in the blood. Two newer biomarkers, NGAL and KIM-1, can rise before creatinine does because they’re released directly by injured kidney cells rather than accumulating passively. NGAL levels measured one day after a kidney insult can be roughly six times higher than in healthy individuals, dropping back to normal as function stabilizes. These markers are used in some hospital settings, particularly after cardiac surgery or kidney transplant, but they haven’t replaced creatinine in routine practice. Their role in everyday clinical diagnosis is still being defined.
AKI Diagnosis in Children
Children are diagnosed using the same KDIGO criteria as adults. The pediatric-specific framework, called pRIFLE, was developed earlier but has since been incorporated into the broader KDIGO system. The key practical difference is that baseline kidney function in children is estimated using height-based formulas rather than the equations used for adults, because children have lower muscle mass and therefore lower baseline creatinine levels. A creatinine value that looks normal by adult standards can actually represent significant kidney injury in a child.