Nursing care
Prerenal vs intrarenal vs postrenal AKI: where the injury starts and what nurses check
Written and reviewed by Nicole Ramirez, BSN, RN, CMSRN · 4 min read · Updated November 2026
Short answer
Prerenal, intrarenal and postrenal describe where acute kidney injury starts. Prerenal means too little blood reaches healthy kidneys, as in dehydration, bleeding or sepsis. Intrarenal means the kidney tissue itself is damaged, most often by acute tubular necrosis. Postrenal means urine cannot drain past a stone, tumor or bladder outlet blockage. The location decides the fix.
The deciding question: before, inside or after the kidney
The three labels are a map, not three separate diseases. Blood arrives at the kidney, is filtered in the nephrons, then leaves as urine through the ureters, bladder and urethra. Prerenal injury is a supply problem before the filter, intrarenal injury is damage to the filter, and postrenal injury is a drainage problem after it.
MSD Manuals lists volume depletion, heart failure, sepsis and drugs such as NSAIDs and ACE inhibitors as common prerenal causes. Acute tubular necrosis and glomerulonephritis are the usual intrinsic causes in hospitalized patients, and MedlinePlus adds nephrotoxic drugs and contrast dye. Postrenal causes include kidney stones, tumors and bladder outlet obstruction such as an enlarged prostate.
Labs that separate prerenal from intrarenal injury
A prerenal kidney is still healthy, so it holds on to sodium and water as hard as it can. MSD Manuals gives the typical pattern as a BUN to creatinine ratio above 20, urine sodium under 10 mmol/L and a fractional excretion of sodium (FENa) under 1 percent. The urine is scant and concentrated because the tubules are doing their job.
Damaged tubules lose that ability. In intrinsic injury, MSD Manuals describes a BUN to creatinine ratio under 10, urine sodium above 40 mmol/L and FENa above 1 percent, with tubular cells and many granular casts, often brown, in the urine sediment. Those brownish casts are the finding to connect with acute tubular necrosis rather than a simple fluid deficit.
Postrenal injury and the early bladder check
Obstruction is the easiest type to reverse and the easiest to miss. MSD Manuals notes that a blockage at the level of the ureter has to involve both ureters to cause significant AKI, unless the client has only one working kidney. A single stone in one ureter of a person with two healthy kidneys usually causes pain, not kidney failure.
That is why a bladder scan or catheter comes early in the workup. MSD Manuals describes measuring residual urine this way, with more than 100 mL suggesting the bladder is not emptying, and MedlinePlus names kidney or abdominal ultrasound as the preferred test for a urinary blockage. Hydronephrosis on ultrasound points to an obstruction above the bladder.
Where students go wrong, and the nursing focus for each
Rising creatinine and falling urine output appear in all three types, so neither can place the problem. The categories also overlap over time. MSD Manuals states that prerenal conditions are usually reversible unless the low perfusion is severe or prolonged, so a client left dry for too long can slide from prerenal injury into tubular damage, and the lab pattern changes with it.
Nursing priorities follow the location. For prerenal injury, the nurse watches blood pressure, fluid balance and daily weights and reports when ordered fluids do not bring urine output back. For intrinsic injury, the nurse reviews the medication list for nephrotoxins, adjusts fluid intake to orders and watches potassium. For postrenal injury, relieving the obstruction comes first, then urine output is tracked closely afterward.
Hypothetical practice scenario
Hypothetical scenario, not a real exam item: a 74-year-old man two days after hip surgery has passed 60 mL of urine in four hours, has lower abdominal fullness and a creatinine that has doubled. Which action comes first: increase the IV rate, request a renal diet consult, or perform a bladder scan?
The bladder scan comes first. Older men are prone to outlet obstruction, and a full bladder is quick to confirm and quick to relieve. Raising the IV rate assumes a prerenal cause and can overload a client whose urine simply cannot drain. A diet consult may help later but does nothing about the cause of the low output.
Sources and further reading
MSD Manual Professional Version: Acute Kidney Injury (AKI). Prerenal, intrinsic and postrenal causes; BUN to creatinine ratio, urine sodium and FENa cutoffs; granular casts; bilateral ureteral obstruction rule; residual urine over 100 mL; prerenal reversible unless severe or prolonged.
MedlinePlus: Acute kidney failure. Causes grouped by low blood flow, direct damage (nephrotoxic drugs, contrast dye) and blockage; ultrasound as the preferred test for urinary blockage.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our renal and genitourinary practice questions are the closest set to what this page covers.
Common questions
What is the most common cause of prerenal AKI?
Prerenal AKI comes from low blood flow to the kidneys. MSD Manuals lists volume depletion, heart failure, sepsis and drugs such as NSAIDs and ACE inhibitors as common causes. Dehydration, bleeding, burns and very low blood pressure after surgery or injury all fit this group.
What BUN to creatinine ratio means prerenal?
A BUN to creatinine ratio above 20 suggests a prerenal cause, according to MSD Manuals, while a ratio under 10 fits intrinsic injury such as acute tubular necrosis. The ratio is read together with urine sodium, FENa and urine sediment, not on its own.
What causes postrenal AKI?
Anything that blocks urine leaving both kidneys, or the only working kidney: kidney stones, tumors, an enlarged prostate or other bladder outlet obstruction, and retroperitoneal fibrosis. Ultrasound looking for hydronephrosis and a bladder residual check are the usual first tests.