The urine is a biopsy you can read in minutes, and in acute kidney injury it is the test that most often changes the plan. A dipstick, a centrifuged sediment, a pair of fractional excretions, and a bedside ultrasound will sort most AKIs into a category before any specialist result returns. Each of these tests is a window onto a mechanism, so reading them well means knowing not just the cut-off but the physiology the cut-off reports.
The urine is a biopsy you can read in minutes
Four cheap tests, used in order, answer most of the question. The dipstick screens for blood, protein, and infection. The sediment shows whether the parenchyma is inflamed. The fractional excretions report how the tubule is handling sodium and urea. The ultrasound excludes obstruction and reads volume. None is definitive alone, but together they triangulate the category the previous chapter demanded. The single discipline that matters most is to examine the sediment fresh: casts and cells degrade within the hour, and a delayed specimen quietly turns an active sediment bland.
The dipstick: what it sees and what it misses
The blood pad detects haem, not red cells, so it lights up for myoglobin and free haemoglobin as readily as for true haematuria. A strongly positive blood pad with no red cells on microscopy is therefore the signature of pigment — rhabdomyolysis or intravascular haemolysis — not glomerular bleeding. The protein pad is just as treacherous in the opposite direction: it reads albumin and is nearly blind to immunoglobulin light chains. A patient with cast nephropathy from myeloma can have a negative or trace protein dipstick while spilling grams of light chain, which is why a dipstick-protein that disagrees with a high urine protein-to-creatinine ratio should prompt a search for a non-albumin protein. Specific gravity, glucose, leukocyte esterase, and nitrites fill in the picture, but each has its own confounder — contrast and glucose inflate specific gravity, and a negative nitrite never excludes infection.
The sediment: bland versus active
The first question of the sediment is binary: bland or active? A bland sediment — a few hyaline casts, little else — fits pre-renal and post-renal AKI, where the parenchyma is intact. An active sediment moves the diagnosis into intrinsic disease, and the particular elements name the compartment. Muddy-brown granular casts and renal tubular epithelial cells are the fingerprint of acute tubular necrosis — sloughed, degenerating tubular cells. Red-cell casts and dysmorphic red cells, deformed as they squeeze through an inflamed glomerular basement membrane, mean glomerulonephritis or vasculitis. White-cell casts with pyuria point to interstitial nephritis or pyelonephritis. Broad, waxy casts speak of chronic, advanced disease. Crystals add their own clues: envelope-shaped calcium oxalate in ethylene-glycol poisoning, uric acid in tumour lysis, and the drug crystals of aciclovir, sulfadiazine, methotrexate, and atazanavir.
Fractional excretion: why the tubule's sodium handling tells the story
The fractional excretion of sodium asks what proportion of filtered sodium the kidney is letting go. FENa equals (urine sodium × plasma creatinine) divided by (plasma sodium × urine creatinine), times 100. In pre-renal AKI the tubule is intact and, driven by angiotensin II and aldosterone, reabsorbs sodium avidly; almost none escapes, so FENa falls below 1%. In established ATN the injured tubule has lost that grip and sodium leaks into the urine, pushing FENa above 2%. The number is not a separate fact to memorise — it is a direct readout of whether the tubular cells are working. It is only interpretable in oliguric AKI, because a non-oliguric kidney has not been asked the question.
When FENa lies, and what to use instead
FENa is confounded often enough that a single value should never overrule the clinical picture. Diuretics force sodium out and falsely raise it in a genuinely pre-renal patient. Chronic kidney disease runs a higher baseline FENa. And it can be deceptively low — under 1% despite real tubular injury — in contrast nephropathy, pigment nephropathy, sepsis, early obstruction, and glomerulonephritis, all of which preserve some sodium avidity. When the patient is on a diuretic, switch to the fractional excretion of urea: urea is reabsorbed largely in the proximal tubule, upstream of where loop and thiazide diuretics act, so a FEUrea below roughly 35% still flags pre-renal physiology when FENa has been corrupted.
Osmolality, urine sodium, and the urea ratio
Three cruder measures back up the fractional excretions. Urine osmolality reads the concentrating mechanism: above 500 mOsm/kg means intact tubules responding to ADH, the pre-renal pattern, while a value stuck near plasma — isosthenuria around 300 — means the medullary gradient and concentrating machinery are damaged, the ATN pattern. A spot urine sodium below 20 mmol/L echoes the avid reabsorption of pre-renal AKI and above 40 the leak of ATN, though it is less reliable than FENa. The BUN-to-creatinine ratio rises above 20 in pre-renal states because urea is reabsorbed alongside the water the kidney is busy conserving; treat it as supportive only, since gastrointestinal bleeding, corticosteroids, a catabolic state, and high protein intake all raise urea independently.
Imaging: ultrasound first, and the POCUS extension
Ultrasound is the first-line image in AKI for one non-negotiable reason: it excludes obstruction. It also reads kidney size and echogenicity — small, bright kidneys betray chronicity, while normal or enlarged kidneys fit an acute or infiltrative process — and Doppler can interrogate the renal vessels. Its blind spot is timing: a freshly obstructed or volume-deplete collecting system may not yet have dilated, so a normal first scan does not exclude obstruction when the clinical suspicion is high; rehydrate and repeat, or move to non-contrast CT, which is also the test of choice for stones. Point-of-care ultrasound has turned this into a bedside, repeatable examination. The inferior vena cava and lung fields gauge volume and congestion, a bladder view gives an immediate post-void residual, and venous-excess (VEXUS) imaging grades the venous congestion that drives cardiorenal AKI — a theme Chapter 6 develops.
Putting it together: from tests to category
Read in concert, the tests collapse onto the categories of Chapter 2. A bland sediment with concentrated urine, FENa under 1%, a high urea ratio, and no hydronephrosis is pre-renal. Muddy-brown casts with isosthenuria and FENa over 2% is ATN. Dysmorphic red cells and red-cell casts are glomerular; white-cell casts with a new drug are interstitial; hydronephrosis is post-renal. The art is not in any single result but in refusing to let one number — a diuretic-inflated FENa, an early-negative ultrasound, a pigment-positive dipstick — override the pattern the others draw.
