Tests
1st tests to order
basic metabolic profile (including blood urea nitrogen [BUN] and creatinine)
Test
Often an acutely elevated serum creatinine may be the initial or only sign of decline in renal function.
Result
acutely elevated serum creatinine, high serum potassium, metabolic acidosis
ratio of serum BUN to creatinine
Test
Consider other causes of elevated BUN (such as drug-induced elevations or gastrointestinal bleeding) when interpreting results.
Result
serum BUN to creatinine ratio ≥20:1 supports prerenal azotemia
urinalysis
Test
Collected as clean-catch specimen.
Patients with glomerular disease typically present with proteinuria and microscopic hematuria with hypertension and edema.
Result
red blood cells, WBCs, cellular casts, proteinuria, bacteria, positive nitrite and leukocyte esterase (in cases of infection)
urine culture
Test
Collected if there is suspicion of infection on initial urinalysis.
Result
bacterial or fungal growth may occur
complete blood count
Test
Anemia is suggestive of possible chronic kidney disease, blood loss, or acute inflammation.
Leukocytosis may support infection.
Thrombocytopenia can be seen in rare disorders such as cryoglobulinemia, hemolytic uremic syndrome, or thrombotic microangiopathy.
Result
anemia, leukocytosis, thrombocytopenia
fractional excretion of sodium
Test
May also be seen in glomerulonephritis, hepatorenal syndrome, and some cases of obstruction, as long as tubular function remains intact.[100][101] Increased levels are also caused by diuretics. The FENa is calculated as follows: (urine sodium x serum creatinine)/(serum sodium x urine creatinine) x 100%.
Result
<1% supports prerenal azotemia; typically <0.2% in hepatorenal syndrome
fractional excretion of urea
Test
Test used if patient has been exposed to diuretics. The fractional excretion of urea is calculated as follows: (urine urea x serum creatinine)/(serum urea x urine creatinine) x 100%. Fractional excretion of urea: calculator Opens in new window
Result
<35% supports prerenal azotemia
urinary eosinophil count
Test
Urinary eosinophil counts have low sensitivity and specificity for acute interstitial nephritis, but may be elevated in patients with pyuria.[99]
Some guidelines (e.g., the American Association for Clinical Chemistry) advise against routine use in the evaluation of AKI.[100] Eosinophiluria may also be seen with atheroembolic disease.
Result
>5% to 7% weakly supports a diagnosis of interstitial nephritis but is not diagnostic
venous blood gases
Test
Anion gap acidosis seen in acute and chronic renal failure due to impaired excretion of nonvolatile acids.
Assists in further evaluation of acidosis, which is often suggested by the low bicarbonate on the basic metabolic profile.
Result
diagnostic for metabolic acidosis and certain intoxications
fluid challenge
Test
May be administered with crystalloid or colloid (but not hydroxyethyl starch solutions), and is both diagnostic and therapeutic in suspected prerenal azotemia.
Result
renal function may improve rapidly in prerenal azotemia
bladder catheterization
Test
Diagnostic and therapeutic for bladder neck obstruction in addition to providing an assessment of residual urine and a sample for analysis.
Result
significant urine volume released after catheter placement (in cases of bladder outlet obstruction); minimal residual urine after catheter placement (in cases of impaired urine production or higher level obstruction)
urine osmolality
Test
Evaluates maintenance of normal tubular function and response to antidiuretic hormone in cases of hypovolemia.
Result
high in prerenal azotemia (the effect of dyes and mannitol must be excluded); close to serum osmolality in acute tubular necrosis
urine sodium concentration
Test
High levels in acute tubular necrosis not exclusive to the diagnosis.
Result
<20 mEq/L (suggests avid sodium retention in renal hypoperfusion and prerenal azotemia); high level (often with acute tubular necrosis)
renal ultrasound
Test
Assists in evaluation of postobstructive causes as well as in the evaluation of renal architecture and size (underlying chronic kidney disease).
Result
dilated renal calyces (suggesting obstruction), reduced corticomedullary differentiation, or small and sclerotic-appearing kidneys (suggesting chronic kidney disease)
chest x-ray
Test
If renal failure is associated with heart failure.
Result
may show signs of pulmonary edema and cardiomegaly
ECG
Test
Changes may occur with severe hyperkalemia.
Result
peaked T waves, increased PR interval, widened QRS, atrial arrest, and deterioration to a sine wave pattern (if severe hyperkalemia)
Tests to consider
antinuclear antibodies
Test
Elevated titer is supportive of a diagnosis of systemic lupus erythematosus, which often has renal manifestations.
Result
normal or elevated
anti-DNA
Test
Elevated titer supports the diagnosis of systemic lupus erythematosus, which often has renal manifestations.
Result
normal or elevated
complement (C3, C4, CH50)
Test
Low complement levels support an active disease process, such as systemic lupus erythematosus.
Result
normal or depressed
anti-glomerular basement membrane antibodies
Test
Elevated antibody titers to the glomerular basement membrane, which may present in diseases of the kidney (e.g., Goodpasture syndrome and antiglomerular basement membrane syndrome).
Result
normal or elevated
antineutrophil cytoplasmic antibodies
Test
Elevated titers are seen in vasculitic syndromes such as granulomatosis with polyangiitis (formerly known as Wegener granulomatosis), eosinophilic polyangiitis, and microscopic polyangiitis.
Result
normal or elevated titers
acute hepatitis profile
Test
The presence of positive serology in active hepatitis C is associated with renal conditions such as membranoproliferative glomerulonephritis and cryoglobulinemia.
Result
positive or negative serology
HIV serology
Test
HIV-associated nephropathy and certain medications used in the management of HIV have renal complications.
Result
positive or negative
cryoglobulins
Test
The presence of cryoglobulins support cryoglobulin-associated renal disease, if AKI is present.
Result
positive or negative serology
erythrocyte sedimentation rate
Test
A normal erythrocyte sedimentation rate argues against the presence of inflammatory renal disease or embolic injury.
Result
normal or elevated
antistreptolysin-O antibody
Test
An elevated titer supports but does not make a diagnosis of an infectious glomerulonephritis.
Result
normal or elevated
abdominal computed tomography or magnetic resonance imaging scan
Test
Sometimes required to further evaluate cases of obstruction suggested on ultrasound.
Result
image of mass or stone may be present
nuclear renal flow scan
Test
May be modified using captopril to evaluate for renal artery stenosis, or furosemide to evaluate for obstruction in cases of hydronephrosis where obvious mechanical obstruction is uncertain.
Result
normal scan reveals appropriate renal perfusion, tracer uptake, and excretion; impaired tracer excretion (supportive of acute tubular necrosis); poor blood flow (supportive of obstruction of blood supply); normal blood flow and tracer excretion with tracer accumulation in the collecting system (supportive of obstruction of the urine outflow tract)
cystoscopy
Test
May be used if obstruction due to stenosis of the ureter is suspected.
Result
direct visualization and treatment of ureteral stenosis if present
renal biopsy
Test
Biopsy is frequently required to further investigate positive serologic studies for suspected glomerulonephritis.
Biopsies are also done when the cause of kidney injury is unclear.
May confirm acute tubular necrosis, but not often performed for this diagnosis.
Result
changes associated with acute tubular necrosis, glomerulonephritis, vasculitis, or other intrinsic renal disease may be present
Emerging tests
novel serum and urinary biomarkers
Test
Various novel serum and urinary biomarkers are showing potential as useful indicators for the diagnosis and classification of AKI and as predictors of mortality after AKI; however, further studies are needed to determine their clinical utility.[102][103][104][105][107][108]
Result
results indicative of renal damage
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