Investigations

1st investigations to order

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Often an acutely elevated serum creatinine may be the initial or only sign of decline in renal function.

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acutely elevated serum creatinine, high serum potassium, metabolic acidosis

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Consider other causes of elevated BUN (such as drug-induced elevations or gastrointestinal bleeding) when interpreting results.

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serum BUN to creatinine ratio ≥20:1 supports prerenal azotemia

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Collected as clean-catch specimen.

Patients with glomerular disease typically present with proteinuria and microscopic hematuria with hypertension and edema.

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red blood cells, WBCs, cellular casts, proteinuria, bacteria, positive nitrite and leukocyte esterase (in cases of infection)

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Collected if there is suspicion of infection on initial urinalysis.

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bacterial or fungal growth may occur

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Anemia is suggestive of possible chronic kidney disease, blood loss.

Leukocytosis may support infection.

Thrombocytopenia can be seen in rare disorders such as cryoglobulinemia, hemolytic uremic syndrome, or thrombotic microangiopathies.

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anemia, leukocytosis, thrombocytopenia

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May also be seen in glomerulonephritis, hepatorenal syndrome, and some cases of obstruction, as long as tubular function remains intact. 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%.[ Fractional Excretion of Sodium ]

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<1% supports prerenal azotemia

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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 external link opens in a new window

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<35% supports prerenal azotemia

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Urinary eosinophil counts have low sensitivity and specificity for acute interstitial nephritis, but may be of some use in patients with pyuria.[78]

Eosinophiluria may also be seen with atheroembolic disease.

Result

>5% to 7% supports a diagnosis of interstitial nephritis

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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.

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diagnostic for metabolic acidosis and certain intoxications

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May be administered with crystalloid or colloid (but not hydroxyethyl starch solutions), and is both diagnostic and therapeutic in suspected prerenal azotemia.

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renal function improves rapidly in prerenal azotemia

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Diagnostic and therapeutic for bladder neck obstruction in addition to providing an assessment of residual urine and a sample for analysis.

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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)

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Evaluates maintenance of normal tubular function and response to antidiuretic hormone in cases of hypovolemia.

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high in prerenal azotemia (the effect of dyes and mannitol must be excluded); close to serum osmolality in acute tubular necrosis

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High levels in acute tubular necrosis not exclusive to the diagnosis.

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<20 mEq/L (suggests avid sodium retention in renal hypoperfusion and prerenal azotemia); high level (often with acute tubular necrosis)

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Assists in evaluation of postobstructive causes as well as in the evaluation of renal architecture and size (underlying chronic kidney disease).

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dilated renal calyces (suggesting obstruction), reduced corticomedullary differentiation, or small and sclerotic-appearing kidneys (suggesting chronic kidney disease)

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If renal failure is associated with heart failure.

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may show signs of pulmonary edema and cardiomegaly

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Changes may occur with severe hyperkalemia.

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peaked T waves, increased PR interval, widened QRS, atrial arrest, and deterioration to a sine wave pattern (if severe hyperkalemia)

Investigations to consider

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Elevated titer is supportive of a diagnosis of systemic lupus erythematosus, which often has renal manifestations.

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normal or elevated

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Elevated titer supports the diagnosis of systemic lupus erythematosus, which often has renal manifestations.

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normal or elevated

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Low complement levels support an active disease process, such as systemic lupus erythematosus.

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normal or depressed

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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).

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normal or elevated

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Elevated titers are seen in vasculitic syndromes such as granulomatosis with polyangiitis (formerly known as Wegener granulomatosis), eosinophilic polyangiitis, and microscopic polyangiitis.

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normal or elevated titers

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The presence of positive serology in active hepatitis C is associated with renal conditions such as membranoproliferative glomerulonephritis and cryoglobulinemia.

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positive or negative serology

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HIV-associated nephropathy and certain medications used in the management of HIV have renal complications.

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positive or negative

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The presence of cryoglobulins support cryoglobulin-associated renal disease, if AKI is present.

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positive or negative serology

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A normal erythrocyte sedimentation rate argues against the presence of inflammatory renal disease or embolic injury.

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normal or elevated

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An elevated titer supports but does not make a diagnosis of an infectious glomerulonephritis.

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normal or elevated

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Sometimes required to further evaluate cases of obstruction suggested on ultrasound.

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image of mass or stone may be present

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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.

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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)

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May be used if obstruction due to stenosis of the ureter is suspected.

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direct visualization and treatment of ureteral stenosis if present

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Biopsy is frequently required to further investigate positive serologic studies for suspected glomerulonephritis.

Biopsies 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

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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;[79][80][81] however, further studies are needed to determine their clinical utility.[82][83][84][85][86][87]

Result

results indicative of renal damage

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