Acute kidney injury laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

Overview

An elevated/reduced concentration of serum/blood/urinary/CSF/other [lab test] is diagnostic of [disease name].

OR

Laboratory findings consistent with the diagnosis of [disease name] include [abnormal test 1], [abnormal test 2], and [abnormal test 3].

OR

[Test] is usually normal for patients with [disease name].

OR

Some patients with [disease name] may have elevated/reduced concentration of [test], which is usually suggestive of [progression/complication].

OR

There are no diagnostic laboratory findings associated with [disease name].

Laboratory Findings

• Laboratory findings consistent with the diagnosis of acute kidney injury include:

Disease specific blood laboratory findings
Blood Laboratory Finding	Related Etiologies
Severe hyperphosphatemia, hypocalcemia, elevated CPK and uric acid	Tumor Lysis Syndrome, Rhabdomyolysis
Increased anion gap and osmolal gap	Ethylene Glycol Poisoning
Low anion gap	Multiple Myeloma
Low complement levels and high titers of ANAs, ANCAs and cryoglobulins	Vasculitides
Severe anemia in the absence of bleeding	Hemolysis, Multiple Myeloma
Anemia, thrombocytopenia, schistocytes on peripheral blood smear, elevated LDH, and low haptoglobin	TTP, HUS, DIC
Peripheral eosinophilia	Acute interstitial nephritis, atheroembolic disease, polyarteritis nodosa, Churg-Strauss
Elevated BNP	Heart Failure
Bacteremia	Sepsis

In prerenal azotemia, tubular function is preserved and sodium reabsorption increases with the associated renal vasoconstriction. Hence the FE_Na is usually <1% in prerenal azotemia. A high FE_Na in the context of prerenal azotemia is possible during diuretic treatment and glycosuria.

Another important index is the fractional excretion of urea (FE_urea) calculated using the same equation for the fractional excretion of sodium. FE_urea is of value in states of reduced effective circulating volume, and in cases where diuretics have been administered. In these situations, a low FEurea (<35%) has a higher sensitivity and specificity than FE_Na in differentiating between prerenal azotemia and renal AKI.

Distinguishing Prerenal azotemia and ATN

Parameter	Prerenal AKI	Acute Tubular Necrosis
Urinary sediment	Normal/Hyaline casts	Epithelial cell casts
Urine specific gravity	>1.020	<1.020
Urine sodium (mmol/L)	<20	>40
FENa	<1%	>2%
FEurea	<35%	>50%
Urine osmolality (mOsmol/kg H2O)	>500	<350
Urine-Plasma creatinine ratio	>40	<10
Plasma BUN-creatinine ratio	>20	<15