ACUTE KIDNEY INJURY
ABDULLAH S . K . AL-ANZI
4th year – medical student
NBU – medical college
Twitter on: @ASKAnzi
KIDNEY FUNCTION STUDIES
increased levels of blood urea nitrogen (BUN) and
creatinine are the hallmarks of renal failure, the
rate of rise depends on the degree of renal insult
and, with respect to BUN, on protein intake.
BUN may be elevated in patients with
gastrointestinal (GI) or mucosal bleeding, steroid
treatment, or protein loading.
The ratio of BUN to creatinine is an important
finding. The ratio can exceed 20:1 in conditions in
which enhanced reabsorption of urea is favored
(eg, in volume contraction); this suggests prerenal
ACUTE OR CHRONIC
The distinction between acute and chronic uraemia depends
in part on the history, duration of symptoms and previous
urinalysis or measurements of renal function.
A rapid rate of change of serum urea and creatinine
with time suggests an acute process.
A normochromic, normocytic anaemia suggests chronic
disease, but anaemia may complicate many of the
diseases that cause AKI, owing to a combination of
haemolysis, haemorrhage and deficient erythropoietin
Several laboratory tests, including the following, are
useful for assessing the etiology of acute kidney
injury (AKI) and can aid in proper management of
Complete blood count (CBC)
Urine analysis with microscopy
In some cases, renal imaging is useful, especially if renal failure is
secondary to obstruction. The American College of Radiology
recommends ultrasonography, preferably with Doppler methods, as the
most appropriate imaging method in AKI.
CBC, PERIPHERAL SMEAR
The presence of the following, along with related findings,
may help to further define the etiology of AKI:
Myoglobin or free hemoglobin - Eg, pigment nephropathy
Increased serum uric acid level-Eg, tumor lysis syndrome
Serum lactate dehydrogenase (LDH) - Eg, renal infarction
The peripheral smear may show schistocytes in
conditions such as hemolytic uremic syndrome (HUS) or
thrombotic thrombocytopenic purpura (TTP).
Possible tests include the following:
Complement levels ( C3-C4 for SLE )
Antinuclear antibody (ANA)
Antineutrophil cytoplasmic antibody (ANCA)
Anti-glomerular basement membrane (anti-GBM)
Hepatitis B and C virus studies
Findings of granular, muddy brown casts are highly suggestive of
tubular necrosis (see the image below).
The presence of tubular cells or tubular cell casts also supports
the diagnosis of ATN. Often, oxalate crystals are observed in
cases of ATN.
Reddish brown or cola-colored urine suggests thepresence
of myoglobin or hemoglobin, especially in the setting of
a positive dipstick for heme and no red blood cells (RBCs)
on the microscopic examination.
The dipstick assay may reveal significant proteinuria as a result of tubular
The presence of RBCs in the urine is always pathologic
The presence of white blood cells (WBCs) or WBC casts suggests
pyelonephritis or acute interstitial nephritis.
The presence of urine eosinophils is helpful in
establishing a diagnosis but is not necessary for allergic
interstitial nephritis to be present.
However, this finding can also be seen in urinary tract
infections, glomerulonephritis, and atheroembolic disease.
The presence of uric acid crystals may represent ATN
associated with uric acid nephropathy.
Calcium oxalate crystals are usually present in cases of
ethylene glycol poisoning
FENA ))OF SODIUM
Urine electrolyte findings also can serve as valuable indicators of
functioning renal tubules. FENa is the commonly used indicator.
the interpretation of results from patients in nonoliguric states,
those with glomerulonephritis, and those receiving or ingesting
diuretics can lead to an erroneous diagnosis
FENa can be a
valuable test for
& useful in AKI only
in the presence of
FRACTIONAL EXCRETION OF UREA
In patients who are receiving diuretics, (FEUrea) can be
obtained, since urea transport is not affected by diuretics.
(FEUrea of less than 35% is suggestive of a prerenal state.)
The formula for calculating the FEUrea is as follows:
Creatinine elevation is a late marker for renal dysfunction and,
once elevated, reflects a severe reduction in GFR
The most promising biomarker to date is urinary neutrophil
gelatinase-associated lipocalin (NGAL), which has been shown to
detect AKI in patients undergoing cardiopulmonary bypass
the markers plasma B-type natriuretic peptide (BNP) and NGAL
and found it to be a strong predictor of early AKI in patients with
lower respiratory tract infection.
The presence of a BNP level of over 267 pg/mL or an NGAL
level of greater than 231 ng/mL correctly identified 15 of 16 early
AKI patients, with a sensitivity of 94% and a specificity of 61%.
the cystatin C level was less sensitive than the creatinine level
for detecting AKI. (confirmation )