Acute Kidney Injury in Adults: An Underdiagnosed Condition

Kristy Washinger; MSN; FNP-BC

Disclosures

Journal for Nurse Practitioners. 2017;13(10):667-674. 

In This Article

Diagnostic Tests

The first step to diagnosing AKI is to establish a patient's baseline level of kidney function. It is best if the creatinine levels 3 months before the episode are known to determine whether a patient has underlying CKD. The diagnosis of CKD is based on the presence of abnormalities for more than 90 days or structural kidney changes.[13] A problem frequently occurs when a baseline serum creatinine level is not available.

Creatinine

Serum creatinine (SCr) is the most widely used marker for assessing kidney function. Creatinine is affected by a patient's age, race, catabolic rate, dietary intake, and muscle mass and is an insensitive biomarker for AKI. An increase in the creatinine level lags the kidney injury by as much as 12 hours and up to 2 days.[14]

Blood Urea Nitrogen

Blood urea nitrogen (BUN) is another commonly used marker in diagnosing AKI. Like creatinine, urea is not produced at a constant rate, and the rate can be influenced by extrarenal factors. The factors that enhance urea production include critical illness, burns, trauma, gastrointestinal bleeding, corticosteroid therapy, sepsis, and a high protein diet. These can limit the usefulness of BUN in assessing kidney function. Normally, the BUN:SCr ratio is approximately 15:1; in patients with AKI, this ratio can exceed 20:1.[15]

eGFR

eGFR is a test to measure kidney function and determine the stage of kidney disease. It can be calculated based on SCr, age, body size, race, and sex. Because of the limitations associated with SCr, this test is also an insensitive biomarker for AKI.

Fractional Excretion of Sodium

The fractional excretion of sodium is measured in terms of plasma and urine sodium. It is the percentage of the sodium filtered by the kidney that is excreted in the urine and is useful in diagnosing prerenal AKI but is an insensitive marker for intrarenal and postrenal AKI.

Complete Blood Count

A complete blood count (CBC) is helpful in evaluating anemia from acute blood loss or decreased erythropoietin production, platelet dysfunction, and/or infection. A CBC is vital in the diagnosis of hematologic manifestations in AKI.

Urine Flow

Urine flow can present as oliguria (urine output < 400 mL/d), anuria (urine output less than 100 mL/d), or normal urine output.[8] Urine flow is also influenced by hydration status and diuretic use. Urinary flow rates often provide helpful information about the cause of AKI. Nonoliguric states may be present in all types of AKI, whereas sustained periods of anuria may suggest urinary tract obstruction.

Urine Microscopy

Urine microscopy is an integral diagnostic tool in AKI and has traditionally been used to characterize prerenal AKI and ATN.[2] Hematuria and/or proteinuria may be seen in glomerulonephritis. Often, white blood cell casts indicate AKI caused by interstitial nephritis. Granular red blood cell casts and/or renal tubular epithelial cells indicate an increased likelihood of ATN (Table 5).

Imaging

A renal ultrasound shows structural imaging such as renal parenchymal size, scarring, calcification, and polycystic kidneys. An ultrasound can be used to differentiate between AKI and CKD. A small kidney size on ultrasound strongly supports a diagnosis of CKD.[2] Enlarged kidneys with a smooth contour indicate ATN.[2] A bladder scan will show the presence of a postvoid residual volume, and a urine volume > 100 mL suggests postrenal AKI.[8]

Noncontrast computed tomographic and magnetic resonance imaging scans analyze kidney structure and renal artery calcification. Noncontrast computed tomographic imaging is the gold standard for detecting ureteric calculi.[2] As noted previously, IV contrast use can cause or worsen AKI.

Electrocardiogram

An electrocardiogram may show peaked T waves, PR prolongation, and QRS widening caused by hyperkalemia during an episode of AKI.

Kidney Biopsy

Consider a kidney biopsy in the absence of an obvious cause of AKI, heavy proteinuria, persistent hematuria, or prolonged course of AKI (greater than 2–3 weeks).[2] Biopsy is considered the gold standard in diagnosing glomerulonephritis or acute interstitial nephritis.[2] A kidney biopsy is important when a disease-specific therapy is indicated. Consultation with nephrology should occur before any planned kidney biopsy.

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