Urinary Obstruction Workup

Updated: Feb 23, 2016
  • Author: Michael A Policastro, MD; Chief Editor: Erik D Schraga, MD  more...
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Workup

Laboratory Studies

As with any renal disease, initial workup should begin with a urinalysis and examination of sediment by an experienced examiner. Whenever an unremarkable urinalysis is observed in a patient with renal failure, obstruction should be considered. In a completely obstructed system, no distal urine flow occurs. A benign urinalysis may be seen with calculi or tumor.

Microscopic hematuria can be found with a calculus or tumor. Pyuria should suggest the possibility of UTI. Crystalluria may be found in nephrolithiasis or intratubular crystal formation. Proteinuria and casts rarely are found in acute obstruction.

Urinary diagnostic indices

Urine sodium, creatinine, and osmolality often are difficult to interpret in the face of obstruction.

Early in the course of obstruction, prior to severe renal impairment, prerenal indices commonly are observed where urine sodium is depressed, osmolality is elevated, and fractional excretion of sodium is less than 1%.

With progressive obstruction and renal failure, indices consistent with acute tubular necrosis are observed, with elevated urine sodium, depressed osmolality, and fractional excretion of sodium greater than 1%.

Serum electrolytes

Serum electrolytes (sodium, potassium, chloride, bicarbonate, BUN, creatinine) as well as calcium, phosphate, magnesium, uric acid, and albumin should be measured.

A hyperkalemic nonanion gap metabolic acidosis (type IV RTA) may be an important clue to urinary obstruction.

Elevations of BUN and creatinine may be caused by renal failure secondary to obstruction.

Complete blood count

Elevated white blood cell (WBC) count may be consistent with UTI or hematologic malignancy.

Polycythemia can be a complication of obstruction; anemia commonly is described in chronic renal failure.

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Imaging Studies

Goals of any imaging study are to distinguish anatomic etiologies from functional forms of collecting system dilation and to identify the site of blockage. Once the decision is made to use an imaging study, the patient's condition (eg, pregnancy and radiation risks, renal failure and radiocontrast risks) must be taken into consideration.

Dilation of urinary tract - Obstruction versus functional

Dilation without obstruction (functional) - Vesicoureteral reflux and one of the following:

  • Chronic massive diuresis
  • Extrarenal pelvis
  • Calyceal diverticula
  • Congenital megacalyces
  • Ileal conduits

Obstruction without dilation - Intermittent obstruction with one of the following:

  • Partial obstructions
  • Intrarenal crystals
  • Nephrocalcinosis
  • Staghorn calculi
  • Retroperitoneal obstruction

CT scan

Compared with ultrasonography, CT scan is particularly adept at diagnosing obstruction at the ureteric level. A dilated urinary collecting system is well visualized even without the use of intravenous dye. CT scan can identify calcified and noncalcified calculi.

Ionizing radiation and the nephrotoxicity of radiocontrast dye are of particular importance in patients with chronic renal failure, diabetes mellitus, or multiple myeloma.

Helical CT scan (especially without contrast) rapidly is replacing kidneys, ureter, bladder (KUB) x-rays as the first step in the radiologic evaluation of the urinary system. Although this technique is sensitive for diagnosing hydronephrosis, its real utility may be in delineating the anatomy of obstruction when ultrasonography and intravenous pyelography (IVP) fail to identify an etiology. CT scanning may replace invasive pyelography as the secondary procedure of choice to define the anatomy of obstruction after ultrasonography and IVP.

MRI

Advantages include the following:

  • Sensitivity of 100% and specificity of 96% for urinary obstruction
  • No ionizing radiation
  • Paramagnetic contrast agents - Allow renal functional assessment without risks of iodinated contrast materials
  • Can identify calcified and noncalcified calculi
  • Can differentiate acute from chronic obstruction by demonstrating perinephric fluid accumulation, which is correlated highly with acute obstruction

MRI can identify only obstruction with dilation and cannot differentiate functional dilation from anatomic obstruction.

Where available, MRI quickly is becoming the imaging study of choice for urinary obstruction.

Intravenous pyelography

IVP provides detailed morphology of the obstruction, and it is well accepted and easily accessible.

Nephrotoxicity of radiocontrast dye is of particular importance in patients with chronic renal failure, diabetes mellitus, or multiple myeloma. In patients with low GFR, excretion of the dye may take 12-24 hours after injection. Moreover, this technique cannot differentiate urinary dilation due to functional obstruction from that due to anatomic obstruction.

IVP is the procedure of choice for defining the extent and anatomy of obstruction.

Invasive pyelography

It provides the same information as IVP without dependence upon renal function and does not expose the patient to nephrotoxicity from intravenous dye.

It cannot differentiate urinary dilation from functional versus anatomical obstruction, and the invasive techniques have the added risk of infection.

This technique can be used when the risks of IVP are considered too great; dye can be injected directly into the renal pelvis (antegrade) or cystoscopically into the ureters (retrograde).

Ultrasonography

Sensitivity is higher than that of IVP; sensitivity for detecting hydronephrosis is 90%. Ultrasonography is particularly useful in identifying hydronephrosis in patients with low GFR. Patients are not exposed to ionizing radiation or intravenous dye (ie, no nephrotoxicity), and cost is low. Additionally, bladder wall thickness, bladder weight, and prostate size may aid in diagnosing bladder outlet obstruction.

The study's effectiveness is operator dependent; false-positive rate is as high as 20%; ureters cannot be visualized. Reliability of bladder wall thickness index and variable anatomic sites needs to be confirmed in larger studies.

Ultrasonography is the procedure of choice to determine the presence of hydronephrosis.

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Other Tests

Electrocardiogram (ECG) should be completed rapidly in these patients to identify the effects of hyperkalemia, a common complication of obstructive nephropathy.

Postvoid residual (PVR) urine volume test can provide important information if bladder outlet obstruction is suspected. Perform test by placing a urinary catheter after a voiding attempt. Urologists often observe the voiding attempt, looking for signs of bladder outlet obstruction, as follows: hesitancy, stream strength, and need to double void. PVR urine volume greater than 125 mL is abnormally large and may indicate obstruction.

Radionucleotide scan

Radionucleotide scan has high sensitivity (90% for detecting hydronephrosis) and involves no exposure to intravenous dye (ie, no nephrotoxicity).

It lacks the resolution of IVP to adequately define the exact site of anatomic obstruction.

Injection of a diuretic agent during the study helps in differentiation of functional and anatomic obstruction. Increased postdiuretic radionucleotide excretion points to a functional cause rather than an obstructive cause. Renal blood flow measurement by radionucleotide scan is becoming the technique of choice to assess the potential for renal recovery after relief of obstruction.

Nonvisualization of the kidney does not conclusively rule out the potential for recovery after relief of obstruction.

In pediatric patients with dilated urinary tract or suspected vesicoureteral reflux (VUR), perform voiding cystourethrography. Alternative imaging study methods include CT, MRI, and echo-enhanced cystosonography.

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