(White/Red) blood cell casts are a classic feature in acute pyelonephritis.
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A 45-year-old man comes to the emergency department with a gunshot wound to the abdomen. His wife says that he was getting dressed in his closet when his loaded handgun fell from the top shelf and unloaded one round when it struck the floor. His temperature is 37.2°C (99.1°F), pulse is 132/min, respirations are 22/min, and blood pressure is 65/45 mm Hg. The patient is non-responsive to verbal and painful stimuli. Physical examination shows that skin is clammy and cool to touch with dry mucous membranes. A bullet entrance wound is visible in the epigastric region. Diffuse ecchymosis and distention of the abdomen are also present. A Foley catheter is placed and dark brown urine accumulates in the collection bag. Which type of renal cast is most likely to be seen on urinalysis?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
With acute pyelonephritis, pyelo- means pelvis, and -neph- refers to the kidney, so in this case it’s the renal pelvis, which is the funnel-like structure of the kidney that drains urine into the ureter, and -itis means inflammation. So acute pyelonephritis describes an inflamed kidney that develops relatively quickly, usually as a result of a bacterial infection.
Now a urinary tract infection, or UTI, is any infection of the urinary tract, which includes the upper portion of the tract—the kidneys and the ureters, and the lower portion of the tract—the bladder and urethra. So acute pyelonephritis is a type of upper urinary tract infection.
Acute pyelonephritis is most often caused by ascending infection, meaning bacteria start by colonizing the urethra and bladder, which would be a lower urinary tract infections, and make their way up the ureters and kidney, therefore upper UTI shares a lot of the same risk factors as lower UTI, things like female sex, sexual intercourse, indwelling catheters, diabetes mellitus, and urinary tract obstruction.
One major factor that increases the risk of an upper UTI from a lower UTI spreading upward is vesicoureteral reflux, or VUR, which is where urine is allowed to move backward up the urinary tract, which can happen if the vesicoureteral orifice fails. The vesicoureteral orifice is the one-way valve that allows urine to flow from each ureter into the bladder, but not in the reverse direction.
VUR can be the result of a primary congenital defect or it can be caused by bladder outlet obstruction, which increases pressure in the bladder and distorts the valve.
As kind of a double-whammy, obstruction also leads to urinary stasis, where urine stands still, which makes it easier for bacteria to adhere and colonize the urinary tract.
Now, it’s also possible that kidneys get infected via hematogenous infection, or spread through the bloodstream, although this is a lot less common.
Usually pyelonephritis from hematogenous spread is a consequence of septicemia or bacteremia—which is bacteria in the blood, as well as infective endocarditis, an infection of the inner layer of the heart. In these situations, the most common organisms are Staphylococcus species and again E. coli.