USMLE Review Aids: Acute Infections of the Urinary Tract

Integrated basically as part of USMLE review for USMLE Step 2 CS, acute infections of the urinary tract are relatively common in pediatric practice. Such infections are rarely limited to a single portion of the urinary tract and for this reason the term “pyelitis,” commonly applied is scarcely applicable. Better appreciated in USMLE videos and photos from USMLE downloads, infections of the renal pelvis usually extend into the renal parenchyma, producing a pyelonephritis and there is usually some inflammation of the ureters and the bladder.

The presence of an acute infection of the urinary tract may be suspected from the clinical symptoms, but the diagnosis is established only by the demonstration of pus cells and bacteria in the urine, which have not been acquired from some extra-urinary source such as vaginitis or “container contamination” During a temporary urinary obstruction of a unilateral infection there may be neither pus cells nor bacteria. In other instances one or the other of them may be absent.

Urine is normally sterile but it is readily contaminated during and after voiding. Urine also contains a small number of white blood cells under normal circumstances. Addis counts of healthy children demonstrate about 1,000,000 white blood cells in the urine excreted in a twelve-hour period. In a fresh centrifuged specimen obtained by similar means there should not be more than one to two white blood cells per low power microscopic field.

During acute febrile diseases in girls there is often an increased number of white blood cells in the urine, which disappear with the subsidence of the fever. In most instances this pyuria arises from the vaginal secretion and not from an infection of the urinary tract.

The incidence of the acute form of pyelonephritis is highest during the diaper age, being somewhat less during the first few weeks of life than during the remainder of infancy. As might be expected, the incidence of chronic infections is highest in older children. Pyelonephritis occurs about six times as frequently in the female as in the male, except during the neonatal period, when there is about an equal incidence in the two sexes.

Many different bacteria infect the urinary tract, but the colon bacillus group is responsible for about 80 percent of all cases. Less frequently the staphylococcus, hemolytic streptococcus, Streptococcus fecalis and still less commonly other organisms such as those of the salmonella, dysentery groups,typhoid and tubercle bacilli. Tubercle bacilli are present in the urine only when the urinary tract itself is infected.