How does an active infection occur in the urinary tract? Which factors in the ur

Question

How does an active infection occur in the urinary tract? Which factors in the urinary tract predispose humans toward urinary tract infections?

Explanation / Answer

Urinary tract infection (UTI) probably affects about one-half of all people during their lifetimes. By some estimates, UTI accounts for more than 10 million office visits and 1 million hospitalizations each year in the United States at a total cost exceeding 1 billion dollars. Up to 10% of UTIs result in serious complications such as gram-negative sepsis or chronic pyelonephritis with loss of renal function. Understanding some key definitions and their limitations facilitates a knowledgeable approach to clinical management. Urinary tract infection means the finding of bacteria (or other microorganisms, such as yeasts) in bladder urine with or without clinical symptoms and with or without renal disease. Thus “UTI” refers to a diverse group conditions. Complicated UTI implies the presence of predisposing anatomic, functional, or metabolic abnormalities. Uncomplicated UTI is encountered most frequently in healthy, young, non-pregnant women (some authorities hold that UTI in all other patient groups is by definition complicated). Complicated UTI is more difficult to treat and usually requires more aggressive evaluation and follow-up. However, the definition of “complicated” is often imprecise. Labeling patients as having “complicated” TI sometimes leads to unnecessary interventions. Significant bacteriuria traditionally refers to the laboratory finding of >105 colony-forming units (CFU) of bacteria per mL of urine. Urine cultures from patients with symptomatic UTI usually show >105 CFU/mL of urine, whereas asymptomatic patients whose cultures have been contaminated usually show < 103 CFU/mL of urine. Limitations of the “>105 per mL rule” have become increasingly apparent. In brief (and as will be discussed more fully below), fewer than 105 CFU/mL often assume significance when the pre-test probability of UTI is high because of the clinical setting. These cases are sometimes called low colony-count UTI. Stated differently, 104 or even fewer bacteria per mL of urine represent “significant bacteriuria” when there is strong clinical evidence of UTI. Asymptomatic bacteriuria denotes significant bacteriuria (> 105 CFU/mL of urine) without clinical symptoms or other abnormal findings. Lower urinary tract infection refers to infection at or below the level of the bladder. In clinical practice, “lower UTI” is often used synonymously with “cystitis,” a syndrome characterized by dysuria, frequency, urgency, and variable suprapubic tenderness. Because one cannot say with certainty that infection involves mainly or exclusively the urinary bladder, some authorities suggest that “cystitis” should be abandoned. “Lower UTI” also encompasses prostatitis, urethritis, and infection of the periurethral glands. Upper urinary tract infection refers to infection of the urinary tract above the level of the bladder; that is, the ureters, kidneys, and peri-renal tissues. This term is used mainly in reference to pyelonephritis. “Upper UTI” also encompasses intrarenal abscess (“renal carbuncle”) and perinephric abscess. Renal papillary necrosis refers to infarction of the papillae (sometimes with sloughing into the ureters), which can be caused by pyelonephritis or analgesic abuse and which often occurs in the setting of diabetes mellitus, sickle cell disease, or ureteral obstruction. Acute bacterial pyelonephritis denotes a clinical syndrome of fever, flank pain, and tenderness, often with constitutional symptoms, leukocytosis, leukocyte casts in the urine, and bacteriuria with or without signs of concomitant inflammation in the bladder. Newer imaging techniques such as CT scanning allow more precise diagnosis than was possible in the past, but are unnecessary in most cases. Chronic bacterial pyelonephritis indicates long-standing infection with active bacterial growth in the kidney, or the presence of residual lesions in the kidney caused by such infection in the past. Chronic interstitial nephritis is a term now used to indicate histologic findings resembling chronic bacterial pyelonephritis but in which evidence for an etiologic role for bacterial infection is lacking. Drugs (not only prescription drugs but also non-prescription drugs as in analgesic nephropathy) are common causes of chronic interstitial nephritis. Obstructive uropathy refers to obstruction of urine flow at any level. Ascending UTI implies that bacteria (or other microorganisms) colonize the periurethral tissues and then spread upwards into the bladder and, in some cases, the kidneys. Descending (hematogenous) UTI implies that microorganisms, having gained access to the bloodstream from another source, infect the kidneys and then other parts of the urinary tract secondarily. Urethral syndrome is a term coined for the presence of classic symptoms of urinary tract infection: dysuria (burning, pain, stinging, or other unpleasant sensations on urination), urgency, and frequency, without significant bacteriuria. Some of these cases may be due to bacterial infection of the urethra; others are due to non-bacterial agents or possibly have non-infectious etiologies. Prostatitis, cystitis, urethritis, epididymitis, and perinephritis imply inflammation (usually due to infection) of the prostate, bladder, urethra, epididymides, and peri-renal spaces, respectively. Pyuria indicates the presence of pus (white blood cells; leukocytes) in the urine, which may or may not be caused by urinary tract infection. The preferred method for quantitative evaluation of pyuria is to enumerate the number of WBC per mL of unspun urine using a counting chamber. In office practice, the leukocyte esterase dipstick test is often used as a simple method for determining the presence or absence of pyuria. Urosepsis denotes UTI as the basis for the sepsis syndrome. Blood cultures are often (but not always) positive for the same microorganism found in urine culture.

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