Diagnosis

Factors that are independent predictors of a UTI in a patient with dysuria are advanced age, history of a UTI, back pain, pyuria, hematuria, and bacteriuria. 23 and 4 Among sexually active women, the incidence of symptomatic UTIs is high, and the risk is independently associated with recent sexual intercourse, recent use of a diaphragm with spermicide, and history of a UTI.56

If UTI is suspected, the first step in establishing the diagnosis is the careful collection of urine for a urinalysis and potentially for culture. The midstream voiding specimen is as accurate as urine obtained by catheterization if the patient is given and follows careful instructions. Instruct the woman to remove her underwear, sit facing the back of the toilet, spread the labia with one hand, cleanse from front to back with povidone-iodine swabs or liquid soap, pass a small amount of urine into the toilet, and then urinate into a sterile cup. Instruct the man to carefully cleanse the urethral meatus, retracting the foreskin if uncircumcised, and obtain a midsteam specimen as described above.

If the sample is properly collected, it should contain no or few epithelial cells. The many sources of contamination include material in the collection bottle, menses, vaginal discharge, urethral or periurethral tissue, and organisms multiplying in the urine after collection. Bacteria in urine double each hour at room temperature; therefore urine should be refrigerated if not sent directly to the laboratory. In addition to special care in cleansing, the use of a tampon also helps women to obtain a clean-catch specimen if menstruation or profuse discharge is present.

Catheterization is indicated if the patient cannot void spontaneously, is too ill or immobilized, or is extremely obese. It may also be performed as part of a urologic evaluation and to relieve obstruction. Many authors promote the ease and accuracy of "minicath"-obtained urine in women, especially with a vaginal discharge or bleeding. However, unnecessary catheterization should be avoided because 1 to 2 percent of patients develop UTI after a single catheter insertion. This seems to be a problem especially if the catheter insertion is done just prior to delivery.

Although blood or bile may be detected by gross examination of the urine, visual inspection or the smell of the urine is generally not helpful in determining infection. Cloudiness is usually not due to white blood cells (WBC) or bacteria, but to large amounts of protein or amorphous phosphate crystals. Malodorous urine may be caused by diet or medications and is not a reliable sign of infection.

Current emphasis is on the detection of pyuria and bacteriuria in the initial examination of the urine to confirm the diagnosis of a UTI. However, the assessment of pyuria is imperfect. Variables include the specific gravity of the urine, method of centrifuging the specimen, the amount of supernatant in which the sediment is resuspended, and the final volume of urine under the coverslip that is examined. Laboratories that use a WBC counting chamber diminish some of this variability and increase accuracy in assessing both centrifuged and uncentrifuged urine. Using a WBC counting chamber, abnormal pyuria can be defined as the presence of 8 leukocytes or more per mL of uncentrifuged urine. This figure roughly corresponds to 2 to 5 leukocytes per high-power field (hpf or 400*) from a centrifuged specimen.

While some authors feel that low-level pyuria (<10 WBC/hpf) is clinically important, others have suggested that pyuria in women is significant only if there are more than 10 WBC/hpf, and only if bacteria is also present on the microscopic examination. Though the combination of pyuria and bacteruria is likely to be true with typical coliform infection, lower degrees of pyuria with or without bacteriuria may be significant, especially with regard to infection with Chlamydia.

As knowledge of UTI in adult women evolves, it is clear that women with symptoms and low-level pyuria (<10 WBC/hpf) do have significant infection that will symptomatically and bacteriologically respond to antimicrobial therapy. In the past, these women were not treated initially and their cultures often did not contain more than 105 CFU/mL. Sensitivity to causes of lower UTI other than typical coliforms has brought the designation of the dysuria-pyuria syndrome (also referred to as the acute urethral syndrome), which almost always benefits from treatment.1 It is in this subgroup of women that the urinalysis may well be more useful than the urine culture. In addition, a positive urinalysis would dictate more immediate management interventions than would awaiting a urine culture.

In men, more than 1 to 2 WBC/hpf can be significant in the presence of bacteria.3 Again, it must be remembered that urethritis and prostatitis are far more likely causes of pyuria in young males who are sexually active and complain of dysuria, whether or not a urethral discharge is present.

Bacteriuria is also felt to be a sensitive tool for detection of UTI in the symptomatic patient. The presence of any bacteria on a Gram stain of uncentrifuged urine (>1 bacteria per oil-power field or 1000*) is significant and highly correlates with culture results of >10 5/mL. For Gram-stained centrifuged specimens, more than 15 bacteria per oil-powered field (1000*) is significant. Both of these methods fail to detect low colony count UTI or infection caused by Chlamydia. False-positives can occur when vaginal or fecal contamination is present.

Several studies have evaluated urinary dipstick nitrite and leukocyte esterase tests in the diagnosis of UTIs, correlating the results with urinalysis. 9 The urine nitrite reaction has a very high specificity (>90 percent) and a positive result is very useful in confirming the diagnosis of a UTI. However, the urine nitrite sensitivity is low (about 50 percent), rendering it much less useful as a screening examination and a negative result does not exclude the diagnosis of a UTI. The urine leukocyte esterase has been evaluated as an indicator of the presence of pyuria, one marker of UTI. As noted above, there is an inexact correlation between clinical symptoms, results of urine culture, and pyuria as detected by microscopic examination. Initial reports of high sensitivity (near 88 percent) supported use of leukocyte esterase as a screening tool for pyuria. However, these studies were done on symptomatic women with high levels of pyuria. Studies from the emergency department have found a low sensitivity (48 percent) for leukocyte esterase with more common levels of pyuria (6 to 20 WBC/hpf).9 When the clinical presentation suggests UTI, a positive leukocyte esterase test supports the diagnosis, and treatment should be initiated without the need for microscopic examination. If the leukocyte esterase test is negative, a microscopic examination should be performed to detect lower levels of clinically significant pyuria.

Unfortunately, the clinician is sometimes faced with women who complain of dysuria, have no pyuria or demonstrable pathogen on culture, and who do not respond to antimicrobial treatment. The absence of pyuria in these patients is useful because it indicates that antimicrobial treatment is probably unnecessary. Presuming that vulvovaginitis or cervicitis (gonorrhea, chlamydia, and herpes) has been excluded, causes of dysuria may include inflammation of the urethra from physical trauma or due to the use of chemical agents, such as spermicides, cleansing douches, or other feminine hygiene products.

In a symptomatic patient who has fewer than 2 to 5 WBC/hpf, other causes of false-negative pyuria should be considered. These include ingestion of large amounts of fluids, which wash out the bladder and produce a dilute urine, and, more likely, old or leftover medication, or a drug belonging to another person, being taken by the patient on a self-directed basis, or systemic leukopenia. It should be remembered that, in the case of an infected and obstructed kidney, pyuria may be intermittent or absent.

For the patient with typical symptoms of an uncomplicated UTI and a "positive" urinalysis (pyuria on microscopic examination, positive leukocyte esterase test, bacteriuria on Gram's stain, and/or positive urine nitrite test), urine culture is not required. The vast majority of these patients respond to empiric therapy. Most authors agree that a urine culture should be obtained in these settings: acute pyelonephritis; patients with epidemiologic risk factors for subclinical pyelonephritis; any patient who needs to be hospitalized; those patients who have a chronic indwelling catheter; and all pregnant women, children, and adult males. 23 and 4 If the patient is symptomatic, a single positive culture is significant. For ABU, two or three positive cultures are necessary before treatment is undertaken, with the exception that treatment for ABU is always indicated in pregnancy.

It is commonly recommended to obtain blood cultures in cases of acute pyelopnephritis, and while they are positive in about 30 percent of patients, the results rarely change management.1 1! Renal imaging studies are not indicated in otherwise healthy patients with acute pyelonephritis that can be managed as an outpatient. Elderly, diabetic, or severely ill patients with acute pyelonephritis should be considered for imaging, particularly if there is a poor initial response to antibiotic therapy. The kidneys can be imaged with portable ultrasound at the bedside, evaluating for obstruction and focal parenchymal abnormalities. 7 Plain film radiology and ultrasound have poor sensitivity for detection of intrarenal gas formation in emphysematous pyelonephritis; CT is the best imaging modality. 8

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