The clean-catch, midstream voiding specimen is as accurate as urine obtained by catheterization if the patient follows instructions carefully. If the sample is properly collected, it should contain no or few epithelial cells. Bacteria in urine double each hour at room temperature, so urine should be refrigerated if not sent directly to the laboratory. Catheterization is indicated if the patient cannot void spontaneously, is too ill or immobilized, or is extremely obese. Avoid unnecessary catheterization, because 1% to 2% of patients develop a UTI after a single catheter insertion.
Visual inspection or assessment of the odor of the urine is generally not helpful in determining infection because cloudiness and odor are caused by noninfectious etiologies. Table 91-3 lists normal reference values for urinalysis. UTI often results in positive dipstick test for protein in the urine, but this finding is not specific enough to be useful in diagnostic decision making to rule in infection.
TABLE 91-3Normal Urinalysis Results ||Download (.pdf) TABLE 91-3 Normal Urinalysis Results
|Value ||Normal Range ||Specimen Type |
|RBCs, female ||0–5/HPF ||Centrifuged specimen |
|RBCs, male ||0–3/HPF ||Centrifuged specimen |
|WBCs ||0–4/HPF ||Centrifuged specimen |
|Bacteria ||None/HPF ||Centrifuged specimen |
|Leukocyte esterase ||None—dipstick test ||Fresh urine |
|Nitrite ||None—dipstick test ||Fresh urine |
Values for individual laboratories may differ from the listed norms in Table 91-3. Dipstick testing is performed on a fresh uncentrifuged urine specimen and is quick and easy to perform at the bedside. Urine for microscopic analysis is routinely centrifuged prior to analysis. If examination of uncentrifuged urine is desired, make a specific request to the laboratory to account for different normal values between centrifuged and uncentrifuged specimens.
NITRITE REACTION BY DIPSTICK TEST
The urine nitrite reaction has a very high specificity (>90%), and a positive result is very useful in confirming the diagnosis of a UTI caused by bacteria that convert nitrates to nitrite, primarily the coliform bacteria, including E. coli. Enterococcus, Pseudomonas, and Acinetobacter species do not convert nitrates to nitrites in the urine and therefore are not detected by the nitrite test. Unfortunately the urine nitrite reaction has a low sensitivity (~50%), so it is not always useful as a screening examination because a negative result does not exclude the diagnosis of UTI.
LEUKOCYTE ESTERASE REACTION BY DIPSTICK TEST
Using positive culture results as the criterion standard, the leukocyte esterase urine dipstick test has an overall sensitivity of 48% to 86% and a specificity of 17% to 93% for identifying infection. Performance varies by clinical setting. In the ED, using culture findings of 105 CFU/mL as the criterion, a positive leukocyte esterase reaction result has a sensitivity of 77% and a specificity of 54%. The sensitivity of positive leukocyte esterase reaction result for detecting infection decreases for specimens with less bacterial growth at culture, ranging from a sensitivity of 79.5% when culture growth is >105 CFU/mL to 50.4% when culture growth is 103 CFU/mL. Therefore, if the clinician uses a lower culture threshold to define infection, the leukocyte esterase test performs with lower sensitivity to detect infection. In summary, a positive urinary dipstick nitrite or leukocyte esterase test result supports the diagnosis of UTI, but a negative test result does not exclude it.
URINE WBC COUNT OR PYURIA BY MICROSCOPY
The assessment of pyuria using standard centrifuged urine is imperfect due to variable specimen preparation techniques. A WBC count of >5 cells/high-power field (HPF) in a centrifuged specimen from a symptomatic patient is abnormal. Although the combination of pyuria and bacteriuria is likely to be found with typical coliform infection, lower degrees of pyuria with or without bacteriuria may be clinically significant, especially in the presence of UTI symptoms.
In a symptomatic patient who has <5 WBCs/HPF in a centrifuged specimen, other causes of false-negative pyuria should be considered such as dilute precentrifuged urine, systemic leukopenia, or patient self-treatment with leftover antibiotics. Pyuria may be intermittent or absent if the patient has an obstructed and infected kidney. In men, >1 or 2 WBCs/HPF in a centrifuged specimen can be significant when bacteria are present. Urethritis and prostatitis are far more likely causes of pyuria in young males who are sexually active and complain of dysuria, regardless of the presence or absence of urethral discharge.
BACTERIURIA BY MICROSCOPY
Bacteriuria is a sensitive tool for detection of UTI in the symptomatic patient. The presence of any bacteria on a Gram-stained specimen of uncentrifuged urine (>1 bacterium/HPF or 1000×) is significant and highly correlates with culture results of >105 CFU/mL. For Gram-stained centrifuged specimens, >1 bacterium/HPF (1000×) is 95% sensitive and >60% specific to predict a culture with 104 CFU/mL. Both of these methods of looking for bacteria under the microscope fail to detect low-colony-count UTI or infection caused by Chlamydia. False-positive results can occur when vaginal or fecal contamination is present. Female patients with symptoms suggestive of UTI and vaginal discharge or dyspareunia should have a pelvic examination to investigate for pelvic inflammatory disease.
COMBINED RESULTS OF DIPSTICK TESTING, URINE CELL COUNTS, AND HISTORICAL INFORMATION
Two meta-analyses found that classic historical findings of cystitis were weak predictors of positive culture or failed to predict cystitis.27,28 Vaginal discharge weakly decreased the likelihood of cystitis; however, in both studies, diagnostic accuracy was significantly improved using dipstick testing, particularly a positive test for nitrites.27,28 A 2013 systematic review of four ED-based studies pooled 948 patients using a reference culture threshold of 104 to 105 CFU/mL and found that no single historical variable of cystitis could rule in or rule out infection, but a dipstick test positive for nitrates, moderate pyuria, and/or bacteriuria were accurate predictors of a UTI,26 effectively ruling in the diagnosis of UTI. However, no single test or combination of testing results can effectively rule out UTI in women presenting to the ED with symptoms of cystitis. Therefore, there will be a subset of women for whom test results are equivocal who should either be treated empirically or receive phone-in treatment based on culture, if follow-up cannot be assured in 2 to 3 days.
For the patient with typical symptoms of cystitis or an uncomplicated UTI and "positive" findings on urinalysis—pyuria on microscopic examination, bacteria in a Gram-stained specimen, positive leukocyte esterase test result, and/or positive urine nitrite test result—urine culture is not required. The vast majority of patients respond to empiric therapy. Urine culture should be performed for the following patients: those with complicated UTI, pregnant women, adult males, patients with relapse or reinfection, and septic patients. If the patient is symptomatic, a single positive culture result is significant.
Results of blood cultures in patients admitted for clinical pyelonephritis are positive in 25% to 29% of cases; organisms in blood culture match those in urine culture in 97% of cases, and blood culture results usually do not alter management. The primary indication for blood cultures in patients with suspected UTI is clinical sepsis.