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Volume 2 Issue 2 February 1997

Urinary Tract Infections (UTI) are the most common infections in febrile infants and children without an obvious source of infection. As described in the accompanying algorithm, the signs and symptoms vary with age and can be nonspecific. The inability of the infant and young child to communicate makes even these symptoms elusive. The fact that, in young children, a UTI and the possibly associated urinary tract abnormalities may lead to progressive renal failure makes accurate diagnosis critical. The diagnostic regime for these abnormalities is expensive and therefore should only be undertaken for a proven UTI which requires quantitative culture.

The first step in an accurate diagnosis is the collection of a proper specimen. In infants and children without bladder control, catheterization (being careful to not collect the first amount of urine flow from the catheter) or suprapubic aspiration are acceptable. A urine specimen collected by bagging can only be used to rule out UTI. In children with bladder control, a midstream clean catch may also be acceptable. Analysis is ideally carried out within one hour. Otherwise, the urine should be refrigerated.

Routine microscopic examination of the urine and dipstick urine analysis can be useful in the presumptive diagnosis of UTI permitting early institution of therapy. However, these tests are not sensitive enough to rule out infection. In freshly examined specimens, neonates and early infections may not elicit sufficient inflammatory response to cause pyuria. Leukocytes lyse in improperly stored urine. Asymptomatic bacteriuria (colonization) with fever attributable to another source cannot be easily discriminated by a single study. In addition, pyuria may be nonspecific, occurring without UTI but with vaginal washout, chemical irritation, fever not associated with UTI, dehydration, trauma and viral infections.

The interpretation of bacteria on a microscopic exam or gram stain depends on the specimen. If collected by suprapubic aspiration or catheterization, any bacteria are significant. In a clean catch specimen >100 bacteria per high power field is commonly considered significant. An improperly stored catheterized or clean catch urine may permit bacteria to multiply and make this test unreliable.

Dipstick urinalysis is also useful for presumptive diagnosis. Leukocyte esterase detects enzymes released from lysed leukocytes. It is effective on stored urine specimens but subject to all the other concerns listed above. The nitrite test detects nitrites which have been converted from nitrates by bacteria in the bladder. This reaction requires several hours and therefore is most effective on first morning voids, rarely available in the pediatric setting. If a urine specimen from a midstream collection has been improperly stored, bacteria may multiply and produce false positive results.

In summary, minimizing the risk of kidney damage in children with UTI requires prompt treatment and appropriate workup. A quantitative culture of a fresh or properly stored urine specimen is key to this process. Dip slides are fine. At this time many studies are attempting to develop screening processes to minimize the requirement for culture. The widespread application of these procedures has yet to be tested

URINE CULTURE TURNAROUND TIME

More than 18,000 urine cultures were processed by Saint Luke’s Hospital laboratory in 1995. The existing laboratory policy was to set up two agar plates per culture and incubate them for 48 hours before sending out a final report. A clinical pathologist noticed that more contaminated cultures were being reported at 48 hours than true positive cultures. A review of the literature revealed that this problem could be alleviated by shortening the incubation time from 48 to 24 hours. This plan was discussed with infectious disease specialists and communicated to the medical staff by publication in the Clinical Laboratory Letter. The urine culture procedural change was implemented on September 1, 1995.

In June, 1996, urine culture results were monitored for a two week period. The true positive rate had not changed, but the number of contaminants reported had decreased by 68%.

Urine Culture Results at 48 versus 24 hours

Culture Results

The benefits of this change include faster turnaround time, decreased laboratory workload, and fewer repeat urine cultures. Reporting results one day earlier allowed physicians to optimize antibiotic therapy more quickly. Laboratory workload decreased because 120 fewer urine cultures had to be interpreted each day and fewer repeat cultures were submitted.

References

1. Cavagnolo R. Evaluation of incubation times for urine cultures. J. Clin Microbiol 1995; 33:1954-1956.

2. Murray P, Traynor P, Hopson D. Evaluation of microbiological processing of urine specimens: Comparison of overnight versus two day incubation. J Clin Microbiol 1992; 30:1600-01.

Chlamydia trachomatis (CT) is a common infection of women and men of reproductive age. At least 3-4 million new cases are estimated to occur in the United States each year. Because this infection is often asymptomatic it can go undiagnosed and lead to pelvic inflammatory disease and infertility in women.

Historically there have been a number of ways to test for CT including culture and various immunoassays. Each of these tests had sensitivities from 60-80% with specificity’s of 80 to 100%. New molecular testing techniques have significantly improved sensitivity and specificity. This is due in part to molecular assays not requiring viable organisms but only intact DNA fragments. Testing can be by a simple DNA probe such as the Genprobe system or by DNA amplification such as the Roche Amplicor system. Both systems have a specificity of 98 to 99%*. The Genprobe direct DNA probe assay has a sensitivity of approximately 85%* and the Roche Amplicor PCR system has a sensitivity of approximately 98%*. The difference in sensitivity can be accounted for by the DNA amplification of PCR. Both tests provide superior testing results for CT when compared to other antibody based systems. The Amplicor PCR system is also FDA approved for testing male urine. The PCR assay can also be performed on specimens collected with the Genprobe single swab (when testing for Gonnococcus is also required).

In summary the new molecular based testing systems for Chlamydia trachomatis provide excellent sensitivity and specificity for clinical testing in patients. Use of these tests according to CDC published guidelines is recommended for reducing the incidence of morbidity associated with undiagnosed CT infections such as pelvic inflammatory disease and infertility.

CDC Guidelines for Chlamydia Trachomatis testing in Women include:

1. Any sexually active woman with symptoms consistent with chlamydia infection.

2. Any sexually active woman, ages 21-35 who has had 2 or more partners in the last 2 months or a new partner in the past 2 months or a partner known to have other partners.

3. Any woman with recent acquisition of any sexually transmitted disease.

4. Sexually active woman less than 20 years of age.

* These sensitivity and specificity figures have been arrived at by both published accepted ranges in peer reviewed journals as well as in house data by comparison studies between the Genprobe and Amplicor systems performed at Physicians Reference Laboratory.

References

1. Kerley SW, et al. Evaluation of Chlamydia trachomatis by PCR (Amplicor) using a 1:100 dilution of specimens collected in GenProbe transport media. Modern Pathology,1996;9:172A

2. Cisek JM, et al. Prevalence and Characteristics of infections by Chlamydia trachomatis as detected by Polymerase Chain reaction in a Metropolitan Area. Modern Pathology, 1997 (in press).