Improving UTI Screening

Vol. 24 • Issue 2 • Page 18


Urinary tract infections (UTIs) are one of the most common bacterial infections where one or more parts of the urinary system (kidneys, ureters, urethra and bladder) become infected.

UTIs are generally classified as “uncomplicated,” which is more common among women involving infections of the lower urinary tract. A “complicated UTI,” on the other hand, occurs equally among men and women and is often associated with an underlying anatomical or physiological disorder.

UTI Risk

The severity and occurrence of UTIs varies, but those most at risk for acquiring the more serious complicated UTIs are the elderly, diabetics, pregnant women, patients with bladder and kidney dysfunctions and patients with urinary catheters.1 For the vast majority of the general population, the most prevalent UTI condition is the uncomplicated asymptomatic bacteriuria where significant numbers of bacteria have colonized in the urinary tract with an absence of notable signs and/or symptoms of an infection.2 Demographic studies have shown that, for those younger than 60, women are far more prone to develop uncomplicated UTIs than men.2 Conversely, men aged 60 years and older are more likely to develop complicated UTIs than women.1,2

While the majority of urinary tract infections are caused by Gram negative organisms, Gram types and organism species can vary between the genders and among various age groups.2 The Table presents the most commonly observed uropathogens and their prevalence among uncomplicated and complicated UTIs.3

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UTI Diagnosis

Urine culture remains the gold standard for confirming UTI diagnosis. Culture results define not only the identification, but also the quantification of uropathogenic organisms. However, urine culture testing is a costly and time-consuming methodology and with the majority of urine specimens submitted for testing having a negative result, clinical and hospital laboratories are exploring alternative techniques for more efficient UTI screening.4

Various tests are used to analyze urine – notably dipstick testing, total automated urinalysis (chemistry/ microscopy) and more recently MALDI-TOF Mass Spectroscopy.4,5 Such techniques have been used individually and in combination to selectively screen UTI positive specimens. Results thus far have been mixed. The dilemma facing most laboratories that utilize UTI screening is determining the optimal urine analyte and particle criteria to avoid false negative outcomes while reducing the number of negative samples sent for culture.

In recent years, manufacturers of medical diagnostic instrumentation have enhanced the capacity of automated urine analyzers, enabling better UTI screening. Many institutions have implemented reflex testing of urine specimens that restrict urine culture testing to samples that meet specific urinalysis criteria. The criteria used may vary among clinical and hospital laboratories, but the most often used urine chemistry ðanalytes for UTI ðscreening are positive nitrite – resulting from the reduction of nitrate by predominately Gram negative organisms – and a positive result of the neutrophil enzyme, leukocyte esterase. Urine sediment particles commonly used for UTI detection are RBC, WBC and bacteria. Microbiologists, however, caution that bacteria will be present in a contaminated sample but not always detected in urine specimens at various stages of a urinary infection.6 Thereby, the mere presence of bacteria in a urine specimen is not itself indicative of an infection and such a general reliance often leads to false positive results.

Studies on Automated Analyzers

The challenge that the clinical laboratory continues to face is correlating urinalysis UTI criteria with outcomes of the urine culture. An all too often occurrence is that even with UTI screening there remains a significant number of negative specimen results or, of greater concern, continued findings of false negative specimens. Various studies have been published testing the viability of automated urine analyzers for screening UTI specimens. C. Fox released a study that evaluated the Iris iQ200 automated microscopic urine analyzer for reflex testing of adult male urine specimens. In this particular study, the authors used only urine microscopy results to screen for suspected urinary tract infections. Using only the WBC count as an indicator with an abnormal threshold of >5 WBC/hpf, the authors found that of 874 patients, 176 (20 percent) were confirmed positive for UTI when using a threshold of CFU ≥104 /mL of uropathogens. The authors reported a PPV (positive predictive value) of 47 percent and NPV (negative predictive value) of 97 percent when using the iQ200 with WBC as a
single indicator.8

A broader study involving UTI screening criteria using the Iris iRICELL was performed by S. Riedel, et. al., and evaluated the benefits of a reflex testing policy of urine culture specimens. Urine chemistry analytes, nitrite, leukocyte esterase and clarity were used in conjunction with microscopic parameters; WBC, RBC, Bacteria/Yeast, and ASP (All Small Particles) were used to evaluate the UTI screening efficiency using total automated urinalysis. The study found that of 1,376 specimens tested, 15 percent were positive for presence of uropathogens, with the vast majority of specimens having either no growth or contamination. The authors observed that single parameters for urinalysis yielded low sensitivity and specificity, whereas complete urinalysis and, in particular, routine urine microscopy ðsignificantly improved the negative predictive value and reduced the omission of false negative specimens.9

An effective screening method is a valuable tool for improving workflow and lowering costs, but even more importantly reduces the time required for a negative result from days to within minutes, thereby saving a potential hospital stay and/or unnecessary antibiotic treatment. The primary objective of any screening method is to improve not only laboratory productivity, but provide more efficient healthcare for the patient.

Greg Scott is clinical staff scientist, Beckman Coulter Diagnostics.


1. Foxman B. The epidemiology of urinary tract infection. Nat. Rev. Urol. 7. P. 2010; 653-660.

2. Hooton TM and Stamm WE. Diagnosis and treatment of uncomplicated urinary tract infection. Infect Dis. Clin. North Am. 1997; 11:551-581.

3. Stamm, WE, Hooton TM. Management of urinary tract infections in adults. N Engl J. Med. 1993;329:1328-34

4. Foxman BR, Barlow H, d’Arcy B, Gillispie B, Sobe JD. Urinary tract infection: Estimated incidence and associated costs. Ann. Epidemol. 2000; 10:509-515.

5. Ferreria L, Sanchez-Juanes F, Gonzalez-Avila M, Cembrero-Fucinos D, Herrero-Hernandez A, Gonzalez-Buitrago JM, Munoz-Bellido JL. Direct Identification of Urinary Tract Pathogens from Urine Samples by Matrix-Assisted Laser Desorption Ionizaion-Time of Flight Mass Spectrometry. Journal of Clinical Microbiology. 2010. June Vol 48. No.6 p.2110-2115

6. European Urinalysis Guidelines (pg. 27) Scan J Clin Lab Invest 2000

7. Iris iQ200 Operators Manual. 2012 Rev C. Chatsworth, CA: Iris Diagnostics

8. Fox C, Fitzgerald M, Turk T, Mueller E, Dalaza L, and Schreckenberger P. Reflex testing of male urine specimens misses few positive cultures may reduce unnecessary testing of normal specimens. Urology 2010; (1) p74-76.

9. Eisinger S, Schwartz M, Rosol M, Dam Lisa, and Riedel, S. Utility of reflex urine culture based on results of urinalysis and automated microscopy. 2012. American Association for Clinical Chemistry. Annual Conference, Los Angeles, CA.

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