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Letters, 9/13

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Urine cultures

We read with interest your article in the June issue, “To reduce UTIs, one lab takes a long, wide look.”

We strongly agree that there is inappropriate prescribing of antibiotics for patients whose urine cultures are reported with organism identification and antibiotic susceptibilities but who do not have urinary tract infection. This is because many physicians send urine for culture inappropriately and then equate a positive result with infection; they believe that these laboratory tests are diagnostic for UTI.

However, urine culture should not be considered a diagnostic test. The presence of bacteriuria does not differentiate infection from asymptomatic bacteriuria or contamination. The diagnosis of urinary tract infection should be made clinically. Even in the catheterized patient who often presents with nonspecific symptoms/signs, the diagnosis should be made clinically, after first excluding all other infectious and non-infectious causes.

Only after there is strong clinical suspicion of UTI should urine be sent for culture for the purposes of identifying the causative pathogen and determining its antimicrobial susceptibility, not to diagnose a UTI.

As reported in the article, traditionally only high counts of ≥105cfu/mL of uropathogens were considered significant. This cutoff was established 50 years ago by looking at otherwise healthy patients with pyelonephritis and the bacterial counts in their urine, collected four hours after their last void. More recent studies have shown that patients with cystitis, who have frequency of micturition, commonly have lower counts, owing to a shorter incubation time of urine in the bladder.

Counts as low as 103cfu/mL in young men, and 102cfu/mL in young women, may therefore be significant in symptomatic patients.1–6 The cutoff of 105cfu/mL, though specific, has poor sensitivity, and using that cutoff can result in missing up to 50 percent of UTIs.3

The most common predisposing factor in hospitalized patients for developing UTI is the presence of long-term (>14 days), indwelling urethral catheters. According to the Infectious Diseases Society of America, low counts of 103cfu/mL may be significant in catheter-associated UTI.4 Therefore, how can a cutoff of 105cfu/mL, as proposed in the article, be appropriate for these patients?

Lance Peterson, MD, says that in his study, the cultures with <105cfu/mL were from patients who had no symptoms of UTI. We agree that these cultures were sent inappropriately. However, we disagree that increasing the cutoff level to ≥105cfu/mL reduces the number of nosocomial UTIs, since a culture result does not equate to the presence or absence of infection; this can only be determined clinically.

The way to reduce the number of “false-positive” urine culture results is to ensure cultures are sent appropriately, from patients with symptoms/signs compatible with UTI, and that the specimen is collected so as to minimize contamination, and stored and transported to the laboratory so as to prevent replication of bacteria before processing. Health care workers should be encouraged to write relevant clinical information on the requisition, including symptoms and signs, recent and proposed treatment, and antibiotic allergies. Laboratory staff will use this to perform appropriate workup and produce a more meaningful result for each patient. We agree that the one-size-fits-all reporting strategy is not conducive to optimal patient care.

Appropriate use of urine culture and antibiotics can be achieved only by educating health care workers on when they should and should not request this test and how to collect and transport the specimen properly and by emphasizing that bacteriuria equates to infection only in symptomatic patients. When the patient is symptomatic, counts of ≥103cfu/mL may be significant.

In our laboratory, we will perform susceptibilities on 103cfu/mL only if symptoms/signs of UTI are indicated on the requisition, since low counts may also be due to contamination with perineal flora. Higher counts of ≥104cfu/mL of pure or predominant uropathogens are worked up even if symptoms are not given, in keeping with guidelines.1,2 However, in the absence of stated symptoms, we add an educational comment discouraging unnecessary treatment of asymptomatic bacteriuria (except in pregnancy and before a urologic procedure).

We agree that using the urinalysis result to reflex to culture has no role in the management of UTI since it can neither rule in nor rule out UTI in most patients. In the presence of symptoms and signs compatible with UTI, urine culture is the relevant laboratory test to guide the selection of the appropriate agent.

In conclusion, we feel that educating health care workers on the diagnosis of UTI and the role of laboratory tests in managing UTI is paramount in improving test and antibiotic usage. Lower counts of <105cfu/mL should be considered significant in patients with symptoms and signs compatible with UTI.

  1. McCarter YS, Burd EM, Hall GS, Zer­vos M. Laboratory Diagnosis of Uri­nary Tract Infections. Cumitech (Cu­mu­lative Techniques and Pro­ce­dures in Clinical Microbiology) 2C, 2009.
  2. Garcia LS, Isenberg HD. Clinical Mi­cro­biology Procedures Handbook, ed 3. 2011.
  3. Burd EM, Kehl KS. A critical appraisal of the role of the clinical microbiology laboratory in the diagnosis of urinary tract infections. J Clin Microbiol 2011; 49:S34–S38.
  4. Hooton TM, Bradley SF, Cardenas DD, Colgan R, et al. Diagnosis, prevention, and treatment of catheter-associated urinary tract infection in adults: 2009 international clinical practice guidelines from the Infectious Diseases So­ci­ety of America. Clin Infect Dis 2010;50:625–663.
  5. www.sign.ac.uk. Management of suspected bacterial urinary tract infection in adults. A national clinical guideline. Scottish Intercollegiate Guidelines Net­work, 2006.
  6. www.urology-textbook.com/urinary-tract-infection-diagnosis.html

Shobhana Kulkarni, MBBS, FRCPath, FRCP(C)
Medical Microbiologist
Assistant Clinical Professor

Jana Nigrin, MD, FRCP(C)
Medical Microbiologist
Assistant Clinical Professor

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