A Simple Method for Selecting Urine Samples that Need Culturing
2001; King Faisal Specialist Hospital and Research Centre; Volume: 21; Issue: 1-2 Linguagem: Inglês
10.5144/0256-4947.2001.104
ISSN0975-4466
Autores Tópico(s)Urinary Tract Infections Management
ResumoBrief ReportA Simple Method for Selecting Urine Samples that Need Culturing Fadil Y. AnadFRCPCH Fadil Y. Anad Search for more papers by this author Published Online:1 Jan 2001https://doi.org/10.5144/0256-4947.2001.104SectionsPDFCite ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail AboutIntroductionUrinary tract infections (UTI) are common in infancy and childhood, and are associated with potentially serious long-term consequences of renal scarring, hypertension and chronic renal failure.1 UTI is essentially a microbiological diagnosis, and thus requires urine culture for confirmation. Collecting, transporting and processing urine samples is financially costly, raises anxiety in, and causes inconvenience to, the children and their parents or caregivers. Rationalization of the whole process of urine testing would, therefore, be highly desirable.As part of an audit of the Children’s UTI Clinic at South Tyneside District General Hospital, England, the reliability of two tests (dipstick for leukocyte esterase and nitrite, and microscopy for pus cells) was assessed. Because of encouraging preliminary results, a prospective study was conducted.Subjects and MethodsThe results of 490 urine samples collected from 279 children (88 boys, 191 girls) aged 4 days to 15 years, were analyzed. Except for two samples, all urine specimens were collected as clean-catch or midstream in younger and older children, respectively, and as clean-catch, by suprapubic aspiration, catheter samples, or by using the Newcastle urine collection pad2–4 in infants and children who were still in diapers. In one ill and febrile 2½-month-old baby, two bag samples yielded a pure growth. Attempts to obtain a suprapubic sample failed, and the baby was treated with IV antibiotics as clinically indicated. The samples were delivered fresh (within 2 hours of voiding) to the laboratory for microscopy and culture.The two tests employed in the study were:Dipstick for leukocyte esterase and nitrite (using Nephur-Test + Leuco, Boehringer Mannheim). The result was recorded as +ve if either or both were positive, and −ve if both were negative.Simple microscopy of fresh uncentrifuged urine for pus cells done by the microbiology laboratory. It was the practice of the laboratory to report the result recorded as positive if there were any number of pus cells/high-power field, and negative if there were no pus cells at all.The results of these two tests were compared with the final urine culture reports. A urine culture was reported as positive when there were >10 colony-forming units (CFU)/mL of urine, or any number of CFU in a urine sample obtained by suprapubic aspiration (SPA). Any growth of <10 CFU/mL of urine, except for those obtained by SPA, was considered negative. Urine cultures which revealed either mixed growth or unusual organisms were either repeated or the child was reviewed. These were excluded from further analysis.ResultsA total of 441 specimens (90%) were negative, and 49 (10%) were positive on culture. Organisms isolated were 39 E. coli, 5 Staphylococcus epidermidis, 2 Proteus spp., 2 Klebsiella spp., and 1 Streptococcus faecalis.To assess the positive and negative predictive values (PPV and NPV) of dipstick and/or microscopy, results were grouped into three categories depending on whether dipstick alone (irrespective of microscopy result), microscopy alone (irrespective of dipstick result), or both tests were considered.Dipstick AloneOf 129 samples with positive dipstick, 34 (26.4%) yielded a positive and 95 (73.6%) yielded a negative culture. Dipstick was negative in 361 samples, with a positive culture in 15 (4.2%), and a negative culture in the remaining 346 (95.8%) samples. This confirms previous observation that dipsticks are better at excluding UTI when the readings are negative than in confirming a diagnosis of UTI when they are positive.5Microscopy AloneUrine microscopy was positive in 120 samples, of which 37 (30.8%) yielded a positive and 83 (69.2%) produced a negative culture. Microscopy was negative in 370 samples, which yielded 12 (3.2%) positive and 358 (96.8%) negative cultures. Using the strict criteria adopted by this study, urine microscopy for pus cells is better at excluding UTI when no pus cells are seen than in confirming the diagnosis when microscopy is positive.Table 1 The positive and negative predictive values (expressed as percentage).Total number of urine samplesNumber (%) of positive urine culturesNumber (%) of negative urine culturesDipstick for leukocytes Positive12934 (26.4)95 (73.6) Negative36115 (4.2)346 (95.8)Microscopy for pus cells Positive12037 (30.8)83 (69.2) Negative37012 (3.2)358 (96.8)Both dipstick and microscopy Positive7828 (35.9)50 (64.1) Negative3196 (1.9)313 (98.1)Dipstick and Microscopy CombinedThere were 78 samples where both tests were positive, and 28 (35.9%) of them yielded a positive and 50 (64.1%) produced a negative culture. It is interesting to note that even if both tests are positive, two-thirds of the samples were negative on culture. For 319 samples where both tests were negative, the vast majority of cultures (313 or 98.1%) were negative too and only 6 samples (1.9%) had positive cultures. These six samples came from children who were asymptomatic and none of them was <1 year old, but they all subsequently received a full course of appropriate antibiotics.DiscussionThe dipstick testing of urine for leukocyte esterase and nitrite has been extensively assessed in adults,6 but there are only a few reports of their use in children. Wiggelinkhuizen et al. compared dipstick testing with formal urine analysis of 1137 children and concluded that the dipsticks were a reliable screening technique.7Over the past few years, a number of studies have been conducted on children to assess the feasibility of various screening protocols to predict positive and negative urine cultures. These studies evaluated the positive and negative predictive values of urine dipstick testing for leukocyte esterase and nitrite combined with: microscopy of a centrifuged urine for pus cells and bacteria,8 semiquantitative gram-stained smear of uncentrifuged urine,9 and microscopy of centrifuged urine for pus cells, bacteria and erythrocytes.10 One study assessed the predictive values of microscopy alone of fresh uncentrifuged urine using three kinds of microscopes.11 Although these studies gave very high predictive values, they employed methods other than a simple microscopy of an uncentrifuged urine.This study was conducted in a district general hospital on 490 fresh urine samples reliably tested for the presence of leukocyte esterase and nitrite on dipstick, and for pus cells on microscopy. When either the dipstick or microscopy alone was positive or when both were positive, the chances of a positive culture were 26.4%, 30.8% and 35.9%, respectively. This suggests that even if both tests are positive, it is much more likely to get a negative culture than a positive one, and will not be of practical value in the process of selecting samples for culture.A negative dipstick test alone (361 samples) and a negative microscopy alone (370 samples) had negative predictive values of 95.8% and 96.8%, respectively. This may be adequate in the asymptomatic older child, but in ill febrile children, a higher negative predictive value is required. In these ill children, the combined negative dipstick and microscopy, which has a negative predictive value of 98.1%), virtually excludes urinary tract infection.The number of infants included in this study was small (22 or 7.9% of the total), but a similar previous study12 conducted on samples collected from children under 18 months of age gave a comparable negative predictive value of 98.5%. The NPV raised to 99.4% when absence of proteinuria was included. The author is of the opinion that provided the samples are collected and tested freshly, this method can be safely recommended for use in infants too.The routine use of urine microscopy in hospital clinics and wards was advocated by Robins et al. more than two decades ago.14 Widespread use of the new dipsticks, coupled with microscopy for pus cells performed on fresh samples by adequately trained doctors, would dramatically cut down the number of samples sent for culture and this, in turn, would cut down on costs. To achieve this, access to a simple basic microscope and a counting chamber should be provided for medical staff working on children's wards and outpatient clinics with suitable prior training. This could be extended to general practitioners working in large practices.ARTICLE REFERENCES:1. White RHR. "Management of urinary tract infection and vesicoureteric reflux in children" . BMJ. 1990; 300:1391-4. Google Scholar2. Vernon S, Redfearn A, Pedler SJ, Lambert HJ, Coulthard MG. "Urine collection on sanitary towels" . Lancet. 1994; 344:612. Google Scholar3. Ahmad T, Vickers D, Campbell S, Coulthard MG, Pedler S. "Urine collection from disposable nappies" . Lancet. 1991; 338:674-6. Google Scholar4. Vernon S. "Urine collection from infants: a reliable method" . Paediatr Nurs. 1995; 7:26-7. Google Scholar5. Fowlis GA, Waters J, Williams G. "The cost effectivenes of combined rapid tests (Multistix) in screening for urinary tract infections" . J Roy Soc Med. 1994; 87:681-2. Google Scholar6. Hurlbut TA, Littenberg B. The Diagnostic Technology Assessment Consortium. "The diagnostic accuracy of rapid dipstick tests to predict urinary tract infection" . Am J Clin Pathol. 1991; 96:582-8. Google Scholar7. Wiggelinkhuizen J, Maytham D, Hanslo D. "Dipstick screening for urinary tract infection" . S Afr Med J. 1988; 74:224-8. Google Scholar8. Goldsmith BM, Campos JM. "Comparison of urine dipstick, microscopy and culture for the detection of bacteriuria in children" . Clin Pediatr. 1990; 29:214-8. Google Scholar9. Weinberg AG, Gan VN. "Urine screen for bacteriuria in symptomatic pediatric outpatients" . Pediatr Infect Dis J. 1991; 10:651-4. Google Scholar10. Shaw KN, Hexter D, McGowan KL, Schwartz JS. "Clinical evaluation of a rapid screening test for urinary tract infections in children" . J Pediatr. 1991; 118:733-6. Google Scholar11. Vickers D, Ahmad T, Coulthard MG. "Diagnosis of urinary tract infection in children: fresh urine microscopy or culture?" Lancet. 1991; 338:767-70. Google Scholar12. Lejeune B, Baron R, Guillois B, Mayeux D. "Evaluation of a screening test for detecting urinary tract infection in newborns and infants" . J Clin Pathol. 1991; 44:1029-30. Google Scholar13. Jodal U. "Urinary tract infection: significance, pathogenesis, clinical features and diagnosis" . Clinical Paediatric Nephrology, 2nd edition. 1994;151-9. Google Scholar14. Robins DG, White RHR, Rogers KB, Osman MS. "Urine microscopy as an aid to detection of bacteriuria" . Lancet. 1975; 1:476-9. Google Scholar Previous article Next article FiguresReferencesRelatedDetails Volume 21, Issue 1-2January/March 2001 Metrics History Received10 June 2000Accepted18 November 2000Published online1 January 2001 InformationCopyright © 2001, Annals of Saudi MedicineThis work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.PDF download
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