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Best Practice No 167 the Laboratory Diagnosis of Urinary Tract Infection

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Best Practice No 167 the Laboratory Diagnosis of Urinary Tract Infection J Clin Pathol 2001;54:911–919 911 Best Practice No 167 J Clin Pathol: first published as on 1 December 2001. Downloaded from The laboratory diagnosis of urinary tract infection J C Graham, A Galloway Abstract women develop a UTI during their lifetime; the Urinary tract infection is common, and it incidence increases at puberty and remains is not surprising that urine specimens high throughout adult life, only after the age of make up a large proportion of those sam- 50 years is a similar incidence seen in males. ples submitted to the routine diagnostic UTI accounts for approximately 23% of all laboratory. Many of these specimens will hospital acquired infections.2 Although the show no evidence of infection and several incidence of infection is high, most specimens methods can be used to screen out received will show no evidence of infection and negative samples. Those that grow bacte- several methods have been developed to screen ria need to be carefully assessed to out negative samples to minimise expense and quantify the degree of bacteriuria and improve turnaround times. These will also be hence clinical relevance. To influence reviewed. treatment, a final report should be pro- Most infections at all ages are the result of duced within 24 hours of specimen re- enteric bacteria, especially Escherichia coli, ceipt, with turnaround times continuously which colonise the perineum and then ascend monitored. Much work needs to be done to the urethra to multiply and infect the bladder, http://jcp.bmj.com/ determine the cost eVectiveness involved kidney, and adjacent structures.3 The most in processing urine specimens and the common site of infection is the bladder. evidence base for the final report pro- Haematogenous infection of the urinary tract vided. occurs most notably with Mycobacterium tuber- (J Clin Pathol 2001;54:911–919) culosis and Salmonella spp, and direct introduc- Keywords: laboratory diagnosis; urinary tract infection tion of organisms during instrumentation of the urinary tract is also well recognised. Struc- tural and functional abnormalities result in a on September 27, 2021 by guest. Protected copyright. The aim of the microbiology laboratory in the wider range of possible infecting organisms. management of urinary tract infection (UTI) is Urinary tract infection may occur with or to reduce morbidity and mortality through without symptoms; the latter is known as accurate and timely diagnosis with appropriate antimicrobial sensitivity testing. Although opti- covert or asymptomatic bacteriuria. Because mal specimen collection, processing, and urine must pass through the distal urethra and interpretation should provide the clinician with in women over the perineum, it may become a precise answer, no single evaluation method contaminated by the normal flora of these is foolproof and applicable to all patient regions. Isolation of more than one bacterial groups. In practice, laboratories will not be able strain suggests such contamination, but even to approach each specimen individually, and when a single strain is isolated, quantitative standard operating procedures are generated to culture is required to determine whether it cover the processing of most samples, with the indicates true bacteriuria. Kass, in his original aim of detecting the abnormal presence of bac- studies validating the midstream urine speci- teria and fungi within the urinary tract.1 The men (MSU), showed that 95% of hospitalised Clinical Microbiology interpretation of results requires an under- patients with acute pyelonephritis had more 5 Laboratory, Royal standing of the limitations of local laboratory than 10 colony forming units (cfu)/ml of Victoria Infirmary, protocols and of the clinical context in which urine, whereas only 6% of asymptomatic Queen Victoria Road, patients had this degree of bacteriuria.45 Newcastle upon Tyne the specimen was taken. NE1 4LP,UK Urine specimens make up a large proportion Subsequent studies have shown that lower bac- J C Graham of the samples submitted to a routine diagnos- terial counts can be important; this applies A Galloway tic laboratory. A large laboratory may examine both to men and symptomatic women in whom 200–300 urine samples each day. This heavy 30–50% have fewer than 105 organisms/ml.67 Correspondence to: Dr Galloway workload reflects the frequency of UTI both in These cut oV values can be applied to all angela.galloway@ general practice and in hospital settings. In rapidly growing bacteria but not fungi or nuth.northy.nhs.uk children, infection is more common in young fastidious organisms. Patients with frequency Accepted for publication girls, except in the neonatal age group, where dysuria syndrome in whom urine cultures show 31 May 2001 boys predominate. It is estimated that 20% of no appreciable growth should be investigated www.jclinpath.com 912 Graham, Galloway for other agents that cause non-specific urethri- used by patients with neurogenic bladder and tis, such as Chlamydia trachomatis.8 It should be these specimens should be treated as CSUs. noted that above the distal urethra the urinary Increasingly, samples are being received tract is normally sterile, and any bacteria from patients with urological problems, includ- isolated from urine samples taken directly from ing patients with ileal conduits and those who the bladder, ureter, or kidney must be viewed have undergone bladder reconstruction, and J Clin Pathol: first published as on 1 December 2001. Downloaded from as clinically relevant. The detection of poly- these can present diYculties with interpret- morphonuclear cells (pyuria) and red blood ation. Specimens should only be taken if there cells (haematuria) in urine is useful for the are clinical signs of infection (for example, diagnosis of infection or other renal tract malaise, pyrexia, or vomiting) and they should pathologies. be obtained via careful catheterisation of the Each laboratory should aim to have available stoma or reconstructed bladder using an asep- a final report with microscopy, culture, and tic technique. The interpretation of the cul- sensitivity on a substantial number of speci- tures should then be as for any catheter speci- mens (for example, > 90%) within 24 hours men. after the receipt of the specimen, and laborato- Suprapubic aspirates (SPAs) were often ries should monitor their turnaround times as obtained from babies and young children and part of quality assurance. The ability to screen are still considered the “gold standard” and are out negative specimens as quickly as possible used in diYcult cases. Any isolate should be should also be considered; ideally this is done considered clinically relevant. Obtaining an at source. The physician can make a clinical SPA involves an invasive procedure; however a judgement as to whether a negative screen (for sterile adhesive bag or pad (that is, a sanitary example, by reagent strip testing or bedside towel with the appropriate absorption charac- microscopy) is suYciently accurate to rule out teristics, lining a nappy) is much simpler but infection in that individual patient, given the can still achieve a definitive answer in 50–75% limitations of the method used. of cases.9 After cleansing the perineum, the This broadsheet deals specifically with the baby is maintained in an upright position until diagnosis of bacterial UTI causing cystitis or urine is passed into the bag, pad, or alterna- pyelonephritis. It does not include the labora- tively via a clean catch into a container. tory investigation of prostatitis, infection with Urine samples collected from the ureter (at mycobacteria, or the examination of urine for cystoscopy) or from the kidney (via a nephros- parasites. tomy) should be treated in the laboratory as a fluid from a sterile site and all bacterial and Collection and transport of specimens fungal growth viewed as clinically relevant. For Rigorous care during the collection of urine is the diagnosis of prostatitis, urine specimens http://jcp.bmj.com/ vital to prevent contamination by commensal may be collected after massage of the prostate flora, especially in female patients and children. via the rectum because this is said to release Most samples are MSUs, and patients should any sequestered bacteria or inflammatory cells be given clear instructions on discarding the from this site. first part of the stream before collection in an As can be seen from the above, not all speci- appropriate sterile container. Female patients men types are the same and correct interpret- should be instructed to part the labia while ation of urine cultures requires accurate data on September 27, 2021 by guest. Protected copyright. passing urine to avoid contamination. The ini- being clearly present on the request form. tial few millilitres of urine wash away distal Urine will permit growth of bacteria and if urethral organisms and hence the MSU is rep- there is to be a delay in transport (> 2 hours) resentative of bladder urine. It requires good or in setting up cultures in the laboratory then control of micturition and an adequate volume it should be stored refrigerated at 4°C; this will of urine in the bladder. It may prove diYcult to also preserve the white cell count.10 11 Boric get such a sample in the elderly or those with acid is often used to retain the bacterial count, hip joint problems. Early morning samples may but its antibacterial activity can reduce the harbour greater bacterial counts but are less number of organisms present, especially if an amenable to outpatient clinical practice. How- inadequate volume of urine is dispensed into ever, they are recommended for the diagnosis the container.12 Hence, rapid transportation/ of renal tuberculosis. processing with refrigeration (if necessary) is Catheter specimens of urine (CSUs) are the preferred method. A dip slide or dip inocu- often obtained from patients with long term lum is useful in overcoming delays in culturing indwelling catheters. Bacteria are frequently urine specimens, but because no microscopy recovered but only a few are important and can be performed on the specimen, it is most samples should only be taken when signs and useful for the follow up of patients.
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