URINARY TRACT INFECTION

Patient 3
Name: Maisy Wong
Sex: Female
Age: 66 yrs
Complaints: Fever, chills, bladder distension, on indwelling catheter
Diagnosis: Urinary tract infection
Antibiotic treatment: Nil

DEFINITION
A urinary tract infection, or UTI, is an infection that can happen anywhere along the urinary tract -- the kidneys, the ureters (the tubes that take urine from each kidney to the bladder), the bladder, or the urethra (the tube that empties urine from the bladder to the outside).

CAUSES, INCIDENCE AND RISK FACTORS
Cystitis, a common condition, is usually caused by a bacterium from the anus entering the urethra and then the bladder. This leads to inflammation and infection in the lower urinary tract.
Certain people are more likely to get UTIs. Women tend to get them more often because their urethra is shorter and closer to the anus. Elderly people (especially those in nursing homes) and people with diabetes also get more UTIs.

BACTERIAL CULTURE
The most common cause of urinary tract infection is E coli. In hospital practice, other bacterial species commonly seen include enterobacter, klebsiella, proteus, pseudomonas, enterococci, and staphylococci. Staphylococcus saprophyticus is a common cause of infection in young sexually active women. The laboratory must also quantify culture results to determine the clinical relevance of an isolate.

CULTURE METHODS
Before culturing the urine should be mixed by inverting the container.
Choice of media:The media chosen must be able to support the growth of urinary pathogens and possible contaminants, inhibit Proteus spp from swarming, and distinguish lactose and non-lactose fermenters. Cysteine lactose electrolyte deficient medium (CLED) fulfils these criteria. Culture plates should be incubated overnight at 35–37°C in air. More recently, a new chromogenic agar has been described for the detection of urinary tract pathogens that may provide better differentiation of bacteria than conventional media. Anaerobes rarely cause UTI but culture should be considered in a selected group of patients, such as those with persistent pyuria or foul urine and symptoms of UTI. In immunosuppressed patients (including those on intensive care or neonatal units), culture for Candida spp should be performed because the urine may be positive before, or may indicate, the development of fungaemia. Screening for methicillin resistant Staphylococcus aureus should include urine samples from catheterised patients. An appropriate selective medium, such as mannitol salt agar with oxacillin (2 mg/litre), should be used.

Media and culture conditions to aid the isolation of more fastidious organisms such as lactobacilli, corynebacteria, Gardnerella vaginalis, Mycoplasma spp, and Haemophilus spp may be worth pursuing for symptomatic patients with pyuria and negative routine culture. The use of multipoint inoculation to perform routine anaerobic culture has been suggested by some authors as being clinically valuable.A microtitre method may be used in the identification of enterobacteriaceae, and this technique may also be applied to sensitivity testing using an automated reader.

IDENTIFICATION OF BACTERIA
Clear protocols for the identification of bacteria and fungi should be in place. In general, it is adequate to report coliforms as such without full identification. Proteus spp are urease positive and resistant to nitrofurantoin. Pseudomonas aeruginosa is an oxidase positive lactose non-fermenter, resistant to most first line antibiotics. If clinical details suggest that a non-lactose fermenting coliform may be a Salmonella spp and the urease and oxidase tests are negative then slide agglutinations with "O" and "H" antisera should be performed from cultures on blood (or other non-selective) agar plates. Any positive results should be followed up with biochemical confirmation. If typhoid fever is suspected, 5–10 ml of uncentrifuged urine should be inoculated into double strength selenite, incubated overnight at 37°C in air, then subcultured on to deoxycholate citrate agar, which in turn is incubated overnight at 37°C in air. Any suspect isolate should be dealt with in containment level 3 accommodation.
One advantage of using blood agar alongside CLED is that Gram positive bacteria are more easily characterised. If uncertainty exists, a catalase test will distinguish streptococci (negative) from staphylococci (positive). Staphylococcus aureus is DNase, slide, and tube coagulase positive. Staphylococcus saprophyticus can be identified by its resistance to novobiocin and this makes a useful distinction from other coagulase negative staphylococci, which are usually only important in specific situations such as instrumented or catheterised patients. If the appearance of the colony is typical of Enterococcus faecalis report the organism as such; if uncertain, perform a bile aesculin test. ß-Haemolytic streptococci can be readily identified by Lancefield group testing.
Other isolates are identified using standard laboratory techniques, all multiply antibiotic resistant organisms need to be fully identified.
Fungi need only be identified if there is evidence to suggest that the isolate is clinically relevant. Because cut off values vary from author to author, we recommend that repeat sampling is performed to determine that there is persistent funguria (catheters should be changed). Candida albicans is germ tube positive and usually sensitive to fluconazole, itraconazole, and amphotericin. Sensitivity testing and the identification of other candida and fungal species are only necessary in selected patients, such as those who are severely immunocompromised.
The detection of antimicrobial substances is not routinely recommended but should be incorporated in the multipoint set to exclude false negative culture results. The detection of antibody coated bacteria in urine is not recommended in the routine diagnostic laboratory but may be useful to distinguish between upper and lower urinary tract infection in selected patients.

SENSITIVITY TESTING
The choice of agents to test will depend upon local antibiotic policies and resistance patterns. In general, the primary agents tested target coliforms and enterococci, and second line sensitivities need only to be performed if less common bacteria or resistant isolates are encountered. The suggested first line agents include amoxicillin, trimethoprim, cefalexin (or other oral cephalosporins), nitrofurantoin, co-amoxiclav, and ciprofloxacin. Urine is used as the primary inoculum when there is evidence of infection (pyuria and/or bacteriuria) so as to permit rapid reporting. This method may be more representative than picking individual colonies for subculture, particularly given the heterogenous nature of urinary tract infection.The degree of pyuria that triggers the performance of direct sensitivity testing should be decided locally depending on the patient group examined. It is suggested that all urines that show bacteria on microscopy and those with pyuria > 100 white cells/mm3 should be tested. Recent recommendations for disc content and zone size interpretation have been published by the British Society for Antimicrobial Chemotherapy.
Each sensitivity report is tailored to guide clinicians to the most appropriate agents and it is often necessary to suppress antibiotics if the isolate is not deemed to be clinically relevant. Suppressing antibiotic sensitivities on the results of positive specimens may be a particularly useful way of educating users that treatment of a positive catheter urine is not normally warranted. In addition, the presence or absence of pyuria may be used to decide which sensitivities are reported. However because the definition of UTI is based on bacterial counts and not on the presence or absence of pyuria, performing sensitivities should be related to the number of bacteria present and the relevant clinical situation. Specific agents may be unsuitable in particular situations—for example, the reporting of intravenous antibiotics to general practitioners—and certain antibiotics are relatively contraindicated in pregnancy. If sensitivity testing is not performed (for example, on mixed cultures) then culture plates should be kept for five days so that further testing may be performed if necessary.

References:
http://jcp.bmj.com/cgi/content/full/54/12/SEC3
http://www.nlm.nih.gov/medlineplus/ency/article/000521.htm

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