Urinary Tract Infection, UTI - Laboratory diagnosis
Laboratory diagnosis of Urinary Tract Infection (UTI)
The laboratory diagnosis of Urinary Tract Infection (UTI) begins with the collection of specimens.
Specimen
Urine
Proper collection of urine must be done as it discriminates between contamination, colonization, and infection.
For purity/prevention of contamination of urine collection, normal vaginal, perineal, and anterior urethral flora is an important consideration.
A clean-catch midstream urine (CCMU) is best for optimal results in able patients
urine can be collected by invasive procedures- straight catheterized urine, suprapubic bladder aspiration, indwelling catheters
Transport
As urine is an excellent supportive medium for the growth of most bacteria, it must be immediately refrigerated or preserved but can be maintained at 4°C for 24 hours.
Screening
Screening methods such as microscopic methods, colorimetric filtration, luminescence, electric impedance, enzymatic methods, photometric detection of growth, and enzyme immunoassay can be used for the detection of bacteriuria and/or pyuria faster.
Gram stain
Gram-stained slides are examined under oil immersion (OIF) at 100x for the presence of 1 or 5 bacteria per oil immersion field (OIP). In cases of bacteriuria (>105 CFU/ml), using either 1 or 5 bacteria/OIF has a sensitivity of 96% and 95% respectively, and a specificity of 91%.
* It does not efficiently detect polymorphonuclear (PMN) leukocytes in urine because leukocytes deteriorate quickly in urine.
Pyuria
Although this syndrome is not specific to UTIS and is associated with other clinical diseases such as vaginitis, the number of PMNs (400,000) in urine per hour may be an indication of infection.
Indirect indices
Rather than enumerating microorganisms or PMNs, screening tests for bacteriuria or pyuria include the detection of the presence of bacterial enzymes or PMN enzymes.
Nitrate Reductase (Greiss) test: This test indicates UTI by detecting the presence of urinary nitrite.
Leukocyte esterase test: The presence of PMNs in the urine indicates a host response to infection. Inflammatory cells (eg. PMNs) produce leukocyte esterase.
Catalase test: This test is based on the detection of catalase present in somatic (pertaining to the body) cells and in most bacterial species causing UTIs except Streptococci and enterococci.
Automated and Semiautomated systems
iRICELL systems are capable of analyzing a urine or body fluid sample in one instrument; both microscopic components and urine chemistries are analyzed by this method.
The Sysmex UF-100 is able to recognize many cellular structures, including leukocytes and bacteria.
* Screening methods are insensitive at levels below 105 CFU/ml and are not acceptable for urine specimens collected by suprapubic aspiration, catheterization, or cystoscopy.
Screening methods also fail to detect UTIs in symptomatic patients with low colonies i.e. 102 – 103 CFU/ml such as young, sexually active females with acute urethral syndrome.
Urine culture
Inoculation, incubation
After thoroughly mixing the urine, the plate is inoculated with a sterile calibrated loop which is designed to deliver a known volume i.e. 0.01 or 0.001 ml. Calibrated loops delivering a larger volume of urine (0.01 ml) are recommended to detect lower no. of organisms in certain specimens.
Media
Cystine-lactose electrolyte deficient (CLED) agar
5% sheep blood agar
MacConkey agar
Colistin-nalidixic acid agar (CNA), Phenyl alcohol agar - selective media for gram gram-positive
The sample is streaked and incubated for complete 24 hours.
* Recently, chromogenic media have been commercially introduced allowing for more specific direct detection and differentiation of urinary tract pathogens on primary plates such as BDCHROM agar. This medium uses enzymatic reactions to identify urine specimens. It also provides presumptive identification of S. saprophytics, Streptococcus agalactiae, Klebsiella-Enterobacter-Serratia, and the Proteus-Morganella-Providencia groups.
Interpretation of urine cultures
As voided urine may be contaminated with normal flora including Enterobacteriaceae, determining what colony count represents true infection from contamination is important.
Another major problem is distinguishing between infection and contamination as a criterion for the positive report is lowered from 105 CFU/ml to 102 CFU/ml.
If yeast is isolated in any number, it must be reported. All isolates should be enumerated and those present in numbers greater than 104 CFU/ml should be described morphologically and identified by other methods such as biochemical tests.