Urinary Tract Infections

The commonest method of diagnosing urinary tract infections is the examination of a mid-stream specimen of urine, often referred to simply as an MSU. Specimen collection is very important, and clear instructions to patients should be provided. The external genitalia must be washed properly using soap and water. The first portion of urine is voided to wash out any microbes from the distal part of the urinary tract. It is the middle section of the urinary flow that is collected for laboratory analysis

Catheter samples of urine are also frequently examined. Collecting urine samples from babies poses a particular problem. To avoid contamination problems associated with bags, supra-pubic aspirates can be performed. If a patient is suspected of suffering from renal TB, then the number of organisms in the sample will be low. To help in the diagnosis three consecutive early morning specimens of urine are examined.

Having collected the specimen, for routine examination, urine is subjected to a microscopic examination and culture. Urine microscopy reveals the presence of leukocytes, red blood cells, bacteria and "casts". These are proteinaceous deposits formed within the diseased kidney, and shed in the urine. They may be clear (hyaline casts) or may have leukocytes or red cells stuck to their surface. Urine sample containing squamous (skin-type) epithelial cells are considered contaminated. Squamous epithelial cells are not found in the urinary tract.

If there is microscopic evidence of infection, a direct antibiotic sensitivity test can be performed on the sample. This will save a day in reporting the sample.

A small range of bacteria cause urinary tract infections. Nearly all grow on a selective and indicator medium such as CLED agar. A semi-quantitative culture of urine is most often performed. A standard known volume of urine is inoculated, and the number of colonies growing from the sample is used as a guide in diagnosis of urinary tract infections. Typically 1 microlitre of urine is plated, and if more than 100 colonies of a single species are grown from an MSU sample, then the sample is considered infected, i.e. more than 100,000 cfu/ml. For supra-pubic aspirates, lower microbial counts are considered significant. Growth of more than one species in a sample is taken as an indication of contamination. Catheter specimens of urine (particularly those from catheters that have been in place for more than a few days) are likely to contain bacteria, sometimes in high numbers. If the patient remains asymptomatic these should not be treated with antibiotics.

It is not uncommon to find microscopic evidence of infection and yet fail to isolate a pathogen. The most common explanation is that the patient is self-medicating. Up to 25% of GP urine samples contain detectable antibiotics. Occasionally, fastidious microbes may cause culture-negative infections. These are rare.

Urine is an excellent bacteriological growth medium. To prevent growth of bacteria in samples, many specimen jars contain measured quantities of boric acid, used to prevent bacterial growth. Alternatively, urine may be refrigerated until it can be examined.