Urine is the only body substance that is routinely cultured in a truly quantitative fashion. It has been shown that urinary tract infections are more likely when a certain quantity of bacteria are present, usually defined as 108(or 100 x 106) organisms (usually referred to as colony-forming units or CFU) / L.
Urine is also a good culture medium for bacteria so, when one considers that the generation time of E. coli is about 20 minutes, it is easy to understand that the urine that sat on the nursing station countertop for 5 hours has a few more bacteria in it than it did when it was in the patient. How do we circumvent this problem? Firstly, we don’t let urine sit on the nursing station countertop for 5 hours before we bring it to the lab! There are collection devices that can help mitigate this problem (they employ a substance that limits bacterial growth while not killing them).
The method of collection is another variable that is essential for proper processing of urine cultures. Mid-stream (“clean catch”), in-out catheter, indwelling Foley catheter, pediatric bag, suprapubic tap samples are all processed and interpreted differently. Any amount of bacteria found in a sterile suprapubic tap sample may be of import, whereas large numbers of bacteria from a long-term indwelling Foley catheter may be the “normal” state and not represent a clinical infection.
The other component of urine analysis that is vital in the diagnosis of urinary tract infection is chemical and microscopic exam commonly referred to as “routine and micro” or R & M. Generally, what this entails is a chemical method (often a manually read colorimetric dip-stick) of determining the presence of red blood cells, white blood cells (more specifically neutrophilic polymorphonuclear leukocytes) nitrates, protein, glucose and perhaps other substances by enzymatic means. True microscopic exam is usually limited to those samples showing an abnormality on chemical screen.
It is impossible to interpret urine culture results, particularly from improperly collected and transported specimens, without R & M results. In the absence of pyuria (denoting inflammation), an acute urinary tract infection is very unlikely to be present. In many places urine cultures are screened using a chemical technique and only those that are positive for WBCs or nitrites are cultured.
The correct way to culture urine is as follows:
Decide whether culture is indicated and will influence your therapeutic decision.
If a woman of childbearing age has typical cystitis and you have elected to treat her empirically with antibiotics there is no need to culture her urine at that time.
Select the most appropriate method of collection and give patient explicit instructions if they will be self-collecting.
Do a dipstick urinalysis or send urine for urinalysis.
Label it properly, and indicate method of collection and time of collection.
If sending for culture make sure it is transported promptly or a urine collection device such as a “Uriswab” is used.
See Lab – Urine Culture for detail on laboratory processing.