Urine culture procedure question

Graham Shepherd muhero at globalnet.co.uk
Sun May 31 05:54:36 EST 1998

Fergus McClelland wrote in message <35728a8d.184247 at news.demon.co.uk>...
>I have got the impression that the standard culturing tests for urine
>may miss a large percentage of positives.

< case details snipped>

Typically the diagnosis of apparent UTI in adults without complicating
factors (eg urinary catheterisation) is done by culturing a mid-stream
specimen of urine, and the threshold level is about 100,000 cfu per mL of
urine - or about 500 colonies from a 5 microlitre sample. Less than that is
usually not significant.  Overnight incubation is pretty standard, and
preservatives may be used to stop bacterial growth in the sample in transit
to the lab. The reason for using a threshold value and preservatives is that
any clean-catch sample procedure can pick up normal flora on the way out,
and any sample showing a mixed culture (3 or more colony types) and less
than the threshold total will usually not be considered to indicate UTI.
Bugs grow happily in urine and if the sample does not reach the lab quickly
you get false positives because the pupoluation has increased. This
procedure may not pick up some type of organism which take longer to grow
(eg fungi, lactobacili).

In cases where the sample is obtained directly from the bladder
(catheterisation or suprapubic stab), ANY growth is considered suspicious.

This type of testing will not detect STDs or other GU infections, and
concurrent antibiotic treatment can make lab diagnosis difficult and
unreliable. Your doctor should know that. It is essential to give the lab
(1) the right sample (2) the right information. About 50% of a typical lab's
workload is urine samples - production-line microbiology. This case sounds
like something other than UTI, and clearly needs proper investigation.
That's what urologists are for.


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