The majority of urines sent to the microbiology laboratory are labelled as “MSU”, short for midstream urine. But do you know what an MSU is, how it is taken and why? No? Then read on….
Midstream urine is taken by voiding the first part of the urine stream (10-20mls at least) and then collecting the next portion (approx. 10-20mls), this eliminates the urine that has been in the urethra and should therefore only be urine from the bladder. In men it is important to retract the foreskin and in women part the labia. This is done because the skin is not sterile and is often contaminated with the same types of bacteria that can cause urinary tract infections (UTIs) leading to false positive urine cultures. The patient can then finish urinating as normal. This is not the easiest sample to obtain in bedbound, frail patients, or in pregnancy. So why do we do it?
Midstream urine is taken by voiding the first part of the urine stream (10-20mls at least) and then collecting the next portion (approx. 10-20mls), this eliminates the urine that has been in the urethra and should therefore only be urine from the bladder. In men it is important to retract the foreskin and in women part the labia. This is done because the skin is not sterile and is often contaminated with the same types of bacteria that can cause urinary tract infections (UTIs) leading to false positive urine cultures. The patient can then finish urinating as normal. This is not the easiest sample to obtain in bedbound, frail patients, or in pregnancy. So why do we do it?
The urine in the urethra is usually not sterile. Bacteria such as E. coli can swim up the column of urine so if this part of the urine stream is collected and cultured it will almost always grow bacteria. This causes false positive results which often lead to unnecessary antibiotic treatments. Discarding this part of the urine sample reduces the risk of a false positive culture result.
The majority of samples sent to the laboratory are just urine samples of non-specific type, including those taken from a bedpan! I suspect I do not want to know how many urine samples are taken in this manner ...it will just upset me. It is not a recognised or acceptable practise. Non-specific urine samples usually contain the first part of the urine stream ecause this is the easiest bit to get. Labelling these as MSUs is incorrect and leads to misinterpretation of the results by the laboratory and doctors.
The majority of samples sent to the laboratory are just urine samples of non-specific type, including those taken from a bedpan! I suspect I do not want to know how many urine samples are taken in this manner ...it will just upset me. It is not a recognised or acceptable practise. Non-specific urine samples usually contain the first part of the urine stream ecause this is the easiest bit to get. Labelling these as MSUs is incorrect and leads to misinterpretation of the results by the laboratory and doctors.
Another common mistake is to collect the urine sample directly into the boric acid (red topped) container. All urines should be tested for leucocytes and bacterial nitrites before being sent to the laboratory. “Dip stick” testing from a boric acid container should not be done as the acid can interfere with the result. The sample should be taken in a different sterile container before being decanted into the boric acid container.
Other common methods of obtaining a urine sample:
• Catheter specimen
These are only usually taken from an existing catheter or when a catheter is inserted for other reasons. A specimen from a brand new catheter is of value but once the catheter has been in for a few days it will start to become colonised with bacteria including the normal causes of UTIs as well as
Pseudomonas sp, and Enterococcus sp. They also often provoke a white blood cell response and therefore catheter urine samples often mimic a UTI even when the patient is fine. In general catheter urines should not be sent for culture unless there is good clinical reason to suspect a UTI such as suprapubic pain, fever and cloudy urine
• Suprapubic aspirate (SPA)
This is only done on very small infants (or when a suprapubic catheter is inserted in adults) and is the gold standard sample for diagnosing infection. The urine sample is collected via a sterile needle and syringe directly from the bladder through the abdominal wall. Contamination is not an issue and any white blood cells or bacterial growth is significant. The procedure looks a lot worse than it actually is!
• Clean catch sample (In a child only)
Used in children who cannot be made to void at a specific time and therefore are stripped of heir nappies and when they urinate an attempt is made to catch this in a clean
container. This takes time and some accuracy but is a good way of getting a urine sample for laboratory testing
• Bag urine (In a child only)
A sterile “colostomy-style” bag is placed over the external genitalia of a baby and urine collected into the bag. Although these are convenient they are prone to contamination because the urine sloshes around against the perineal skin. Parents love them, microbiologists don’t, but they are in common use
Taking a urine sample is not as straight forward as peeing in a bottle. If you are asking a patient to take a urine sample, the doctor or nurse must explain how to take the sample properly, and why eliminating contamination is important. This will ensure more accurate and helpful results.
Other common methods of obtaining a urine sample:
• Catheter specimen
These are only usually taken from an existing catheter or when a catheter is inserted for other reasons. A specimen from a brand new catheter is of value but once the catheter has been in for a few days it will start to become colonised with bacteria including the normal causes of UTIs as well as
Pseudomonas sp, and Enterococcus sp. They also often provoke a white blood cell response and therefore catheter urine samples often mimic a UTI even when the patient is fine. In general catheter urines should not be sent for culture unless there is good clinical reason to suspect a UTI such as suprapubic pain, fever and cloudy urine
• Suprapubic aspirate (SPA)
This is only done on very small infants (or when a suprapubic catheter is inserted in adults) and is the gold standard sample for diagnosing infection. The urine sample is collected via a sterile needle and syringe directly from the bladder through the abdominal wall. Contamination is not an issue and any white blood cells or bacterial growth is significant. The procedure looks a lot worse than it actually is!
• Clean catch sample (In a child only)
Used in children who cannot be made to void at a specific time and therefore are stripped of heir nappies and when they urinate an attempt is made to catch this in a clean
container. This takes time and some accuracy but is a good way of getting a urine sample for laboratory testing
• Bag urine (In a child only)
A sterile “colostomy-style” bag is placed over the external genitalia of a baby and urine collected into the bag. Although these are convenient they are prone to contamination because the urine sloshes around against the perineal skin. Parents love them, microbiologists don’t, but they are in common use
Taking a urine sample is not as straight forward as peeing in a bottle. If you are asking a patient to take a urine sample, the doctor or nurse must explain how to take the sample properly, and why eliminating contamination is important. This will ensure more accurate and helpful results.
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