A midstream specimen of urine (MSU) is tested to look for infection.
What is the purpose of a midstream specimen of urine test?
- To confirm the diagnosis of a urine infection. The usual symptoms of a urine infection are pain when you pass urine and passing urine frequently. However, symptoms are not always typical, particularly in children and the elderly, so a urine test may be needed.
- To decide the best antibiotic to use. Some germs (bacteria) are resistant to some antibiotics. If the test shows that bacteria are in the urine then the bacteria are tested against various antibiotics. This finds which antibiotics will kill the bacteria in the urine.
How do I do a midstream specimen of urine?
The aim is to obtain a sample (specimen) of urine from the middle of your bladder. Urine does not normally have any germs (bacteria) in it (urine should be sterile). If bacteria are found in the sample, it means that the urine is infected. A midstream specimen of urine (MSU) is best, as the first bit of urine that you pass may be contaminated with bacteria from the skin.
Before doing an MSU, wash your hands and ideally your genitals as well.
Women - hold open the entrance to the vagina (your labia). Men - pull back your foreskin. Pass some urine into the toilet. Then, without stopping the flow of urine, catch some urine in a clean (sterile) bottle. (The bottle is usually provided by a doctor or nurse.) Once you have enough urine in the bottle, finish off passing the rest of your urine into the toilet.
Do not open the sterile bottle until you are ready to take the sample. Avoid touching any part of your genitals with the bottle, as this will increase the risk of contamination. Put the cap back on the container. You do not need to fill the bottle to the top; a small amount will do. Some specimen bottles contain a powder which helps the sample last longer for testing (a preservative). If this is the case, a mark on the bottle will indicate the ideal amount of urine. However, if that is difficult, any amount is better than none.
The sooner the sample is given in to the doctor's surgery, or to the laboratory, the better. Within two hours is best. If that is not possible, put the sample in the fridge until you take it to the doctor or laboratory.
If it is difficult to aim your urine stream into the bottle, you may use another container such as a jam jar or a disposable plastic cup. You can then pour the urine into the sterile bottle. If you do this, make sure the container you pass water into is as clean as possible. Wash it well and rinse it with boiling water. You should still pass the first part of your urine stream into the toilet. In this way, you are collecting the urine from the bladder.
The result of an MSU takes 2-7 days.
Urine specimens and children
It is not easy to obtain a pure midstream specimen of urine (MSU) in young children and babies. The following methods may be used:
The clean catch method
The usual way is to catch some urine in the specimen bottle whilst the child is passing urine. This is called the clean catch method. Just be ready with the open bottle as the child passes urine. (Be careful not to touch the open rim of the bottle with your fingers, as this may contaminate the specimen with germs (bacteria) from your fingers.)
For babies the following might work: take the nappy off about one hour after a feed. Tap gently with a finger (about once a second) just at the bottom of the tummy (abdomen) above the genitals. Have the open bottle ready. Quite often, within about five minutes, the baby will pass urine. Try to catch some in the bottle.
One method is to place a special absorbent pad in a nappy. Your doctor or nurse will provide the special pad and tell you how to use it. Basically, when you place the pad in the nappy, check the pad every 10 minutes to see if it is wet with urine. Do not leave in for longer than 30 minutes as this may increase the risk of contamination. Therefore, replace the pad with a fresh one every 30 minutes until urine has been passed. Then, use the syringe provided to suck the urine from the pad. Then, transfer the urine in the syringe into a sterile container. Do not use any other type of pads, cotton wool balls or gauze as they could alter the results.
Urine collection bags
These are bags which are placed inside the nappy to collect urine. They are stuck to the skin around the genitals. When the infant passes urine, it collects in the bag. You then take off the bag, cut the corner of the bag and pour the urine into the sterile bottle. Always wash your child's genital area and dry it carefully before sticking the bag on. This is so germs from the skin are not mixed in with the urine.
What do the test results mean?
The MSU test is primarily performed to check for infection (see above). However, there are occasions when the urine is clear of infection, but other abnormalities are detected, such as blood in the urine. These abnormalities may also be detected by other tests such as the urine dipstick test. For more information, see our separate leaflets called Urine Dipstick Test and Blood in Urine.
Further reading and references
Urinary tract infection in under 16s: diagnosis and management; NICE Clinical guideline (Aug 2007, updated September 2017)
Gilbert R; Taking a midstream specimen of urine. Nurs Times. 2006 May 2-8102(18):22-3.
Urinary infection (child) - Patient information; The British Association of Urological Surgeons
Loane V; Obtaining urine for culture from non-potty-trained children. Paediatr Nurs. 2005 Nov17(9):39-42.