Mixed growth on a urine culture: what it means and what happens next

A urine culture is a laboratory test that grows any bacteria or yeast found in a urine sample. The sample is spread on a nutrient plate and left overnight at body temperature so that anything present can multiply into visible colonies, which are then counted and identified. Lab Tests Online UK explains that the aim is to confirm whether you have a urinary tract infection and, if so, which antibiotics will work against the bacteria.

In a true bladder infection one organism, most often E. coli, usually dominates the plate. "Mixed growth" is the laboratory's way of saying that several different organisms grew in smaller numbers and no single one stood out. Lab Tests Online UK says the report may also use the wording "possible contamination". It is a description of what grew, not a diagnosis, and it carries no information about which antibiotic would help because there is no single organism to test.

Because the finding is vague, it is a result that people often find unsettling. Seeing a long list of bacteria, or being told the sample "grew mixed organisms", can sound alarming. In practice it is one of the most routine outcomes in a microbiology laboratory, and it is far more often a comment on the sample than on the person.

The urethra, the tube that carries urine out of the body, opens onto skin that naturally carries bacteria. Lab Tests Online UK notes that bacteria from the skin around the urethra can easily contaminate a sample and give a misleading result. Collecting the first part of the stream, not cleaning beforehand, or touching the inside of the container can all introduce skin bacteria.

The UKHSA diagnostic tool adds two further explanations: that mixed growth may reflect perineal contamination, and that it may reflect a delay in sample transport or processing. Lab Tests Online UK makes the second point in more detail, explaining that bacteria multiply rapidly at room temperature, so a sample left too long before reaching the laboratory can show falsely high counts of whatever was present.

• Skin or genital bacteria entering the container during collection.

• Collecting the first part of the urine stream rather than a midstream sample.

• Touching the inside of the container or lid.

• A long gap between collecting the sample and it reaching the laboratory.

• Blood from a period in the sample, which Lab Tests Online UK says can interfere with microscopy and culture.

There is no single rule, and that is why the result is read alongside the story. The UKHSA tool states that the culture result should be interpreted with symptoms and repeated if its significance is uncertain. It also notes that the presence of epithelial (skin-type) cells in the sample is not necessarily an indicator of contamination, so the microscopy report is only one clue.

If you have no urinary symptoms and the sample grew mixed organisms, the sensible reading is usually that the result is a collection artefact. If you do have symptoms, the clinician may treat on symptoms, repeat the test or ask the microbiologist to advise. The UKHSA tool specifically says that when a person is symptomatic, a re-test and discussion with the microbiologist about the possibility of a genuine mixed infection can be considered.

Context also matters. Lab Tests Online UK points out that urine from a long-term catheter may grow bacteria that have colonised the catheter without causing active infection, and that interpretation needs clinical judgement rather than automatic antibiotics. The NHS makes a similar point for people who are older or frail, where changes such as new confusion or worse leaking can be the presenting signs of infection.

Lab Tests Online UK describes the midstream clean-catch method as the way to reduce the chance of a repeat contaminated result. The steps it lists are to wash your hands first, then start to urinate into the toilet, move the sterile container into the flow to catch a midstream sample, finish into the toilet, seal the container tightly and label it with your name and date of birth. It advises that women hold the labia apart while urinating and that men clean the tip of the penis first.

The sample should reach the laboratory or GP surgery as soon as possible, ideally within one to two hours. If that is not possible, Lab Tests Online UK says to store the container in the fridge for no longer than 24 hours.

One detail differs between sources. the Lab Tests Online UK urinalysis guide advises cleaning the genitalia first, whereas the UKHSA tool says holding the labia apart in women may help reduce contamination and advises not to cleanse with antiseptic before collection because bacterial growth may be inhibited. If the instructions you are given differ, follow those from the service that asked for the sample.

Tell the person arranging the repeat if you have already started antibiotics. Lab Tests Online UK says that even a single dose can suppress bacterial growth and cause a false negative result.

A culture is only one part of the picture. The NHS lists symptoms of a urinary tract infection as pain or burning when peeing, needing to pee more often or more urgently, cloudy pee, blood in the pee, lower tummy or back pain, and a high or very low temperature. If those are present, the NHS advises seeing a pharmacist if you are a woman aged 16 to 64 who is not pregnant, and asking for an urgent GP appointment or contacting NHS 111 in a longer list of circumstances.

That list includes being aged 65 or over, being a man, having diabetes, being pregnant, using a catheter, having a weakened immune system, having symptoms that get worse quickly or do not improve within 48 hours, and having pain in the lower tummy or the back just under the ribs. It adds that some of these could be symptoms of a kidney infection, which can be serious if untreated because it could cause sepsis. Call 999 or go to A&E if you or your child are confused, drowsy or have difficulty speaking.

The NHS also notes that if you keep getting UTIs (two within six months or three within twelve months) you should see a GP, and that long-term UTI symptoms sometimes occur even when urine tests do not show infection. In those cases a mixed growth result should not be the end of the conversation.