What Does It Mean When a Urine Sample Is Contaminated?

A contaminated urine sample is one that picked up bacteria from outside the bladder, usually from the skin or genital area, as it was collected. The lab can’t tell whether any germs in it came from your bladder, so the result neither confirms nor rules out a urinary tract infection. It is an unreadable test, not a diagnosis. It’s also one of the most common reasons a urine culture comes back unusable, even when people follow the instructions, so the usual next step is a better-collected second sample, guided by how you feel.

What the Lab Actually Saw in Your Sample

A urine culture puts a little of your urine on a dish and lets any bacteria in it grow, so the lab can count and name them. Your report may show counts as CFU/mL, the number of bacteria colonies per millilitre. One hospital lab calls a sample “mixed growth” when it grows 2 or more types of bacteria that aren’t known to cause urinary infections.¹ It also counts as mixed if an infection-causing germ is present but outnumbered about 10 to 1 by harmless ones, such as 1,000 versus 10,000 CFU/mL.² When harmless bacteria swamp the dish like that, the lab can’t trust that any germ it finds came from your bladder.

After a day or so on the dish, a technician looks at how many different colony types have grown and how big each group is. One germ growing heavily and alone is read as a likely cause. A scatter of several kinds in modest amounts is read as a sample that picked up bacteria on the way in.

On real reports and messages you may see “mixed growth,” “contaminated,” “probable contamination” or “please repeat.” All of these phrases mean the same thing for you: the sample can’t answer the question.

That puts a contaminated result apart from the other two outcomes. Negative means little or no bacteria grew, which argues against infection. Positive means one infection-causing germ grew in large numbers.

The extra bacteria in contaminated urine are largely types such as lactobacilli, enterococci and gut bacteria.³ Lactobacilli are normal, healthy residents of the vagina, so finding them usually points to contact during collection, not illness.

Does a Contaminated Sample Mean You Have an Infection

Possibly, but the sample can’t say. Your symptoms and a repeat test decide. Typical UTI symptoms include burning when peeing, bladder pain, urgency, needing to go often, new leaking, fever or pain in the side or back.⁴ If you have these, an infection is still possible and worth following up despite the contaminated result.

A quick dipstick test doesn’t fully settle it either. In women with suspected simple UTIs, about 1 in 5 real infections showed a negative dipstick.⁵ So neither a clean dipstick nor a contaminated culture rules an infection out on its own.

Your situation points in different directions depending on what you’re feeling. If you had mild symptoms that have faded, contamination likely changed nothing and a repeat may be enough. With ongoing burning and urgency, an infection may be hidden, so ask for a repeat promptly. Fever, back or side pain, or vomiting suggest the kidneys may be involved and need same-day care regardless of the lab result. And if you had no symptoms and the sample was taken routinely, a repeat is often only needed if your doctor asks. Burning or irritation when peeing is not always a bladder infection; skin irritation or vaginal infections can feel similar, which is one reason a clean sample matters.

While you wait for a repeat, you can usually call the clinic the same day and ask whether a new sample can be collected quickly rather than waiting for a routine appointment. Drinking normally is sensible, but drinking huge amounts beforehand can dilute the next sample and make a real infection harder to detect. Keep a short note of when symptoms began, whether they are getting better or worse, and any fever, so the clinician can judge urgency without relying on the lab result. Symptoms that keep worsening, or that return after fading, are a reason to contact the clinic again.

Why Samples Get Contaminated

Mostly it comes down to how the urine is caught. Catching the first splash raises contamination far more than catching the middle of the stream; switching to midstream cuts the odds by about 77%.⁶ The first urine washes bacteria from the opening of the urethra and surrounding skin into the cup.

Skin cells in urine were once taken as a sign of contamination, but in emergency patients their count predicted it poorly.⁷ So a lab doesn’t decide contamination from skin cells alone; it relies on what grows.

Contamination is common even among people who follow instructions. Body shape, a full bladder, hurry and awkward clinic bathrooms all make clean catching hard, and your period or vaginal discharge can add bacteria and cells to the sample.

Some routine slips do raise the odds. Starting to catch urine too early is the commonest, because it’s hard to judge in the moment when the first part has passed. Touching the rim or inside of the cup, or putting the lid down on a surface, can add bacteria even when the urine itself was caught well. Not holding the labia apart lets urine run over skin on its way out, and wiping back to front can bring gut bacteria toward the urethral opening just before you pee.

Delays matter too. Urine left at room temperature without preservative shows rising bacterial counts.⁸ A sample sitting on a counter for hours can grow enough bacteria to look worse than it was, as a few contaminants in a warm cup turn into a heavy mixed growth.

Many collection tubes contain a preservative, often boric acid, that holds bacteria at their starting level.⁹ Even so, boric acid tubes should reach the lab within 24 hours, since some bacteria start growing after that.¹⁰

How to Give a Clean Sample Next Time

Wash your hands with soap before opening the cup.
Clean the genital area front to back with the wipe provided, or plain water.
Women should hold the labia apart; men should pull back the foreskin if present.
Pee the first part into the toilet, then catch the middle part in the cup.
Finish peeing into the toilet, and don’t touch the inside of the cup or lid.
Close the lid tightly and label it right away.

A few tricks make this easier. Make sure you need to pee, without drinking large amounts, so you have enough urine to spare the first part. Sitting facing the cistern or with your knees apart gives better positioning. If you’re on your period, ask whether you can use a tampon to keep blood out. And if the cup or wipe confuses you, ask staff to repeat the instructions.

Mistakes happen, and some are easy to recover from. If you realise you started catching too early, tell the staff rather than handing it in as if it were perfect, since they may ask you to try again. If you touch the inside of the cup or drop the lid, ask for a fresh cup; an extra cup costs far less than a repeat visit. If you can’t stop and start midstream, begin peeing into the toilet and slide the cup into the stream a few seconds in. It doesn’t have to be exact. If the stream is small or you run out, hand in what you have and say so, because a small midstream sample is more useful than a full one that started with the first splash.

The wipe or water has limits. If you have vaginal discharge or your period, mention it to staff, since the method can’t fully keep those out and they can change how the result is read. Wipe once and discard each wipe rather than reusing it, so you don’t move bacteria back over the area you just cleaned. Keep cleaning gentle: harsh soaps or antiseptics left on the skin can get into the sample and interfere with the culture.

Gadgets help less than technique. In adult emergency patients, a funnel device plus silver wipes didn’t lower contamination.¹¹ In primary care, special midstream devices left about 27-28% of samples contaminated, versus 29% with standard instructions.¹² The method itself, not the equipment, does most of the work.

Then get the sample to the lab. If you can’t drop it off within about an hour or two, keep it in the fridge, because refrigeration slows the bacterial growth that happens at room temperature. If you’re given a tube with preservative, fill it to the line and still return it the same day.¹³

What Happens After a Contaminated Result

Most clinics ask for a fresh sample when the first is contaminated. Getting it right the second time saves you a third trip and the clinic a wasted test.¹⁴

The repeat visit varies. You may be given a new cup and wipes and asked to collect on the spot, or to collect at home and bring it in; clinics differ on which they prefer. Staff may briefly check your symptoms again and sometimes run a quick dipstick, but a dipstick alone doesn’t replace a culture when the picture is unclear. The repeat culture generally takes a day or two, so the clinic may contact you by phone, text or app.

Antibiotics shouldn’t rest on a contaminated result. In primary care, about 23% of patients with symptoms and contaminated cultures were given antibiotics anyway.¹⁵ Treating on contamination can mean side effects, risks to a fetus in pregnancy and more resistant bacteria.¹⁶ As one emergency department cut contamination, unnecessary antibiotics and admissions fell with it.¹⁷

While results are pending, the decision depends on your symptoms and situation. If your symptoms are typical and troublesome, a clinician may prescribe an antibiotic for a likely UTI and use the repeat culture to confirm or adjust it. If they’re mild or fading, many clinicians prefer to wait for the clean result. Pregnancy, a weakened immune system, older age or signs of kidney infection push toward earlier treatment and quicker testing.

If a second sample is also contaminated, the clinic may supervise collection, give more detailed coaching, or consider a catheter sample when a clear answer is important. That’s a sign the collection is difficult for you, not that something is seriously wrong.

Bacteria Without an Infection

Contamination is different from urine that genuinely carries bacteria from the bladder without causing symptoms, which your doctor may call asymptomatic bacteriuria. Contamination means bacteria entered from outside during collection; with bacteria without symptoms, they really live in the bladder. A good repeat sample is what separates the two.

It tends to be found by chance, when a urine test is done for another reason such as a routine check or a hospital admission. Older adults and catheter users have it often, which is why symptoms guide treatment. A result showing bacteria in a clean sample from someone who feels well prompts the clinician to consider the whole situation, not to treat automatically.

In people with a urinary catheter, bacteria without symptoms usually doesn’t need antibiotics.¹⁸ Antibiotics here usually bring no benefit to people who feel well, while still carrying side effects and encouraging resistant bacteria.

Pregnant women and people facing certain urinary procedures are treated even without symptoms.¹⁹ If you’re pregnant, tell whoever orders your test, because the next step differs for you.

Samples From Children and People With Catheters or Stomas

Young children are hard to sample cleanly. Urine bags taped to the skin were contaminated about 63% of the time, versus 9% for catheter samples.²⁰ In preschool children, clean-catch samples were contaminated about 37% of the time, versus about 8% by catheter.²¹ Coaching helps: in children aged 3 to 17, it cut clean-catch contamination from about 45% to 31%.²² For a child with fever and no clear cause, doctors may suggest a catheter sample to get a trustworthy answer.

Young children can’t hold urine, aim, or stop midstream on request, so collection often means waiting for a chance. Bag samples pick up skin bacteria during the long wait, so they’re better for ruling out infection than confirming it.

To help an older child give a midstream sample, practise the stop-start before the appointment. Clean the area first, have the parent hold the cup while the child pees, and catch a later part of the stream. If the child can’t manage, ask staff rather than handing in a splashed sample.

A catheter sample means a thin, soft tube is passed briefly into the bladder to collect urine straight from it, bypassing the skin. It’s uncomfortable but quick, and usually reserved for when the answer will change treatment.

For people with a urinary stoma, urine collected from the stoma by a small catheter is the most reliable.²³ Urine taken from the pouch missed about a quarter of the infections the catheter found.²⁴ So don’t hand in urine from a used pouch; ask how your clinic wants it collected.

With a long-term catheter, urine is generally taken from a sampling port after cleaning it, not from the drainage bag, whose contents carry bacteria that have grown over time. If the catheter has been in a while, clinicians may change it before collecting, so bacteria in the old tubing don’t distort the result. Because bacteria without symptoms are so common in these settings, a sample is normally sent only when you have symptoms such as fever, new pain or feeling suddenly unwell.