Urine culture

What it measures

The lab reports growth in colony-forming units per milliliter (CFU/mL). When one type of germ grows in large numbers, the lab identifies it and tests it against a set of antibiotics. Growth of several different germs usually means the sample picked up skin bacteria during collection. Antibiotics taken before the sample can stop germs from growing and give a falsely negative result.

Who should consider it

• People with burning, urgency, or frequent urination, alongside a clinician visit
• Anyone whose urinalysis showed signs of infection
• People with repeat UTIs who want to know which germ keeps coming back
• Pregnant people, as part of routine prenatal screening ordered by their prenatal care team

How to read your result

• Labs have long used 100,000 CFU/mL or more of a single germ as a threshold. The Infectious Diseases Society of America (IDSA) uses that level to define bacteria in the urine without symptoms. Lower counts can still matter when you have clear symptoms.
• Mixed growth or several organisms usually points to contamination. A repeat clean-catch sample is the usual next step.
• Bacteria in the urine without symptoms (asymptomatic bacteriuria) usually does not need antibiotics. IDSA recommends treating it mainly in pregnancy and before certain urologic procedures.
• A negative culture with ongoing symptoms can come from recent antibiotics or from causes other than a bacterial UTI, such as some sexually transmitted infections.

Reference ranges differ between labs. Use the range printed on your own report.

When to get care right away. Fever, chills, back or side (flank) pain, nausea, or vomiting along with urinary symptoms can mean a kidney infection and need same-day medical care. If you are pregnant and have urinary symptoms, contact your prenatal care team promptly. Confusion, fainting, or feeling very unwell: go to an emergency room or call 911.

What to do next

Share a positive culture with a clinician, who can pick an antibiotic that the sensitivity report shows will work. If symptoms keep returning, a clinician can look for causes such as kidney stones or bladder emptying problems.