Glossary · Laboratory techniques
Urine culture
In one sentence
A urine culture grows a urine sample on culture media to see whether bacteria or fungi are present, count them in colony-forming units per millilitre (CFU/mL) and identify them. If a pathogen grows, antibiotic susceptibility testing follows.
Also known as: urine cultures · urine culture and sensitivity · urine MC&S · urocultivo
What is a urine culture?
It is the reference microbiology test for confirming a urinary tract infection and identifying the organism responsible. A measured volume of urine is plated and, after incubation, colonies are counted so the result can be expressed in CFU/mL. If a uropathogen grows, such as Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis or Enterococcus faecalis, the laboratory identifies it and tests its susceptibility to guide treatment and monitor antimicrobial resistance.
How it is interpreted
- Colony count: Kass’s classic threshold for significant bacteriuria is 10⁵ CFU/mL in midstream urine. Several studies have shown that a proportion of women with acute cystitis have lower counts, so guidelines accept lower thresholds depending on the setting. Criteria differ between guidelines and laboratories.
- Mixed growth: several species usually point to contamination during collection and may warrant a repeat sample.
- Asymptomatic bacteriuria: bacteria in the urine without symptoms is not the same as infection. The 2019 IDSA guideline (Nicolle et al., Clinical Infectious Diseases) advises against screening for or treating it in most groups, with exceptions such as pregnancy and before certain urological procedures.
Limitations
A negative urine culture means nothing grew under the laboratory’s conditions and criteria; on its own it does not explain persistent symptoms. Antibiotics taken before sampling, poor collection or storage, organisms that grow poorly on routine media and non-infectious causes all play a part. Weighing these possibilities is the clinician’s role.
Example
A woman with burning and urgency provides a midstream sample before starting treatment. At 24 hours more than 10⁵ CFU/mL of E. coli has grown, and at 48 hours susceptibility testing shows resistance to one commonly used antibiotic, so her GP adjusts treatment. In a different patient with recurrent symptoms and repeatedly negative cultures, the clinician reviews collection technique, prior antibiotics and other causes before deciding whether further investigation is needed.
Urine culture at Xenogene
Xenogene does not perform conventional urine cultures. Its AMC Advanced Urological Study is an analysis designed to detect the microorganisms present in a urinary tract sample (kidneys, ureters, bladder and, in men, the prostate), with indications such as cystitis, prostatitis and urethritis. In selected cases, such as recurrent symptoms with negative cultures, it may add complementary information through clinical metagenomics. Detecting genetic material does not by itself confirm active infection, and it does not replace urine culture or susceptibility testing; the decision to request it and its interpretation rest with the clinician.
Frequently asked questions
How long does a urine culture take?
A negative urine culture is usually reported within about 24 to 48 hours. If bacteria grow, identification and susceptibility testing can extend this to around 72 hours, depending on the laboratory.
How should I collect a urine sample for culture?
Laboratories usually ask for a first-morning sample, or urine that has been in the bladder for a few hours. The genital area is cleaned, the first part of the stream is discarded and the midstream portion is caught in a sterile container, which should reach the laboratory promptly or be refrigerated. Follow the laboratory's instructions and mention any antibiotics you are taking.
What does a count of more than 100,000 CFU/mL mean?
It is the classic threshold for significant bacteriuria in a midstream sample. However, symptomatic women can have infection at lower counts, and bacteria in the urine without symptoms usually do not need treatment. The result is interpreted by the clinician alongside symptoms.
Related terms
- Blood culture
A blood culture incubates a blood sample in bottles of nutrient broth to find out whether live bacteria or fungi are circulating in the bloodstream. If anything grows, it is identified and tested against antibiotics to guide treatment.
- Antimicrobial resistance
Antimicrobial resistance (AMR) is the ability of bacteria, viruses, fungi or parasites to stop responding to medicines that used to control them, making infections harder to treat.
- Clinical metagenomics
Clinical metagenomics (mNGS) is the untargeted sequencing of the genetic material in a patient sample to detect bacteria, viruses, fungi or parasites without choosing the pathogen in advance. It complements culture and susceptibility testing rather than replacing them.
- Bacteraemia
Bacteraemia is the presence of viable bacteria in the bloodstream, usually confirmed by blood cultures. It can be transient and harmless or part of a serious infection, and it is not the same as sepsis, which is defined by the body's response to infection.
Educational content for healthcare professionals and the general public. It does not replace assessment by a healthcare professional: ordering and interpreting any test is the responsibility of the requesting clinician.
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