Urine Culture: What Is it and What’s it Used For?

10 February, 2021
You’ve probably gotten a urine test before and wondered what happened after you delivered the sample to the professional. Urine culture is one of the techniques medical professionals use to isolate pathogens.

Urinary tract infections (UTIs) are very common. Scientific studies estimate that between 50% and 60% of women will suffer one during their lives. Many germs can cause it. Fortunately, a urine culture can detect the exact causal agent.

In 1995, the direct and indirect costs of these infections in the United States were estimated at $1.6 billion. Of course, knowing how to identify the pathogen that caused the infection is essential, as it allows the patient to recover quickly.

What’s a urine culture used for?

Firstly, we need to highlight that a urine culture and a urine test aren’t the same. The latter consists of a series of tests, including macroscopic and microscopic tests, physicochemical tests, and, if necessary, a urine culture.

Therefore, not all urine tests require a urine culture for diagnostic purposes. According to the National Library of Medicine of the United States, medical professionals consider them when they suspect a urinary tract infection in adults and children.

To learn more, read: What Does a Urinalysis Detect?

How’s the sample taken?

Most of the time, the patient takes the urine sample at home and then takes it to their health care provider’s office. The clinical microbiology committee of the Chilean Society of Infectious Diseases stated each of the steps in a scientific document.

A medical professional holding a urine sample.
Medical professionals process urine samples in labs.

How to take the sample

This is a crucial stage in the urine collection process, as the fluid can become contaminated with bacterial skin commensals and commensals from the patient’s genitourinary ducts. According to the American Society for Microbiology, any sample less than 5% contaminated is considered valid.

There are several types of sample collection, but we’re going to focus our attention on the most common. This is when the patient urinates in a container at home and then takes the sample to a health center. You should take the following guidelines into account for this procedure:

  • Collect your first morning urine, as it’s the most concentrated.
  • Don’t drink too much liquid before collecting the sample, since this dilutes urine.
  • Experts recommend collecting from 25 to 50 milliliters. Urine cultures need at least three milliliters.

According to the journal Annals of Pediatrics, the sample can also be obtained using other techniques, such as suprapubic aspiration or urinary bladder catheterization. Although they’re more aseptic than urination, they’re also much more invasive procedures that professionals only resort to in exceptional cases.

What happens to the sample?

According to the medical texts we mentioned above, several analyzes are conducted on a urine culture. The most common is bacterial growth. We’ll tell you all about it below:

  • A seeding loop is used and the urine sample is spread on a medium conducive to bacterial growth, which usually consists of a mixture of nutrient agar as a base on a Petri dish.
  • The professional incubates the seeded sample for 16 or 18 hours at 95-98.6 °F.
  • Once incubated, the professional counts the bacterial colonies that grew on the dish.

The premise is simple. If bacteria are present in the patient’s urine, they’ll multiply in the urine culture. This allows medical professionals to know if the patient has an infection or not. With further tests, or if it’s the specific medium for that pathogen, they’ll determine the exact agent that’s causing the infection.

Symptoms of a urinary tract infection

It can be hard to know whether or not you need a urine culture to resolve your discomfort. Here are the most common symptoms of a urinary tract infection, according to Mayo Clinic:

  • A strong, persistent urge to urinate.
  • A burning sensation when urinating.
  • Urine that appears red, bright pink, or cola-colored — a sign of blood in the urine.
  • Pelvic pain, in women — especially in the center of the pelvis and around the area of the pubic bone.

The symptoms will vary depending on the part of the genitourinary system the bacterial colonies infected. Even so, the presence of the symptoms we already described, as well as fever and abdominal discomfort, are very common.

Read more: The Presence of Nitrites in Urine

Urine culture results

The results of the test are easy to interpret. Normal values imply that everything is in order, thus ruling out a bacterial infection in the patient. On the other hand, a positive test result implies the presence of bacteria or fungi in some part of the urinary tract.

In these situations, the doctor in charge of your case will prescribe the correct antibiotic or antifungal. Remember to strictly follow your doctor’s recommendations, as stopping treatment early could lead to complications.

A patient collecting the urine sample.
The urine culture sampling process is an important step, as the sample can’t be contaminated.

Urine culture in pregnancy: is it important?

According to the website of the nonprofit organization Inatal, a urine culture during pregnancy is very important. Pregnant women are more prone to bacterial infections, due to the physiological changes that occur during pregnancy.

Furthermore, many are asymptomatic. Even if a urinary infection doesn’t cause clinical signs in the mother, it can lead to unwanted complications, such as an increased risk of preterm labor.

The importance of urine cultures

As you may have seen, urine cultures are an important part of a urine test, when a medical professional suspects an infection in a patient. This simple laboratory technique allows scientists to isolate and identify the pathogen in each case.

Patients can easily collect the sample at home. However, some exceptional cases require suprapubic aspiration and catheters. A negative urine culture indicates the absence of bacteria in the genitourinary tract, while a positive culture requires the use of antibiotics or antifungals.

  • Alós, J. I. (2005). Epidemiología y etiología de la infección urinaria comunitaria. Sensibilidad antimicrobiana de los principales patógenos y significado clínico de la resistencia. Enfermedades Infecciosas y Microbiología Clínica23, 3-8.
  • Urocultivo, medlineplus.gov. Recogido a 3 de diciembre en https://medlineplus.gov/spanish/ency/article/003751.htm#:~:text=Es%20un%20examen%20de%20laboratorio,urinaria%20en%20adultos%20y%20ni%C3%B1os.
  • Triantafilo, V. (2001). Recomendaciones para el diagnóstico microbiológico de la infección urinaria. Rev Chil Infect18(1), 57-63.
  • American Society for Microbiology. Recogido a 3 de noviembre en https://asm.org/
  • Métodos para la recogida de muestras de orina para urocultivo y perfil urinario, analesdepediatría.org. Recogido a 3 de diciembre en https://www.analesdepediatria.org/es-metodos-recogida-muestras-orina-urocultivo-articulo-13111597
  • Infección de las vías urinarias, mayoclinic.org. Recogido a 3 de diciembre en https://www.mayoclinic.org/es-es/diseases-conditions/urinary-tract-infection/symptoms-causes/syc-20353447
  • El urocultivo en el embarazo, inatal.org. Recogido a 3 de diciembre en https://inatal.org/el-embarazo/consultas/pruebas-diagnosticas-durante-el-embarazo/119-el-urocultivo-en-el-embarazo.html