Urinalysis and Urine Culture

General Description

A urinalysis consists of examining a urine sample in the laboratory, assessing its appearance, concentration, and color, as well as its microscopic components and chemical substances.

Chemical analysis evaluates proteins, glucose, ketones, bilirubin, nitrites, and blood, among other substances, and measures urine acidity (pH) and concentration. Microscopic analysis, in turn, identifies red blood cells, white blood cells, crystals, and urinary casts.

A urine culture (uroculture) is a laboratory test used to detect the presence of microorganisms, such as bacteria or fungi, in urine. This allows for the diagnosis of urinary tract infections.

When is it indicated?

Urinalysis is commonly part of a routine medical examination and helps diagnose various disorders, such as diabetes, infection, kidney damage, or liver damage.

A urine culture is usually indicated when the patient presents symptoms consistent with a urinary tract infection, including:

• Pain or burning during urination
• Strong and frequent urge to urinate, often with low urine output
• Changes in urine appearance: cloudy color and strong odor
• Presence of blood in urine
• Fever, chills
• Lower back pain
• Pressure in the lower abdomen
• Findings suggestive of infection in a urinalysis

A follow-up urine culture is indicated to confirm bacterial clearance after antibiotic treatment in complicated urinary tract infections, in cases of symptom recurrence after completing treatment, or when symptoms persist 48–72 hours after initiation.

Urinary tract infections that can be diagnosed include cystitis (bladder infection), acute pyelonephritis (kidney infection), and urethritis (infection of the urethra). The culture allows identification of the causative pathogen and, consequently, selection of appropriate treatment.

How is it performed?

The patient may collect the urine sample at home or at a medical facility. The sample is collected in a sterile container provided during consultation or purchased at a pharmacy. If the patient is unable to collect the sample, it is usually obtained by bladder catheterization, inserting a catheter (a thin, flexible tube) into the bladder through the urethra and collecting urine into a sterile drainage bag attached to the catheter. If this is not feasible, the sample is obtained by suprapubic puncture, inserting a needle through the abdomen into the bladder and aspirating urine with a syringe. In infants, collection may be performed by suprapubic puncture or by attaching a collection bag to the genital area.

For chemical urinalysis, reagent strips (dipsticks) are used plastic or paper strips with small pads impregnated with chemicals. When immersed in urine, the pads react with substances in the sample, producing characteristic color changes.

For microscopic analysis, urine sediment (concentrated urine obtained after centrifugation) is examined under a microscope.

To perform a urine culture, the sample is placed onto one or more culture media plates (Petri dishes) containing a nutrient solution. The samples are incubated for 24–48 hours at a temperature similar to body temperature, allowing any microorganisms present to proliferate as colonies, which can then be identified by their shape, color, and size.

Additional identification tests are often performed, such as Gram staining. This technique involves isolating cultured microorganisms and applying special dyes to classify them according to the color they retain after staining.

In a negative urine culture, no microorganisms are detected, ruling out infection. If the culture is positive, an antibiogram is typically performed to determine the susceptibility of pathogens to specific antibiotics and guide appropriate treatment.

Risks

If the urine sample is collected via catheterization or puncture, there is a minimal risk of bleeding, infection, or bladder, urethral, or intestinal perforation.

What to expect from a urinalysis or urine culture

When collecting the sample, it is recommended to use the first morning urine, although urine cultures can also be performed with samples collected throughout the day. Proper hygiene measures are essential to avoid contamination: first, hands should be washed with soap and water. Then, the genital area should be cleaned with soap and water or with wipes provided by the healthcare facility—separating the labia minora in women or retracting the foreskin in men.

Only midstream urine should be collected. The initial portion of urine should be voided into the toilet. Approximately 20 mL should then be collected in the sterile container, avoiding contact with the skin or toilet. Urine should not be transferred between containers. Once collected, the container must be sealed immediately and delivered to the laboratory as soon as possible. If this is not possible, the sample may be stored for up to two hours at room temperature or up to four hours at 4°C.

Urine collection via bladder catheterization must be performed in a medical facility. With the patient lying supine on an examination table, the area around the urethra is cleaned with an antiseptic solution and a urological lubricant is applied to facilitate catheter insertion. Mild discomfort or pressure during catheter advancement is normal. After urine drainage into the container, the catheter is removed.

If suprapubic aspiration is performed, the patient lies on their back with hips and knees flexed. The puncture site is disinfected and a topical anesthetic is applied. Pressure may be felt during needle insertion and sample collection. After needle removal, a dressing is applied and gentle pressure is exerted to prevent bleeding.

Specialties requesting urinalysis or urine culture

Urinalysis and urine culture are commonly requested in Internal medicine, Primary care, Gynecology and Obstetrics, and Urology.