Culture analysis of a urine sample collected with a clean midstream technique to identify the causative bacterium of a urinary tract infection and the appropriate antibiotic.
Indication
- Complicated or recurrent urinary tract infection
- Leukocyte and nitrite positivity on urinalysis
- Asymptomatic bacteriuria screening during pregnancy
- Investigation of fever etiology in newborns, infants, and children
- Pre-operative evaluation before urinary tract surgery
- Suspicion of infection in catheterized or immunosuppressed patients
- Urinary tract infection unresponsive to antibiotic therapy
Preparation
- If possible, the sample is collected before starting antibiotics or at least 48 hours after
- The genital area is washed with soapy water, rinsed, and dried
- The first morning urine is preferred
- When fluid intake is high, urine may be diluted; this is taken into account during interpretation
- In catheterized patients, the sample is collected directly from the catheter port using sterile technique
How it's performed
- The patient provides urine into a sterile container using the clean midstream technique: the initial flow is discarded, and the middle portion is collected
- The sample is delivered to the laboratory within 1-2 hours or stored at 4°C
- Inoculation onto culture media (blood agar, MacConkey) is performed with a calibrated loop
- Incubated at 37°C for 24-48 hours, with colony counting performed
- The growing microorganism is identified by biochemical/MALDI-TOF methods
- Antibiotic susceptibility (antibiogram) testing is performed to determine the appropriate antibiotic
Post-procedure
- Significant growth: in midstream samples, growth of a single bacterium >10^5 colonies/mL (10^3 colonies/mL is also valuable in symptomatic women)
- Mixed growth is generally contamination; a repeat sample is requested
- The result and antibiogram report are completed within 48-72 hours
- The physician plans treatment based on the antibiogram result or adjusts ongoing therapy
- A follow-up culture is recommended after treatment in indicated cases (pregnancy, childhood)
Risks
- The procedure is non-invasive and carries no physical risk
- Improper collection technique may lead to false growth due to contamination
- False-negative results may occur in patients who have already started antibiotics
- In catheter samples, distinguishing colonization from true infection requires clinical correlation
FAQ
How is it different from a routine urinalysis?
Urinalysis is a screening test; culture is the definitive diagnostic test that identifies the causative agent of the infection and to which antibiotics it is susceptible.
Why does the result take 2-3 days?
Bacterial growth and susceptibility testing require an incubation period. With rapid molecular methods, the duration may be shortened in some laboratories.
Can a culture be done after taking antibiotics?
Whenever possible, the sample should be collected before antibiotics are started; if started, there is a possibility of false-negative results, and this should be discussed with the physician.
What does mixed growth mean?
Growth of more than one bacterium usually indicates that the sample was not collected properly (contamination); a repeat clean collection is recommended.
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