What is Culture Urine?
The Urine culture is a urine test which is used to detect and diagnose a microbial infection of the urinary tract. The waste products are excreted out of the blood by the kidneys in the form of a light straw colored liquid called urine. The path taken by the urine from the kidneys to the outside is called the urinary tract. The urinary tract gets sometimes infected by microorganisms. The urine culture test is performed to detect and diagnose a microbial infection of the urinary tract.
Why is Culture Urine done?
· To detect and diagnose a urinary tract infection (UTI) in the presence of symptoms such as burning sensation during urination, increased frequency of urination, pain in abdomen, fever, blood in urine etc.
· To screen for urinary tract infections in the first trimester of pregnancy
What does Culture Urine Measure?
The urine culture test is performed to detect and diagnose a microbial infection of the urinary tract. The metabolic processes in the body produce products called metabolites which are present in the blood. Some of them are utilized by cells, while others are wastes which are filtered out of the blood by the kidneys in urine. Urine produced by the kidneys passes through ureters into the urinary bladder where it is stored until the urinary bladder is full. Once full, the urinary bladder releases the urine through another tube called urethra to the outside of the body. The pathway followed by urine from the kidneys to the outside is called the urinary tract. The urinary tract may be infected by some microorganisms causing various conditions. Urine is normally sterile, but if urinary tract infection occurs, the pathogenic microorganism may be found in urine. A urine sample is collected and cultured to detect such an infection and to identify the microorganism causing it.
The collected urine sample is placed on an agar medium (nutrient solution mixed with agarose gel) and incubated at body temperature for 24 to 48 hours. This allows the growth of the microorganisms in the sample if any. Growing microorganisms form different types of colonies on the agar which are studied further in a laboratory to determine the exact microbe causing the UTI.
Once the pathogen is identified, antibiotic susceptibility test is performed to guide treatment for the UTI being tested for. This may be done in two ways. The conventional method involves the use of small filter paper discs with antibiotic of known concentration and placing them on agar plate where the pathogen is cultured. The antibiotic creates a small area around the discs where the microorganisms do not form colonies. The radius of this zone is measured to estimate the efficacy and strength of that antibiotic in treating the UTI. The other method involves an automated machine to detect the sensitivity pattern to find out which antibiotic can be used to treat the infection.
Interpreting Culture Urine results
Negative result: No colonies to less than 1,00,000 CFU/ml (Colony Forming Units)
· Usually more than 1,00,000 CFU/ml
· Patient undergoing antibiotic treatment: More than 1,000 CFU/ml
· For gram-positive bacteria like Staphylococcus aureus: Single colony
The negative result indicates no infection of the urinary tract.
The positive result indicates an infection of the urinary tract. The positive result is accompanied by an antibiotic susceptibility test.
In an uncontaminated sample, only a single type of colony is found and indicates a clinically significant positive result. However, if multiple colonies of different types are found, a sample contamination is indicated and the sample may need to be collected again.