Urinary Tract Infection - Diagnosis

Treatment can be started without the need for further tests if the following urinalysis results are present in patients with symptoms and signs of UTIs:

A high white cell count.
Cloudy urine.

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Urine Culture

A urine culture uses a urine specimen that is placed on an agar plate, then incubated in the laboratory for 24 to 48 hours. It is then examined for the presence of bacterial growth. Urinary tract infection is nearly always caused by a single species of bacteria, notably E. coli. Cultures have limitations, however. If a mix of different species is found, the test is considered contaminated and is redone. In addition, even if E. coli is identified, researchers are also looking for variants of this bacteria. Certain types may indicate a higher risk for a second infection, while others may even be protective against recurring infections. Furthermore, some organisms, such as Chlamydia, which is a sexually transmitted organism, may not be detected.

A urine culture is usually performed if the dipstick results are positive, but even if the results are negative, a culture may still be helpful under certain circumstances:

If urinalysis or dipstick is negative but the patient has UTI symptoms, particularly if the patient has recurring infections or is in a high-risk group.
If the doctor suspects complications.
In girls less than two years of age with a high fever of unknown origin that lasts two days or more.

Even if bacteria are present in the culture, a diagnosis of UTI depends on symptoms and gender:

The presence in a culture of at least 100,000 bacteria milliliter of any single type of bacterium per milliliter of urine usually provides conclusive evidence of infection in women with symptoms.
A count of 100,000 bacterial per milliliter in a woman without symptoms indicates asymptomatic bacteriuria. The decision to treat depends on the woman's risk factors for complications.
In young women with symptoms of cystitis, a diagnosis of infection can reasonably be made with counts as low as 1000 bacteria per milliliter.
Men are considered to have an infection with a count of only 1,000.

Gram Stain

If doctors suspect that bacteria other than E. coli may be present, a Gram stain is used to help predict the species. This is a staining procedure used to make bacteria visible through a microscope. Many bacteria are categorized by the terms gram-positive and gram-negative.

Bacteria that turn pink from staining are called gram-negative.
Those that turn blue are called gram-positive.

Escherichia coli is gram negative and the most common cause of UTIs. If doctors suspect that bacteria other than E. coli are causing a UTI, a Gram stain is useful for identifying other species.