Samples that contain large numbers of skin cells are contaminated and probably not useful.
For the culture it is necessary to obtain an uncontaminated sample. The culture sample obtained in the morning is more likely to detect a urinary infection. Samples left at room temperature for> 2 h can give false positivity with high bacterial counts due to continuous bacterial proliferation. Criteria for culture positivity include isolation of a single bacterial species from a sample of the intermediate jet, obtained with a clean technique, or from a sample taken from a catheter.
In asymptomatic bacteriuria, the criteria for positivity that are based on the guidelines of the Infectious Diseases Society of America (see the Guidelines for the Diagnosis and Treatment of Asymptomatic Bacteriuria in Adults) are
- Two consecutive culture samples (for humans, one sample) from which the same bacterial strain is isolated with a colony count> 105 / mL
- In women or men, from a catheter specimen, isolation of a single bacterial species with a colony count> 102 / mL
- In symptomatic patients, the criteria are
- Uncomplicated cystitis in women:> 103 / mL
- Uncomplicated cystitis in women:> 102 / mL (This quantification may be considered to improve sensitivity to E. coli.)
- Acute, uncomplicated pyelonephritis in women:> 104 / mL
- Complicated UTI:> 105 / mL in women; or> 104 / mL in men or a catheter sample in women
- Acute urethral syndrome:> 102 / mL of a single bacterial species
- Any positive culture result, regardless of colony count, in a suprapubic puncture specimen should be considered a true positive.
In intermediate urine, E. coli in mixed flora can be a real pathogen (1).
Sometimes a urinary tract infection is present despite a low colony count, possibly due to previous antibiotic therapy, highly diluted urine (specific gravity <1.003), or obstruction to the flow of macroscopically infected urine. Repeat culture improves the diagnostic accuracy of a positive result, ie it can distinguish contamination from a true positive result. The latest molecular urine tests can sometimes reveal unusual pathogens in patients with refractory or recurrent urinary tract infection.
Localization of the infection
Clinical differentiation between upper and lower urinary tract infections is impossible in many patients and tests are generally not advisable. When the patient has a high fever, pain in the costovertebral angle and severe pyuria with urinary cylinders, it is very likely that it is pyelonephritis.
The best non-invasive technique for differentiating bladder from kidney infections appears to be the response to a short course of antibiotic therapy. If the urine is not sterile after 3 days of treatment, the presence of pyelonephritis should be checked.
Symptoms similar to those of cystitis and urethritis can occur in patients with vaginitis, as these can cause dysuria when urine passes through the inflamed labia. Vaginitis can often be distinguished by the presence of vaginal discharge, vaginal odor and dyspareunia.
Men with symptoms of cystitis who do not respond to regular antimicrobial therapy could be suffering from prostatitis.
Severely impaired patients should be evaluated for sepsis, typically with formula blood count, electrolytes, lactate, BUN, creatinine, and blood cultures. Patients with abdominal pain or tenderness should be evaluated for other causes of acute abdomen.
Patients who have dysuria / pyuria but not bacteriuria should be tested for a sexually transmitted disease, typically using nucleic acid tests on urethral and cervical swabs (see Chlamydial Infections: Diagnosis).
Most adults do not require evaluation for structural abnormalities except in the following cases:
- The patient has ≥ 2 episodes of pyelonephritis.
- Infections are complicated.
- Nephrolithiasis is suspected.
- Gross haematuria in the absence of pain or new onset renal failure.
- Fever that persists for ≥ 72 h.