THE CASE: SIGNS AND SYMPTOMS
A 29–year–old nullipara at 18 weeks’ gestation complains of fevers and back pain. She had a diagnosis of urinary tract infection with sulfonamide-resistant Escherichia coli at 9 weeks of gestation, which was treated with nitrofurantoin, 100 mg by mouth twice a day for 7 days. A test of cure by urine culture was negative.
Now her temperature is 101°F and she has right costovertebral angle tenderness.
How should you proceed?
Anatomy is destiny, in the case of susceptibility to urinary tract infection (UTI). The female urethra is only 3 cm to 4 cm long, and its proximity to the vagina, anus, and rectum facilitates colonization of normal gastrointestinal flora in the bladder.1
Sexual activity also facilitates migration of normal gastrointestinal flora to the female urethra.2
Anatomical features of pregnancy exacerbate the female predisposition to urinary tract infection. In pregnancy, hormonal and mechanical changes that occur in the urinary tract lead to urinary stasis and ureterovesical reflux—setting the stage for urinary tract infection (FIGURE 1).
Who should be screened?
All pregnant women should be screened for UTI early in pregnancy, according to the American College of Obstetricians and Gynecologists.3
I recommend a urine culture screening for all pregnant women at their first prenatal visit.
Screen often if she has risk factors
I recommend frequent screening (at least every trimester) by urine culture, in pregnant women with any of these risk factors:
- diabetes mellitus, including gestational diabetes4;
- urologic abnormalities—specifically, neurogenic bladder;
- prepregnancy (for example, 2 to 3 infections per year) and antepartum history of UTI prior to initiation of prenatal care5;
- sickle cell hemoglobinopathy.5
Which test is best?
The gold standard for detecting bacteria in urine is by culture.
Which threshold to use?
The standard definition of a positive urine culture from a clean-catch, midstream, voided specimen is ≥100,000 colony forming units (CFU) per mL of a single organism. However, in symptomatic patients, the test’s sensitivity is increased by lowering the cut-off to 100 CFU/mL of a single organism.6 In women with urinary symptoms, only 50% of patients had 100,000 CFU/mL by urine culture collected from clean-catch, midstream, voided specimens, though all of them had positive cultures from suprapubic taps.
The clean-catch, midstream, voided specimen is the specimen of choice for practical purposes, since it is noninvasive and easily obtained in the office setting.
For the record: The presence of any organism represents UTI in specimens obtained via suprapubic aspiration of the bladder; 100 CFU/mL of a single organism is positive for specimens obtained by urethral catheterization.
I recommend that, when obtaining urine cultures via clean-catch, midstream, voided specimens:
- for asymptomatic patients, use ≥100,000 CFU/mL of a single organism.
- in symptomatic patients, use ≥100 CFU/mL of a single organism.
What about rapid tests?
Urinary sediment analysis and urine dipstick testing offer speed and low cost, but with lower accuracy than urine cultures, which require 24 to 48 hours for results and cost more.
Urinary sediment analysis can diagnose pyuria, defined as a clean-catch, midstream, voided specimen, which is spun and which has >10 leukocytes per high-power field.
Pyuria can occur without infection due to:
- previous treatment with antibiotics,
- contamination of urine sample by sterilizing solution,
- contamination of urine sample with vaginal leukocytes,
- chronic interstitial nephritis (such as analgesic abuse),
- uroepithelial tumor, and
Bacteria visualized on microscopic examination is more sensitive (75%) but less specific (60%).7
Urinary dipstick testing—fast, convenient, and low in cost—is considered positive if it identifies either leukocyte esterase or nitrite. Positive leukocyte esterase signifies pyuria. Positive nitrite indicates the presence of enteric organisms that convert urinary nitrate to nitrite.
With either finding, dipstick sensitivity is only 50%, although specificity is 97%.7
- If a symptomatic patient’s rapid test is positive, obtain a urine culture, empirically treat for UTI, and then use urine culture results to decide whether to continue treatment.
- If an asymptomatic patient’s rapid test is positive, obtain a urine culture and treat only if the culture is positive.
What urinary tract disorders occur in pregnancy?
First, determine if the patient has urinary tract symptoms and, if so, whether the symptoms are typical of upper or lower urinary tract infections.