Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (248 page)

   Nonspecific symptoms (like failure to thrive or feeding difficulties) may be the only symptoms of UTI in infants and elderly patients.

   In uncomplicated UTI, patients respond rapidly to effective antibiotic therapy. Further evaluation, including urinalysis and culture, is recommended for patients with persistent symptoms or early recurrence to rule out pathogen resistant to initial therapy or to rule out other factors associated with complicated UTI.

   Diagnostic and Laboratory Findings

   Uncomplicated UTI can be reliably diagnosed on the basis of typical symptoms. Urinalysis and urine culture are not routinely needed; patients may be treated empirically.

   Uninalysis and urine culture should be performed for patients if complicated UTI is suspected, and for patients with symptoms of pyelonephritis.

   Diagnostic Tests

   
Urinalysis
(dipstick or microscopic): Dipstick urinalysis performs best when urine culture yields growth >10
⁵
cfu/mL and dipstick shows positive leukocyte esterase and nitrite reactions (sensitivity 84%; specificity 98%). Sensitivity was significantly lower when the urine culture yielded growth <10
⁵
cfu/mL. The urine dipstick is not a reliable screen to rule out UTI.

However, urinalysis has good specificity and may provide evidence to support a diagnosis of UTI. Most patients with UTI have pyuria (WBCs by microscopy or dipstick leukocyte esterase); WBC casts suggest pyelonephritis. Proteinuria and hematuria are also frequent findings. A positive dipstick nitrite reaction is typical for UTI caused by
E. coli
and other
Enterobacteriaceae
, but may be negative for other uropathogens, like
Enterococcus
species,
Pseudomonas
species and
S. saprophyticus
.

Algorithms using dipstick urinalysis have been proposed to reduce unnecessary antibiotic use while awaiting culture results. In patients at low risk for complicated UTI, three variables were used: dysuria, leukocytes greater than trace, and any positive nitrite reaction, including trace. Patients positive for two or three variables were treated without culture; culture was collected for patients with no or one positive variable and antibiotics withheld pending culture results. Using the algorithm, 80% of significant UTIs were detected; unnecessary antibiotic prescriptions were reduced by 23.5% and urine cultures by 59% compared with usual physician care.

   
Gram stain
: Gram stain of an unconcentrated urine may be useful for detecting urine specimens that yield growth >10
⁵
cfu/mL, but are not reliable for detecting specimens that yield lower level, but significant growth. Because of the limited sensitivity for detecting significant cultures, and because of the labor intensity to perform, Gram staining is not recommended for urine specimens.

   
Routine culture
: Quantitative culture is performed by inoculation of 1 microliter of urine onto SBA and selective (e.g., MacConkey or CNA) agar. The lower level of detection, therefore, is 10
³
cfu/mL. The extent of workup (identification and susceptibility testing) depends on several factors, including: type of specimen (clean catch versus invasively collected), number of species isolated (pure culture versus mixed), pathogenic potential of isolate (typical uropathogen versus common contaminant) and quantity of growth.

Laboratory workup is usually limited (descriptive ID only; no susceptibility testing) for cultures that yield mixed growth (3 or more species in comparable quantities), organisms with low uropathogenic potential (like
Lactobacillus
and diphtheroids), or isolates growing in quantities <10
⁴
cfu/mL.

   
Culture for possible complicated UTI
: For symptomatic patients at risk for complicated UTI, bacteriuria at quantities <10
^3-4
cfu/mL may predict significant UTI. For such patients, culture methods using a 10-microliter inoculum allow detection of growth at a lower detection limit of 10
²
cfu/mL. The extent of workup follows similar guidelines used for routine cultures, except that full ID and appropriate susceptibility testing is performed when one or two uropathogens are isolated in quantities >10
³
cfu/mL (versus the 10
⁴
cfu/mL cutoff used for routine cultures).