UTI: Urinalysis and Culture Thresholds

Urinary tract infections (UTIs) are among the most common bacterial infections encountered in primary care, affecting an estimated 50-60% of women at least once in their lifetime. The NP must distinguish true infection from asymptomatic bacteriuria, interpret urinalysis and culture results accurately, and apply diagnostic thresholds that guide evidence-based antibiotic prescribing. Normal urine is sterile. UTIs occur when uropathogenic bacteria colonize the periurethral area and ascend through the urethra to the bladder (cystitis) or continue ascending via the ureters to the kidney (pyelonephritis). Escherichia coli accounts for 75-90% of uncomplicated UTIs. Other common pathogens include Klebsiella pneumoniae, Proteus mirabilis (associated with alkaline urine and struvite stones), Staphylococcus saprophyticus (second most common in young sexually active women), and Enterococcus species. In complicated UTIs (catheter-associated, recurrent, structural abnormalities), a broader spectrum of organisms may be found including Pseudomonas aeruginosa, Enterobacter, and Candida. Urinalysis components critical for UTI diagnosis include: Leukocyte esterase — an enzyme released by lysed white blood cells; positive result indicates pyuria (sensitivity 75-96%, specificity 94-98% for UTI). Nitrites — produced when gram-negative bacteria (particularly E. coli, Klebsiella, Proteus) reduce...