UTI & Pyelonephritis

Nearly every UTI starts the same way: pathogenic bacteria from the perineum and rectum reach the periurethral area, adhere to the urethral epithelium, and ascend into the bladder. E. coli dominates — the large majority of bladder infections and by far the most common cause of pyelonephritis — followed by Klebsiella, Proteus, Enterobacter, Enterococcus, Pseudomonas, and Staphylococcus saprophyticus.

The reason UTIs are roughly 4× more common in women is anatomic, not behavioral: a short urethra whose meatus sits close to the anus gives bowel flora a short climb. An infection confined to the bladder with no systemic symptoms is acute cystitis — the “simple,” uncomplicated UTI. Since the 2025 IDSA update, what makes a UTI complicated for treatment purposes is evidence it has spread beyond the bladder — fever, chills or rigors, hemodynamic instability, flank pain, or CVA tenderness (pyelonephritis is always complicated) — or infection in a patient with a urinary catheter, stent, or nephrostomy tube. Men, people with diabetes or immunocompromise, and patients with urologic abnormalities can still have an uncomplicated bladder infection; older teaching labelled all of them “complicated,” and many references still do.

Most bladder infections stay in the bladder. But bacteria that overcome the host defenses and multiply fast enough progress up the ureter to the kidney. Uropathogenic E. coli are built for the trip: adhesive appendages called P-fimbriae grip receptors on uroepithelial cells, letting the organism colonize and climb against the flow of urine. (A second, less common route — hematogenous seeding of the kidney — is mostly a story of debilitated or immunocompromised patients and ureteral obstruction.)

This gives the nurse a clean dividing line. A lower-tract UTI causes dysuria, frequency, urgency, and suprapubic discomfort — but not fever, chills, nausea, vomiting, or back/flank pain. The appearance of any of those means the infection is no longer confined to the bladder. Pyelonephritis announces itself with the classic triad of fever (often > 103 °F / 39.4 °C), flank pain, and nausea or vomiting — not all need be present — plus costovertebral-angle (CVA) tenderness, commonly unilateral over the affected kidney. Fever + flank pain + CVA tenderness = the kidney is involved; the outpatient problem just became an urgent one.

An indwelling catheter is a highway past every natural defense, and catheter-associated UTI (CAUTI) is the most common healthcare-associated infection — with roughly 450,000 events, ~13,000 deaths, and $340–450 million in cost annually in the US. Bacteria colonize the tube itself, and bacteriuria accumulates at about 3–10% per catheter-day, approaching 100% with long-term catheters; up to a quarter of that bacteriuria progresses to infection. By about 2 weeks, biofilm is established on the catheter — which is why a catheter that has been in ≥ 2 weeks is replaced before the culture specimen is collected.

Because colonization is near-universal, the rules invert at the bedside: a catheterized patient is treated only when symptomatic — by IDSA definition, a catheter in place ≥ 2 days plus at least one symptom or sign (dysuria, urgency, fever, flank pain, CVA tenderness) plus a culture growing ≥ 1,000 CFU/mL of a single species. Treating asymptomatic catheter bacteriuria, or running routine antibiotic prophylaxis, only breeds resistance — and the resistance backdrop is already grim: over 90% of UTI-causing bacteria are resistant to at least one antibiotic, and close to 80% to at least two.

In older adults the classic script often never plays. A UTI — and especially pyelonephritis — may present as altered mental status alone: unexplained lethargy, disorganized speech, altered perception, even what looks like “sudden dementia.” That new confusion is the bridge to urosepsis and demands evaluation. But the counterweight matters just as much: asymptomatic bacteriuria is extremely common in the elderly — at least 15% of women and men aged 65–80 and 40–50% after age 80 — so a positive urine test without symptoms is not, by itself, a UTI. In most adults asymptomatic bacteriuria is neither screened for nor treated: treatment doesn’t reduce symptomatic UTIs or improve survival, and it drives adverse effects and resistance.

Pregnancy is the exception, twice over. Physiologically, progesterone relaxes ureteral smooth muscle (decreased tone, progressive dilation) while the gravid uterus compresses the tract — reflux, rising post-void residuals, and urinary stasis impair clearance, so bacteria that would have been flushed now climb. Clinically, pregnancy is the one common setting where asymptomatic bacteriuria IS screened for and treated (USPSTF grade B): about a quarter of untreated cases progress to symptomatic UTI, and treatment clearly reduces pyelonephritis risk — the historically reported preterm-birth and low-birth-weight benefit is questioned by newer evidence. Pyelonephritis complicates 1–2% of pregnancies, usually needs hospitalization and IV antibiotics, and is a leading cause of severe maternal infection.