Urinary Tract Infection (UTI)

Cystitis in men is uncommon, its prevalence increases with age, and it’s often related to urinary tract abnormalities. Consider prostatitis in men with fever ≥38°C, obstructive urinary symptoms, or prostate tenderness on gentle DRE. Women have a roughly one-in-three lifetime incidence of UTI — about 50 times the rate in men — and prevalence rises with age in both sexes. Ascending infection via the urethra is by far the most common route; haematogenous spread is rare and mainly seen with organisms like S. aureus, Salmonella or M. tuberculosis.

Acute cystitis

Clinical features: dysuria, frequency, urgency, sometimes suprapubic tenderness. Consider pyelonephritis if flank pain, fever >38°C, or vomiting. Consider an STI in higher-risk groups, or vulvovaginitis if vaginal discharge is present.

Pathogens and risk factors: E. coli causes 80–90% of infections; Staphylococcus saprophyticus (5–10%), enterococci, Proteus mirabilis and other Gram-negative enteric organisms (e.g. Klebsiella) account for most of the rest. Host defences matter more than pathogen virulence — risk rises with diabetes, immunosuppression, urinary stone disease, some connective tissue diseases, atrophic vaginitis, and bladder outlet obstruction (prostatic enlargement or stricture in men); indwelling catheters are a common cause of colonisation and infection.

Investigations: urine MC&S is helpful but not necessary for a first episode of uncomplicated UTI in non-pregnant women — everyone else needs testing. UTI is unlikely if pyuria is absent on microscopy. A midstream sample is considered clinically positive at >10³ cfu/mL for acute uncomplicated infection in women, or >10⁵ cfu/mL (women) / >10⁴ cfu/mL (men, or an in–out catheter sample) for complicated infection; pyuria (>100 WBC/ high-power field) is usually present too. Take blood cultures if there are signs of sepsis or an unusual urinary organism (e.g. S. aureus) suggesting a bloodstream source. Ultrasound is worth considering for recurrent infections (upper-tract abnormalities, stones) and in older men (bladder outlet obstruction, post-void residual); macroscopic or persistent microscopic haematuria after a UTI resolves warrants cystoscopy and a CT urogram.

Management:

• Analgesia: paracetamol/NSAIDs
• Urinary alkalinisers may help symptoms, though efficacy isn’t established — don’t combine with nitrofurantoin (reduces effectiveness) or quinolones (bladder crystal risk)
• Cranberry products are not effective treatment
• In most women under 65 with uncomplicated symptoms, UTI settles within a week without treatment — antibiotics are still recommended given pyelonephritis risk
• ~20% of community E. coli is trimethoprim-resistant, but it remains first-line. A 2015 Australian national survey of urinary isolates found E. coli resistance of 43% to ampicillin, 22% to trimethoprim, 16% to cefazolin, 9% to amoxicillin+clavulanate and 7% to ciprofloxacin — local resistance data should still guide empirical choice where available
• Don’t modify therapy based on culture results if symptoms are improving/resolved
• Nitrofurantoin only reaches therapeutic concentrations in the urinary tract (serum levels are negligible), so it treats lower UTI only — it isn’t used for pyelonephritis
• Fosfomycin 3g as a single oral dose is TGA-approved for acute uncomplicated lower UTI in women over 12, for susceptible organisms (E. coli, E. faecalis) — reserve it for when first-line agents aren’t suitable rather than using it routinely, since a randomised trial found single-dose fosfomycin less effective than a 5-day course of nitrofurantoin (58% vs 70% clinical cure)

Antibiotics (women): trimethoprim 300mg nocte for 3 days; nitrofurantoin 100mg 6-hourly for 5 days; cephalexin 500mg BD for 5 days (if the above can’t be used, or in older patients).

Antibiotics (men): same agents, but 7 days for all regimens.

Pyelonephritis

Clinical features: flank pain, nausea/vomiting, fever, costovertebral angle tenderness; may coexist with cystitis symptoms. Consider prostatitis, particularly in older men with obstructive symptoms.

Investigations: urine MC&S (unlikely if pyuria absent). Consider imaging to exclude obstruction, an infected stone, or abscess — particularly if fever persists beyond 72 hours of treatment. Men with pyelonephritis need urological evaluation. Klebsiella pneumoniae is the second most common cause of pyelonephritis after E. coli, followed by Proteus, Pseudomonas, enterococci and staphylococci. Risk factors for a resistant organism include recent broad-spectrum antibiotic use (especially quinolones and beta-lactams), obstructive uropathy, frequent healthcare contact, recurrent UTIs, recent hospital admission, an indwelling catheter, and older age. Consider Candida in patients with diabetes, extensive prior antibiotic exposure, hospitalisation (especially ICU) or an indwelling catheter. Bilateral pyelonephritis tends to present sicker, progresses faster, and carries a higher risk of acute kidney injury than unilateral disease. Obstructive pyelonephritis (e.g. an obstructing stone) is a urological emergency needing urgent drainage. Pregnant patients with pyelonephritis need hospital admission given the risk of preterm labour, with ultrasound as the initial imaging study (CT is avoided in pregnancy). Patients who were hospitalised or received parenteral antibiotics should have a follow-up urine culture 1–2 weeks after completing treatment.

Management:

IV therapy is needed if: unable to tolerate orals, fever >38°C, systemic features (tachycardia, nausea, vomiting), or sepsis/septic shock. Otherwise, oral therapy suffices.

Scenario	Regimen
Oral, first-line	Amoxicillin+clavulanate 875/125mg BD for 14 days (stop at 10 days if rapid response)
Penicillin hypersensitivity	Ciprofloxacin 500mg BD for 7 days
Narrow-spectrum if sensitivities allow	Trimethoprim 300mg nocte, cephalexin 500mg QID, or amoxicillin 500mg TDS
Pseudomonas isolated	Ciprofloxacin 500mg BD for 7 days
IV therapy	Ampicillin 2g 6-hourly + gentamicin (4–5mg/kg non-critical, 7mg/kg critically ill)

These regimens reflect Australian resistance patterns and eTG-style empirical choices (e.g. amoxicillin+clavulanate or narrow-spectrum agents over a fluoroquinolone/cephalosporin first-line) — US-facing references favour a different empirical mix (e.g. ceftriaxone) driven by their own local resistance data, so don’t substitute one for the other without checking local (Australian) guidance.

Catheter-associated UTI and bacteriuria

Consider this diagnosis with fever, rigors, altered mental status, flank pain, haematuria, or pelvic discomfort. Don’t investigate non-specific symptoms with urine MC&S in catheterised patients — asymptomatic bacteriuria, pyuria, and cloudy/malodorous urine are not reliable UTI signs in this group (false positives are common).

Bacteriuria in a catheterised patient is expected within a few days from colonisation, and is usually asymptomatic at that stage; catheterisation beyond 30 days is associated with colonisation by multiple organisms, and an indwelling catheter is also a risk factor for catheter-associated candiduria. Asymptomatic bacteriuria in a catheterised patient should not be treated — treating it mainly increases the risk of a subsequent antibiotic-resistant infection.

Sample collection: collect after placing a new catheter, or after removing the catheter and passing an MSU — never from the drainage bag (this cultures biofilm organisms, not bladder infection). Label the sample as a catheter specimen.

Treatment: replace/remove the catheter; treat as for UTI, but for 7–14 days. Recurrent catheter-associated infection may be reduced by careful catheter handling, removing unnecessary catheters promptly, and changing to a suprapubic catheter where appropriate. Silver-impregnated catheters reduce colonisation only in the first week of use, with some evidence of a reduced risk of symptomatic infection. Low-dose prophylactic antibiotics have a limited role, reserved for patients susceptible to severe infection once reversible causes (poor catheter care, calculi) have been excluded — this should generally only be started with specialist input given the risks of long-term antibiotic use.

For patients with true recurrent UTI (not catheter-related), see Recurrent UTI for the dedicated work-up and prevention approach.