Urinary Tract Infection — Causes, Symptoms & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

A urinary tract infection (UTI) is a bacterial infection of any part of the urinary system — the kidneys, ureters, bladder, or urethra. Most UTIs involve the bladder (cystitis) and urethra (lower UTI) and do not progress to involve the kidneys. Upper UTI (pyelonephritis — kidney infection) is more serious, can cause permanent renal damage, and may lead to sepsis. UTIs are among the most common bacterial infections globally — affecting approximately 150 million people per year — and are the most common infection seen in primary care. Women are disproportionately affected: the shorter female urethra (4 cm versus 20 cm in men) provides a shorter route for bacteria to ascend to the bladder. Escherichia coli (E. coli) from the gastrointestinal flora accounts for 80% of community-acquired UTIs; other organisms include Staphylococcus saprophyticus (young sexually active women), Klebsiella, Proteus, and Enterococcus. Complicated UTI is defined as UTI occurring in the presence of structural or functional abnormalities, in men, pregnant women, or immunocompromised patients.

The most common causative organisms: Escherichia coli (80% of community UTIs — uropathogenic strains with adhesin fimbriae allowing mucosal adherence), Staphylococcus saprophyticus (15% — young sexually active women), Klebsiella pneumoniae, Proteus mirabilis (urease-producing — associated with struvite kidney stones), Enterococcus faecalis, and Pseudomonas aeruginosa (healthcare-associated). Risk factors in women: sexual intercourse ('honeymoon cystitis' — mechanical introduction of periurethral bacteria into the bladder), use of diaphragms or spermicide (alters vaginal flora), postmenopausal oestrogen deficiency (loss of protective lactobacilli flora), previous UTI, urinary catheters (catheter-associated UTI — CAUTI), urinary retention, and immunosuppression (diabetes, HIV, corticosteroids). Risk factors in men: urinary obstruction (benign prostatic hyperplasia — impairs bladder emptying), urinary catheters, renal calculi, and rectal intercourse. Risk factors in all: kidney stones, structural abnormalities (vesicoureteric reflux, ureteric stricture), neurogenic bladder (spinal cord injury, multiple sclerosis), diabetes mellitus (glycosuria provides bacterial growth substrate; neuropathy causes urinary retention), and chronic kidney disease.

Lower UTI (cystitis and urethritis): dysuria (burning, stinging pain on urination — the cardinal symptom), urinary frequency (needing to urinate more often than usual), urgency (sudden strong urge to urinate), nocturia (waking at night to urinate), suprapubic discomfort or lower abdominal pressure, haematuria (blood in urine — macroscopic or microscopic), and turbid (cloudy) or malodorous urine. Absence of systemic features (fever, rigors, loin pain). Upper UTI (pyelonephritis — kidney infection): all of the above plus systemic features — fever (above 38°C), rigors (shaking chills), loin pain (flank tenderness — unilateral or bilateral), nausea and vomiting, and general malaise. Costovertebral angle tenderness on examination. Urosepsis: pyelonephritis complicated by bacteraemia — sepsis criteria (tachycardia, hypotension, tachypnoea, altered consciousness) requiring emergency hospital admission. Asymptomatic bacteriuria: significant bacterial counts in urine without symptoms — common in elderly, pregnant, and catheterised patients. Treatment is generally required only in pregnancy and before urological procedures.

Urine dipstick: a rapid in-clinic test — positive nitrites (produced by gram-negative bacteria reducing urinary nitrates) and leucocyte esterase (produced by neutrophils indicating pyuria) are 70-80% sensitive and 60-80% specific for UTI. In young women with classic symptoms and a positive nitrite, empirical antibiotic treatment without culture is appropriate in uncomplicated cases. Urine microscopy, culture, and sensitivity (MC&S): mid-stream urine (MSU) sample for culture — the gold standard. Significant bacteriuria is conventionally defined as 10^5 CFU/mL of a single organism. Culture identifies the organism and antibiotic sensitivity pattern — essential for complicated UTI, upper UTI, recurrent UTI, and antibiotic treatment failure. Significant pyuria (white cells above 10/hpf) without bacteriuria suggests sterile pyuria — consider chlamydia, tuberculosis, renal calculi, or interstitial nephritis. Blood tests for upper UTI or urosepsis: FBC (leucocytosis), CRP (elevated), blood cultures (bacteraemia — taken before antibiotics), renal function (assess acute kidney injury). Renal ultrasound or CT scan: for upper UTI not responding to antibiotics (to exclude perinephric abscess, obstructed infected kidney — a urological emergency), recurrent UTI in men, and first febrile UTI in young children (assess for structural abnormality).

Uncomplicated lower UTI in non-pregnant women: nitrofurantoin 100mg modified-release twice daily for 5 days (first-line — effective; low systemic levels reduce resistance selection — avoid with eGFR below 30), or trimethoprim 200mg twice daily for 7 days (first-line if local E. coli resistance to trimethoprim below 20%), or pivmecillinam 400mg twice daily for 5 days (alternative). Cefalexin 500mg twice daily for 7 days is an alternative. Symptom relief: ibuprofen and paracetamol for dysuria; phenazopyridine (urinary analgesic — not available in the UK) may reduce dysuria. Uncomplicated UTI in men: cefalexin 500mg twice daily or trimethoprim 200mg twice daily for 7 days — shorter courses may not be adequate as ascending prostatitis is more common. Catheter-associated UTI (CAUTI): remove or change catheter if possible; treat symptomatic CAUTI; asymptomatic bacteriuria in catheterised patients does not require treatment. Acute pyelonephritis (mild-moderate — able to take oral medication): ciprofloxacin 500mg twice daily for 7 days, or co-amoxiclav 625mg three times daily for 14 days, or cefalexin 500mg twice daily for 14 days — guided by culture. Severe pyelonephritis or urosepsis: hospital admission, IV antibiotics (co-amoxiclav 1.2g every 8 hours, or gentamicin IV, or ceftriaxone 2g daily), IV fluids, regular observations. Recurrent UTI prevention (3 or more episodes per year): post-coital single-dose prophylaxis (trimethoprim 100mg, cefalexin 125mg), continuous low-dose antibiotic prophylaxis (trimethoprim 100mg nightly for 6-12 months), or self-start therapy; topical vaginal oestrogen cream for postmenopausal women (restores protective flora); D-mannose powder (2g daily — some evidence for reducing E. coli adherence); methenamine hippurate (urinary antiseptic).

Ascending infection causes pyelonephritis (kidney infection) in 2-3% of untreated lower UTIs — particularly dangerous in pregnancy where asymptomatic bacteriuria progresses to pyelonephritis in 40% without treatment, risking preterm labour, low birth weight, and maternal sepsis. Recurrent pyelonephritis in children with vesicoureteric reflux causes renal scarring (reflux nephropathy) and chronic kidney disease. Renal or perinephric abscess from inadequately treated upper UTI requires CT-guided drainage or surgical intervention. Urosepsis (systemic sepsis from UTI) is the most common cause of gram-negative bacteraemia, carrying 20-30% mortality without prompt management. Infected hydronephrosis (pyonephrosis — pus under pressure behind an obstruction) is a urological emergency requiring emergency nephrostomy. Chronic recurrent UTI causes anxiety, reduced quality of life, antibiotic resistance, and social disruption. Antibiotic resistance is an increasing complication — E. coli trimethoprim resistance now exceeds 25-30% in UK community isolates, limiting first-line treatment options.

Hydration: drink adequate water (at least 1.5-2 litres per day) to ensure regular urination, which physically flushes bacteria from the bladder — dehydration is a risk factor for UTI. Urination after sexual intercourse: voiding within 30 minutes of intercourse reduces post-coital UTI risk by flushing bacteria introduced during intercourse. Wiping technique: wipe from front to back after bowel movements to avoid introducing gastrointestinal bacteria (E. coli) to the urethral meatus. Avoid spermicidal products: spermicides and diaphragms disrupt the protective Lactobacillus vaginal flora and significantly increase UTI risk — consider alternative contraception. Topical vaginal oestrogen: for postmenopausal women with recurrent UTI — restores Lactobacillus-dominant protective vaginal flora and reduces UTI frequency by 50% in clinical trials. Cranberry products: some evidence that proanthocyanidins in cranberry prevent E. coli from adhering to bladder mucosa — 36mg proanthocyanidins (standardised extract) daily may reduce recurrent UTIs in some women; evidence is stronger than for acute treatment. Avoid prolonged catheter use: remove urinary catheters as soon as clinically safe; catheter care bundle (hand hygiene, closed drainage system, regular bag changes) reduces CAUTI. Prompt treatment of constipation (reduces bacterial contamination of periurethral area).

See a GP for first-time UTI symptoms, symptoms not resolving with antibiotic treatment within 48-72 hours, or any UTI in a man, pregnant woman, or immunocompromised patient. Seek emergency care immediately for: features of upper UTI (loin pain, fever above 38°C, rigors) — particularly if pregnant or diabetic — as this may indicate pyelonephritis requiring IV antibiotics; signs of urosepsis (confusion, rapid heart rate, very low blood pressure, high fever); and inability to tolerate oral fluids. GP review is needed for: haematuria (blood in urine) that does not resolve after treatment — macroscopic haematuria in adults over 45 should be investigated to exclude urological malignancy; recurrent UTIs (3 or more per year) warrant investigation for structural abnormality and a preventive strategy; UTI in a child under 3 years requires urgent assessment to exclude vesicoureteric reflux. Any UTI in a man requires investigation for underlying urological cause.