Surgery chapters
Urinary tract infection: classification, pathogenesis and management
How urinary tract infections are classified, why they happen, the organisms and diagnostic counts, and the treatment of cystitis, pyelonephritis, recurrent infection, infection in pregnancy, children and catheterised patients, plus renal abscess, emphysematous pyelonephritis and genitourinary TB.
- A UTI is uncomplicated in a healthy woman with a normal tract and complicated when there is an abnormal tract, a man, a child, pregnancy, a catheter, diabetes or upper tract involvement.
- Most infections ascend from bowel flora; E. coli causes about 70%, using fimbriae to stick to urothelium; Proteus splits urea and makes alkaline urine and struvite stones.
- Treat cystitis with nitrofurantoin or trimethoprim; treat pyelonephritis with IV antibiotics and an urgent ultrasound, draining any obstructed infected kidney by nephrostomy.
On this page
Overview
Urinary tract infection (UTI) is the presence of multiplying microorganisms in the urinary tract with an inflammatory response. It is very common in women; about half will have one in their lifetime. In men and children it is less common and often means an underlying abnormality.
Most UTIs are simple bladder infections that settle quickly. The surgical importance lies in the complicated cases: infection plus obstruction can destroy a kidney and cause Gram-negative septic shock within hours.
Definitions and classification
| Term | Meaning |
|---|---|
| Bacteriuria | Bacteria in the urine (may be symptomatic or not) |
| Asymptomatic bacteriuria | Significant growth on two samples with no symptoms |
| Cystitis (lower UTI) | Infection of the bladder: frequency, urgency, dysuria, suprapubic pain |
| Acute pyelonephritis (upper UTI) | Infection of the kidney: fever, rigors, loin pain and tenderness |
| Uncomplicated UTI | Healthy, non-pregnant woman with a normal urinary tract |
| Complicated UTI | Higher risk of serious complications or treatment failure (see below) |
| Isolated UTI | A single episode |
| Recurrent UTI | Repeated episodes: relapse or re-infection |
| Relapse | Same organism returns within 7 days of finishing treatment; suggests a persisting focus |
| Re-infection | New infection after urine has been sterile for at least 14 days; same or different organism |
Features that make a UTI complicated:
- Male sex, children, pregnancy, older age.
- Structural problems: stones, obstruction, reflux, neurogenic bladder, residual urine, diverticulum.
- Catheter or recent instrumentation.
- Diabetes, immunosuppression, kidney transplant.
- Upper tract infection or sepsis.
About 80% of recurrent UTIs are re-infections, not relapses. Relapse points to a hidden source such as a stone, scarred kidney, polycystic kidney or chronic bacterial prostatitis.
Pathogenesis
Routes of infection
- Ascending (by far the commonest): bowel organisms colonise the perineum and vulva, then climb the urethra. The short female urethra close to the vagina and anus explains why women are affected far more often.
- Haematogenous: rare; e.g. Staphylococcus aureus renal carbuncle, renal tuberculosis.
- Lymphatic or direct spread from neighbouring organs (e.g. colovesical fistula).
Bacterial factors
- Fimbriae (pili) are adhesins that attach bacteria to urothelium. P fimbriae are linked with pyelonephritis.
- Flagella (motility), haemolysin and aerobactin (iron capture) add virulence.
- Urease producers (Proteus, Pseudomonas, Staphylococcus) split urea into ammonia, making the urine alkaline and forming struvite (magnesium ammonium phosphate) stones. E. coli does not cause struvite stones.
Host defences
- Regular urine flow and complete emptying wash bacteria out.
- Urine with very high osmolality or extremes of pH inhibits bacteria.
- Tamm–Horsfall protein and the mucus layer stop bacteria binding.
- Normal vaginal and periurethral flora (lactobacilli) keep E. coli in check.
- Neutrophils, complement and mucosal IgA.
- Women who do not secrete ABH blood group antigens have 3–4 times more recurrent UTIs.
Risk factors
| Category | Examples |
|---|---|
| Stasis | Outflow obstruction (BPH, stricture), diverticulum, neurogenic bladder, reflux, pregnancy-related ureteric dilatation |
| Foreign body | Catheter, stent, stone, tumour |
| Host | Female sex, sexual activity, spermicide or diaphragm use, menopause (oestrogen deficiency), diabetes, immunosuppression, dementia |
Organisms
| Organism | Approximate share in community UTI | Note |
|---|---|---|
| E. coli and other coliforms | ~70% | Commonest overall |
| Proteus species | ~12% | Alkaline urine, struvite and staghorn stones |
| Staphylococcus saprophyticus / epidermidis | ~10% | Young sexually active women (novobiocin-resistant) |
| Enterococcus faecalis | ~6% | More in hospital |
| Pseudomonas | ~5% | Catheters, instrumentation |
| Klebsiella | ~4% | Hospital, diabetes |
Mixed and resistant organisms are typical of long-term catheters and neurogenic bladders.
Clinical features
- Cystitis: frequency by day and night, urgency, dysuria, suprapubic pain, cloudy smelly urine, sometimes haematuria. No fever or rigors.
- Pyelonephritis: fever above 38 °C, rigors, loin pain, costovertebral angle tenderness, nausea and vomiting; cystitis symptoms may be absent.
- Elderly: new confusion may be the only sign.
- Small children: fever, irritability, vomiting, failure to thrive.
Diagnosis
Clinical diagnosis
In women up to 65 with no risk factors, two of three cardinal symptoms (dysuria, urgency, frequency) with no vaginal discharge are enough to diagnose uncomplicated cystitis and treat.
Dipstick
Nitrite plus leucocyte esterase together strongly suggest infection (sensitivity about 75%, specificity about 82%).
Culture: what counts as significant?
| Patient | Significant count |
|---|---|
| Symptomatic woman | ≥10² coliforms/mL plus pyuria, or ≥10⁵ of any pathogen/mL |
| Symptomatic man | ≥10³ organisms/mL |
| Asymptomatic patient | ≥10⁵ organisms/mL on two samples |
| Suprapubic aspirate | Any growth |
Sterile pyuria
Pus cells with no growth on routine culture. Causes:
- Genitourinary tuberculosis (always exclude: three early-morning urines).
- Partly treated UTI, chlamydia, gonorrhoea.
- Stones, carcinoma in situ of the bladder, papillary necrosis, interstitial cystitis.
Imaging
- Uncomplicated cystitis: no imaging.
- Investigate: a man with a UTI, any child, recurrent UTI in women, pyelonephritis needing admission, haematuria persisting after treatment, diabetes or immunosuppression.
- Ultrasound first (obstruction, stones, residual urine). Contrast CT for complicated infection and abscess. Cystoscopy if haematuria or bladder pathology is suspected.
Management
Uncomplicated cystitis in women
- Nitrofurantoin (100 mg twice a day, 5–7 days) or trimethoprim (or co-trimoxazole), guided by local resistance.
- Encourage fluids (about 2 L a day).
- Men with a simple lower UTI need a longer course (7–14 days) and investigation.
Acute pyelonephritis
- Blood and urine cultures, U&E, CRP.
- IV antibiotics (e.g. cefuroxime, ciprofloxacin or gentamicin), then switch to oral treatment once improving.
- IV fluids for sepsis.
- Urgent ultrasound in anyone presenting with fever and loin pain, to exclude an obstructed kidney (pyonephrosis).
- An obstructed infected kidney is a urological emergency: drain it with a percutaneous nephrostomy or a ureteric stent. In a very sick patient, nephrostomy is preferred because it drains pus better with less manipulation.
Infection + obstruction = emergency drainage. Antibiotics alone will not sterilise an obstructed kidney. Definitive stone treatment waits until the infection has settled.
Nitrofurantoin reaches useful levels only in urine, not in kidney tissue or blood. Do not use it for pyelonephritis, and avoid it when eGFR is low.
Recurrent UTI
- Confirm with cultures before and after treatment and decide: relapse or re-infection?
- Relapse: look for a cause (stone, scarred kidney, prostatitis); longer treatment.
- Re-infection: good fluid intake, regular and post-coital voiding, avoid spermicides, treat constipation; vaginal oestrogen in postmenopausal women; low-dose night-time prophylaxis for 6–12 months (e.g. trimethoprim, nitrofurantoin, cefalexin); single dose after intercourse if linked to it.
UTI in pregnancy
- Asymptomatic bacteriuria affects about 2–6% of pregnant women. It must be treated because it can progress to pyelonephritis and preterm labour.
- Progesterone relaxes ureteric muscle, so the ureters dilate (more on the right), encouraging ascent.
- Pyelonephritis usually appears at about 20–28 weeks.
- Safe: amoxicillin/ampicillin, oral cephalosporins, nitrofurantoin (avoid at term).
- Avoid: tetracyclines, trimethoprim (folate antagonist, first trimester), sulphonamides and quinolones.
UTI in children and vesicoureteric reflux
- Symptomatic UTI affects roughly 3–5% of girls but only 1–2% of boys. Under 3 months it is commoner in boys; after 1 year, in girls.
- Investigate after the first confirmed UTI.
- Vesicoureteric reflux (VUR) is present in 30–40% of children who have had a UTI. Confirm with a micturating cystourethrogram; assess scars with DMSA.
- The main long-term danger is renal scarring (reflux nephropathy), causing hypertension and chronic kidney disease. Reflux nephropathy begins in childhood, not adult life.
| VUR grade | Appearance |
|---|---|
| I | Into ureter only |
| II | Into ureter and pelvis, no dilatation |
| III | Mild to moderate dilatation |
| IV | Blunting of fornices |
| V | Loss of papillary impressions |
Grades I–III usually resolve with growth. Surgery (re-implantation or endoscopic injection) is for breakthrough infections despite prophylaxis or severe reflux with a correctable anomaly.
Asymptomatic bacteriuria is treated in pregnancy and before urological procedures that breach the lining, but not in catheterised or elderly patients without symptoms.
Catheter-associated UTI
- Bacteriuria is almost universal after a few days of catheterisation (biofilm).
- Do not treat asymptomatic catheter bacteriuria.
- Treat only if symptomatic, and change the catheter; give antibiotic cover when changing a long-term catheter.
Special and severe infections
| Condition | Key points |
|---|---|
| Renal cortical abscess (carbuncle) | S. aureus, blood-borne; diabetics, IV drug users; may have no urinary symptoms; CT diagnosis; antibiotics ± percutaneous drainage |
| Corticomedullary and perinephric abscess | Ascending Gram-negative infection with obstruction or reflux; CT; percutaneous drainage |
| Pyonephrosis | Pus in an obstructed collecting system; urgent nephrostomy |
| Emphysematous pyelonephritis | Gas-forming organisms (E. coli, Klebsiella); up to 90% in diabetics; gas on X-ray or CT; antibiotics, drainage, sometimes emergency nephrectomy |
| Xanthogranulomatous pyelonephritis | Chronic destructive infection with obstruction, often a staghorn stone (Proteus, E. coli); lipid-laden foamy macrophages; enlarged non-functioning kidney mimicking tumour; nephrectomy |
| Emphysematous cystitis | Gas in the bladder wall; poorly controlled diabetes |
| Malakoplakia | Yellow bladder plaques of foamy macrophages; immunosuppressed patients with E. coli |
| Alkaline encrusting cystitis | Urea-splitting organisms deposit phosphate on the bladder lining |
In a child, a febrile UTI is not "just an infection". Missing reflux or obstruction risks renal scarring, so investigate after the first confirmed UTI.
Genitourinary tuberculosis
- About 14% of extrapulmonary TB. Always secondary, by blood-borne spread from an earlier focus.
- Starts in the kidney near the glomeruli → caseation, papillary necrosis, calyceal strictures, then calcification. The end-stage calcified non-functioning kidney is a putty kidney (autonephrectomy).
- Spreads down to the vesicoureteric junction (strictures) and the bladder around the ureteric orifice; fibrosis gives a small contracted thimble bladder with golf-hole ureteric orifices.
- Genital TB: craggy epididymis, beaded vas.
- Clues: frequency, sterile pyuria, haematuria, loin ache.
- Diagnosis: three early-morning urines for AFB (ZN stain and culture); CT urography.
- Treatment: standard short-course anti-TB drugs (they concentrate in urine); surgery for strictures, destroyed kidneys or a small bladder (augmentation). In Pakistan, follow the National TB Programme regimen; guidance can change, so check the current national guideline.
Sterile pyuria + small thimble bladder + calcified kidney = TB until proven otherwise.
Other lower tract syndromes
- Urethral syndrome (frequency–dysuria syndrome): cystitis symptoms with negative cultures; exclude CIS, TB and interstitial cystitis.
- Interstitial cystitis: middle-aged women; pain on a full bladder relieved by voiding; sterile urine; Hunner's ulcer on cystoscopy.
- Acute haemorrhagic cystitis: viral, mycoplasma, or drug-induced (cyclophosphamide).
Expect: E. coli as the commonest organism and S. saprophyticus in young women, Proteus and struvite stones, the ascending route and the short female urethra, criteria for complicated UTI, relapse versus re-infection, significant colony counts, drugs safe and unsafe in pregnancy, VUR and DMSA in children, emergency drainage of an infected obstructed kidney, emphysematous pyelonephritis in diabetics, XGP with staghorn stones, not treating catheter bacteriuria, and sterile pyuria pointing to TB.
Quick revision
- Ascending infection; short female urethra; fimbriae aid adhesion.
- E. coli ~70%; S. saprophyticus in young women; Proteus → alkaline urine → struvite.
- Complicated: men, children, pregnancy, catheters, stones, obstruction, diabetes, upper tract.
- Relapse within 7 days (same bug); re-infection after 14 days (80% of recurrences).
- Cystitis: nitrofurantoin or trimethoprim; pyelonephritis: IV antibiotics + urgent USS.
- Infected obstructed kidney → nephrostomy or stent now.
- Pregnancy: treat asymptomatic bacteriuria; avoid tetracycline, trimethoprim, quinolones.
- Children: investigate first UTI; VUR in 30–40%; DMSA for scars.
- Sterile pyuria → EMU ×3 for TB.
Written by The House Job from standard references (Bailey & Love; Kumar and Clark's Clinical Medicine).
FAQ
What is the commonest cause of urinary tract infection?
Escherichia coli from the patient's own bowel flora, which reaches the bladder by climbing the urethra. It causes about seven in ten community infections.
When does a UTI need urgent imaging?
When there is fever with loin pain, because an obstructed infected kidney must be found on ultrasound and drained quickly.
Should bacteria in a catheter urine sample be treated?
Not if the patient has no symptoms. Treat only symptomatic infection, and change the catheter at the same time.
What does sterile pyuria mean?
Pus cells without growth on standard culture. Tuberculosis must be excluded with three early-morning urine samples, then consider chlamydia, partly treated infection, stones and carcinoma in situ.
Why is pyelonephritis commoner in pregnancy?
Progesterone relaxes the ureters and the enlarging uterus slows drainage, so urine stagnates and bacteria ascend more easily, more often on the right.