尿路感染症（UTI）は世界中で年間1億5000万人以上に影響を及ぼし、臨床現場において最も一般的な細菌感染症の一つです。それ以外に健康な女性における非複雑性下部尿路感染症（膀胱炎）から、重篤な上部尿路感染症（腎盂腎炎）および入院を要するカテーテル関連UTIまで多岐にわたります。本ガイドは、原因、診断、抗菌薬治療プロトコル、予防を含むUTIの臨床スペクトラムを、EAUおよびIDSAガイドラインに沿ったエビデンスに基づく臨床的知見とともに解説します。
What is a Urinary Tract Infection?
A urinary tract infection (UTI) is an infection that affects any part of the urinary system — including the kidneys, ureters, bladder, and urethra. Most infections involve the lower urinary tract, primarily the bladder and the urethra.
UTIs are among the most common bacterial infections in the world. While they can affect anyone, women are significantly more susceptible due to their shorter urethra, which allows bacteria to reach the bladder more easily.
When to see a doctor
If you experience burning urination, pelvic pain, cloudy urine, or a persistent urge to urinate, consult a healthcare professional. Untreated UTIs can spread to the kidneys.
Urinary tract infections represent a spectrum of conditions ranging from uncomplicated cystitis in healthy premenopausal women to complex urosepsis in catheterized, immunocompromised, or structurally abnormal patients. Classification is critical for guiding empirical therapy.
| Classification | Definition | Key Considerations |
|---|---|---|
| Uncomplicated UTI | Lower UTI in non-pregnant women without structural/functional abnormalities | Short-course antibiotics; no imaging required |
| Complicated UTI | Any UTI associated with functional/structural urinary tract abnormality, or in special populations (men, pregnant women, immunocompromised, catheterized patients) | Urine culture mandatory; longer treatment course |
| Recurrent UTI | ≥3 episodes per year or ≥2 episodes in 6 months | Investigate underlying cause; consider prophylaxis |
| Catheter-associated UTI (CAUTI) | UTI in patients with urinary catheter in place for >2 days | Remove catheter if possible; distinct antibiogram |
| Asymptomatic Bacteriuria | ≥10⁵ CFU/mL on 2 consecutive specimens without symptoms | Treatment only in pregnancy and pre-urological procedures |
Anatomy of the Urinary Tract
Kidneys
Filter blood, produce urine. Kidney infections (pyelonephritis) are serious and require prompt treatment.
Ureters
Tubes carrying urine from kidneys to bladder. Can be affected by stones or strictures.
Bladder
Stores urine. Cystitis (bladder infection) is the most common UTI form.
Urethra
Carries urine outside the body. Urethritis often presents with discharge and burning.
Clinical Note — Ascending Infection Pathway
The most common route of infection is ascending, with periurethral colonization by uropathogens (predominantly E. coli expressing type 1 and P fimbriae) followed by bladder invasion. Vesico-ureteral reflux (VUR), bladder outlet obstruction, ureteral stenting, and nephrostomy access all create conditions that promote retrograde ascent into the upper urinary tract. The female urethra (3–4 cm) versus the male (20 cm) accounts for the marked sex disparity in uncomplicated UTI incidence.
Causes & Risk Factors
UTIs are most commonly caused by bacteria from the gut, particularly Escherichia coli (E. coli), which accounts for around 80–85% of cases. These bacteria can enter the urinary tract and multiply rapidly.
| Pathogen | Frequency | Key Notes |
|---|---|---|
| Escherichia coli | 80–85% | Virulence factors: fimbriae, hemolysin, cytotoxic necrotizing factor |
| Staphylococcus saprophyticus | 5–15% (young women) | Novobiocin-resistant; seasonal peaks in summer/autumn |
| Klebsiella pneumoniae | 5–8% | Emerging ESBL producers; nosocomial setting |
| Proteus mirabilis | 2–5% | Urease-producer; associated with struvite stones |
| Pseudomonas aeruginosa | <3% | Predominantly nosocomial/catheter-associated; high resistance rates |
| Enterococcus faecalis | <5% | Post-urological procedure; consider VRE in hospital settings |
Risk Factors
Signs & Symptoms
Lower UTI (Cystitis / Urethritis)
- Burning or painful urination (dysuria)
- Frequent, urgent need to urinate
- Passing only small amounts of urine
- Cloudy, dark, or strong-smelling urine
- Blood in urine (haematuria)
- Pelvic pressure or discomfort
Upper UTI (Pyelonephritis) ⚠️
- Fever and chills
- Flank, back, or side pain
- Nausea and vomiting
- High temperature (>38°C / 100.4°F)
- Costovertebral angle tenderness
- Requires immediate medical care
Clinical Pearl — Atypical Presentations
Elderly patients (especially those with dementia or delirium) and immunocompromised individuals frequently lack classic dysuria/frequency, presenting instead with unexplained delirium, falls, or haemodynamic instability. Catheterized patients may be asymptomatic or present only with suprapubic discomfort. Overdiagnosis of UTI in the elderly (attributing delirium to UTI without meeting formal NHSN/IDSA criteria) is a recognised quality metric and contributes to unnecessary antibiotic exposure.
Diagnosis
Your doctor will likely ask about your symptoms and request a urine sample. A dipstick test can detect signs of infection quickly, while a urine culture identifies the specific bacteria involved and guides antibiotic selection.
Symptom Assessment
Medical history & physical exam
Urinalysis
Dipstick + microscopy
Urine Culture
Identifies organism & sensitivity
Urinalysis Interpretation
- Leukocyte esterase: Sensitivity 75–96%; specificity 94–98%. Positive suggests pyuria (>5 WBC/hpf)
- Nitrites: Specificity ~95%; sensitivity ~50%. Only gram-negative Enterobacteriaceae reduce nitrates
- Combined LE + nitrite: Best discriminator in uncomplicated cystitis; negative dipstick has high NPV to exclude UTI
- Microscopy: Pyuria threshold: ≥10 WBC/mm³ on uncentrifuged specimen; bacteriuria: ≥10²–10⁵ CFU/mL depending on context
Imaging Indications
- Ultrasound: First-line for suspected obstruction, abscess, or paediatric UTI
- CT urogram: Diagnostic standard for pyelonephritis complications (abscess, emphysematous pyelonephritis), nephrolithiasis
- VCUG/MAG3: Evaluate vesico-ureteral reflux in paediatric recurrent UTI
- Cystoscopy: Recurrent haematuria, suspected bladder lesion, or structural abnormality
UTI治療：抗菌薬、水分補給、再発性UTI管理
Most UTIs are effectively treated with a short course of oral antibiotics. It's important to complete the full course even if symptoms improve early. Your doctor will choose the antibiotic based on the bacteria identified and local resistance patterns.
Important: Never self-medicate
Using the wrong antibiotic or stopping treatment early contributes to antibiotic resistance and may not cure the infection. Always follow your doctor's prescription.
| UTI Type | First-Line (EAU/IDSA) | Duration | Notes |
|---|---|---|---|
| Uncomplicated Cystitis | Nitrofurantoin, Fosfomycin, Pivmecillinam | 3–5 days | Avoid fluoroquinolones as first-line; preserve for complicated UTI |
| Uncomplicated Pyelonephritis (mild) | Ciprofloxacin (if local resistance <10%) or TMP-SMX guided by cultures | 7–14 days | Culture before starting; adjust to sensitivity results |
| Complicated UTI / Hospitalised Pyelonephritis | IV ceftriaxone or piperacillin-tazobactam (initial empirical) | 10–14 days (step-down oral when appropriate) | Blood cultures; urology consult if obstruction suspected |
| CAUTI | Per local antibiogram; remove/replace catheter | 7 days (14 days if slow response) | Do not treat asymptomatic bacteriuria in catheterized patients |
| Recurrent UTI (prophylaxis) | Nitrofurantoin 50mg nocte or post-coital, D-mannose, intravaginal oestrogens (post-menopausal) | 3–6 months | Exclude anatomical cause first; review after 6 months |
💧 Hydration
Drinking plenty of water helps flush bacteria from the urinary tract. Aim for 1.5–2 litres daily, more during hot weather or fever.
🌿 Symptom Relief
Paracetamol or ibuprofen may help with discomfort. Some patients find urinary alkalinising agents (e.g. potassium citrate) soothing, though evidence is limited.
Prevention
Stay well hydrated
Adequate fluid intake dilutes urine and encourages frequent voiding, reducing bacterial colonisation time in the bladder.
Wipe front to back
This prevents transfer of gut bacteria from the anal area to the urethra — a simple but highly effective hygiene practice.
Urinate after sexual intercourse
Voiding after sex helps flush bacteria that may have been introduced into the urethra during sexual activity.
Avoid irritating products
Perfumed soaps, douches, and deodorant sprays near the genital area can disrupt natural flora and increase infection risk.
Cranberry products & D-Mannose
Some evidence supports the use of cranberry extract (PACs) and D-Mannose supplements in reducing recurrent UTIs by inhibiting bacterial adhesion to the urothelium.
Manage underlying conditions
Good glycaemic control in diabetes, prompt treatment of urinary obstruction, and minimising unnecessary catheterisation all reduce UTI risk significantly.
Clinical Insights & Current Guidelines
EAU 2024 Urological Infections Guidelines — Key Updates
- • Fluoroquinolones are no longer recommended as empirical first-line therapy for uncomplicated cystitis due to ecological collateral damage and resistance pressure.
- • Fosfomycin trometamol (3g single dose) remains a preferred option for uncomplicated cystitis with favourable resistance profile.
- • Post-procedural antibiotic prophylaxis (e.g. ureterorenoscopy, PCNL) is recommended only when pre-operative urine culture is positive or contamination risk is high.
- • Catheter-associated bacteriuria: treat only if symptomatic, pregnant, or prior to urological instrumentation.
Antimicrobial Stewardship Considerations
- • Reserve carbapenems for documented ESBL/carbapenem-susceptible organisms; meropenem for KPC/NDM-producing Enterobacteriaceae only if no alternative.
- • Urine cultures from catheterised patients frequently represent colonisation — clinical context is paramount.
- • Repeated quinolone exposure promotes fluoroquinolone-resistant E. coli and MRSA — consider resistance monitoring in high-risk populations.
Urological Device Considerations
Ureteral access sheaths (UAS), nephrostomy drainage catheters, and JJ stents all represent foreign body surfaces that predispose to biofilm formation. Continuous suction sheaths (e.g. Manawa FANS series) reduce intra-renal pressure build-up during ureteroscopy, potentially lowering pyelovenous backflow and infectious complications. Appropriate antibiotic prophylaxis before endourological procedures and timely removal of drainage devices remain the cornerstones of post-procedural UTI prevention.
よくある質問：尿路感染症
UTIの主な症状は？
下部UTI（膀胱炎）の最も一般的な症状には、排尿時の灼熱感または疼痛（排尿痛）、頻尿・尿意切迫感、混濁尿または悪臭尿、骨盤部の圧迫感が含まれます。上部UTI（腎盂腎炎）では追加症状として38℃を超える発熱、悪寒、側腹部痛、悪心・嘔吐が現れます。腎盂腎炎は直ちに医療的評価が必要です。
尿路感染症はどのように治療されるか？
非複雑性UTIのほとんどは3〜5日間の経口抗菌薬療法で治療されます。現行のEAUガイドラインでは、ニトロフラントイン、ホスホマイシン、またはピボメシリナムを第一選択薬として推奨しています。耐性の懸念からフルオロキノロン系薬は第一選択として使用を避けるべきです。複雑性または上部UTIは7〜14日間の治療が必要な場合があり、重篤な腎盂腎炎では静脈内抗菌薬投与と入院が必要になる場合があります。
再発性UTIの原因は何か？
再発性UTI（年間3回以上）は解剖学的要因から女性に最も多く見られます。主な原因には、性行為、閉経、不完全な膀胱排出、腎結石、尿管狭窄、長期的な尿道カテーテル使用が含まれます。治療は基礎原因への対処、低用量予防的抗菌薬、Dマンノース補充、または閉経後女性への腟内エストロゲン投与を含みます。
膀胱炎と腎盂腎炎の違いは何か？
膀胱炎は発熱を伴わない排尿痛、尿意切迫感、頻尿を引き起こす膀胱（下部尿路）の感染症です。腎盂腎炎は腎臓の感染症（上部尿路）であり、より重篤で、発熱、側腹部痛、悪心、全身症状を呈します。腎盂腎炎は速やかな抗菌薬治療が必要であり、特に妊婦、高齢者、免疫不全患者では入院が必要な場合があります。
UTIの予防方法は？
主なUTI予防戦略には、十分な水分摂取（1日1.5〜2リットルの水）、性行為後の排尿、トイレ後の前から後へのふき取り、外陰部への芳香製品の使用回避、糖尿病や尿路閉塞などの基礎疾患の管理が含まれます。クランベリーエキス（PACs）とDマンノースサプリメントは、尿路上皮への細菌付着を阻害することによる再発低減のエビデンスがあります。