Definition

A urinary tract infection is microbial infection of the urinary system, usually the bladder or kidney, causing a clinical syndrome that may range from lower UTI to pyelonephritis, prostatitis, urosepsis or infection in an abnormal urinary tract.

Epidemiology

UTI is common across all ages, with lower UTI frequent in women and infection in men, pregnancy, children, older adults and people with urinary abnormalities requiring more careful culture and complication assessment.

Pathophysiology

Bacteria usually ascend from the perineum through the urethra to the bladder and may travel to the kidneys. Incomplete emptying, reflux, obstruction, stones, catheters, pregnancy and immunosuppression increase bacterial persistence and the risk of upper-tract or systemic infection.

First principles

Localise the infection before prescribing

Dysuria, frequency and suprapubic discomfort suggest lower UTI; fever, rigors, flank pain, vomiting or systemic illness suggest upper UTI or sepsis. Nitrofurantoin is a lower-tract drug and should not be used when pyelonephritis or prostate involvement is suspected.^1,2,3

Use the right urine test for the patient

Culture is important in pregnancy, men, children, pyelonephritis, recurrent or treatment-resistant infection and suspected sepsis. In children, urine collection and dipstick interpretation are age-specific; do not extrapolate an adult dipstick rule to an infant.^4,1,2

Antibiotic choice is a local and patient-specific decision

Use the current NICE table, previous cultures, local resistance, renal function, pregnancy, allergy, recent antibiotics and the severity of illness. Review culture results and narrow or change treatment when resistance is identified.^1,2,4,3,5

Failure to improve is a diagnostic problem

Worsening or no improvement within 48 hours should trigger reassessment for pyelonephritis, sepsis, obstruction, stone, prostatitis, resistant bacteria, another diagnosis or inadequate adherence rather than an automatic repeat prescription.^1,2,6

Presentation

UTI ranges from uncomplicated cystitis to pyelonephritis, urosepsis and infection in a structurally abnormal urinary tract. Symptoms, age, pregnancy, comorbidity, catheter status, obstruction and systemic physiology determine the urgency and antibiotic pathway.^1,2,4,6

Cardinal features

Dysuria, urinary frequency, urgency, suprapubic discomfort or haematuria
Fever, rigors, flank pain, renal-angle tenderness, vomiting or systemic illness suggesting upper UTI
Non-specific fever, poor feeding, lethargy, vomiting or jaundice in an infant or young child
Pregnancy, male sex, recurrent UTI, structural/functional urinary abnormality, diabetes or immunosuppression
Recent antibiotics, resistant previous culture, urinary catheter or possible obstruction

Red flags

Sepsis, shock, altered consciousness, severe pain, rigors or rapidly worsening systemic illness
Pregnancy with suspected pyelonephritis, inability to take oral fluids or antibiotics, or significant dehydration
Child under 3 months with suspected UTI, or any child at high risk of serious illness
Suspected infected obstruction, renal abscess, stone, urinary retention or a solitary/bilateral obstructed kidney
Persistent fever or symptoms despite 48 hours of treatment, recurrent infection or a resistant organism
Male patient with fever or pelvic/perineal symptoms suggesting prostatitis rather than simple cystitis

Investigations

Clinical localisation and severity assessment

Ask about dysuria, frequency, urgency, suprapubic pain, haematuria, fever, rigors, flank pain, vomiting, vaginal or urethral symptoms, pregnancy, catheter, stones, prostate symptoms, prior cultures and antibiotic exposure.

Expected finding: Lower UTI, pyelonephritis, prostatitis, sepsis or an alternative diagnosis becomes more likely; the result determines the urine and referral pathway.

^1,2,6

Urine sample and culture

Send a midstream or clean-catch sample before antibiotics where possible in men, pregnancy, children, pyelonephritis, recurrent or complicated infection, treatment failure and suspected sepsis. Do not delay emergency treatment in a systemically unwell patient.

Expected finding: Culture and susceptibility identify the pathogen and permit narrowing or changing antibiotics; a negative culture requires review of sampling, prior antibiotics and alternative diagnoses.

^1,2,4,6

Age-specific urine testing in children

For babies and children under 3 months, seek paediatric specialist care and use the fever/sepsis pathway. For children aged 3 months to 3 years, use the NICE urine-testing strategy; from 3 years, interpret leukocyte esterase and nitrite together and send cultures when NICE criteria are met.

Expected finding: A positive or discordant test is confirmed or cultured as indicated; a negative strategy redirects the search for another cause of fever.

^4,7,2

Renal function and sepsis blood tests when indicated

Check renal function before or during treatment when illness is upper-tract, complicated, severe, recurrent or associated with dehydration, and perform blood cultures, lactate, FBC, CRP and other sepsis tests when clinically indicated.

Expected finding: Renal impairment changes antibiotic choice or dose; organ dysfunction or raised lactate increases urgency and may require admission and IV antibiotics.

^2,6,3,5

Ultrasound or further imaging for obstruction or recurrence

Do not image every uncomplicated lower UTI. Use ultrasound or specialist imaging when obstruction, stone, abscess, structural disease, recurrent upper UTI, unusual organism or failure to respond is suspected.

Expected finding: Hydronephrosis, abscess, stone or structural abnormality requires urology or renal management alongside antibiotics.

^4,2

Management

Step	Detail	Source
Assess for sepsis, pyelonephritis and obstruction first	Use ABCDE and the current sepsis pathway for a systemically unwell patient. Admit or seek urgent specialist advice for shock, organ dysfunction, severe pain, infected obstruction, inability to take oral treatment, pregnancy with significant upper-tract illness, or a child under 3 months with suspected UTI.^2,4,6,7	NICE NG111 pyelonephritis; NICE NG224 UTI in under 16s; NICE NG253 suspected sepsis
Collect urine appropriately without delaying emergency care	Send urine for culture before antibiotics in men, pregnancy, children, pyelonephritis, recurrent or complicated infection, treatment failure and suspected sepsis. In a stable uncomplicated non-pregnant woman, treatment can follow the lower-UTI pathway without waiting for culture when a sample is not otherwise indicated.^1,2,4,6	NICE NG109, NG111 and NG224
Treat uncomplicated lower UTI with the current NICE pathway	For a non-pregnant woman, consider a back-up prescription if symptoms are mild or an immediate antibiotic if symptoms are more severe, with review if symptoms do not improve within 48 hours. Nitrofurantoin is a first choice only when renal function and the clinical syndrome make it suitable; trimethoprim requires a low risk of resistance. Men generally need an immediate prescription and a longer course, and nitrofurantoin is unsuitable if prostate involvement is suspected.^1,3,5	NICE NG109 lower-UTI recommendations and antibiotic tables; BNF
Use pregnancy and paediatric antibiotic pathways	Treat suspected UTI in pregnancy with a urine sample for culture and the current NICE pregnancy table, usually for a 7-day course, taking trimester, allergy and renal function into account. Children aged 3 months and over follow the NICE paediatric table; children under 3 months need paediatric specialist assessment and parenteral treatment according to the fever/sepsis pathway.^1,2,4,7,8	NICE NG109, NG111 and NG224; BNF/BNFC
Treat acute pyelonephritis promptly and review route	Start an antibiotic after obtaining urine where feasible, using the current NICE table, previous cultures, local resistance and patient factors. Refer adults with sepsis or serious illness; consider hospital referral for pregnancy, significant dehydration, inability to take oral medicines, high complication risk or failure to improve. Review IV antibiotics by 48 hours and step down to oral treatment when possible.^2,6,8,9	NICE NG111 pyelonephritis recommendations
Review cultures and avoid ineffective or over-broad treatment	When susceptibility results are available, check the patient is improving, narrow or change treatment if resistance is present and use the narrowest effective antibiotic. Do not use amoxicillin empirically for lower UTI unless culture shows susceptibility, and do not use nitrofurantoin for pyelonephritis or suspected prostatitis.^1,2,10,3	NICE NG109 and NG111 antimicrobial prescribing tables
Support symptoms and reduce avoidable harm	Advise adequate fluids to avoid dehydration and use paracetamol for pain or fever where suitable. Explain antibiotic adverse effects, allergy and diarrhoea. Avoid routine cranberry or urine-alkalinising products as treatment claims are not supported by NICE evidence, and consider renal function and drug interactions.^1,2,11,12	NICE NG109 and NG111 self-care recommendations; NHS UTI information
Safety-net and reassess non-response	Tell the patient to seek help if symptoms worsen rapidly, they become systemically unwell, vomiting prevents oral treatment, or symptoms do not start to improve within 48 hours of antibiotics. Reassess diagnosis, culture, resistance, adherence, obstruction, stone, prostatitis and sepsis rather than automatically repeating the same antibiotic.^1,2,6	NICE NG109 and NG111 reassessment recommendations

Illustrations

Lower UTI versus pyelonephritisDiagram contrasting bladder symptoms with upper-tract fever, flank pain, vomiting, sepsis risk and antibiotic route.PassFinals · original

Age-specific urine-testing pathwayFlow diagram showing urine collection and dipstick/culture strategies for infants, young children and children aged 3 years or more.PassFinals · original

Culture-led antibiotic reviewFlow diagram linking sample collection, empiric treatment, susceptibility results, narrow-spectrum switch, treatment failure and sepsis escalation.PassFinals · original

Differentials

Uncomplicated lower UTI

Dysuria, frequency or urgency with suprapubic symptoms and no systemic illness, pregnancy, male prostate features or major urinary abnormality.

Acute pyelonephritis

Fever, rigors, flank pain, renal-angle tenderness, vomiting or systemic illness from upper-tract infection.

Acute prostatitis

Male patient with pelvic/perineal pain, fever, obstructive urinary symptoms or a tender prostate; treatment differs from simple cystitis.

Vaginitis, urethritis or STI

Vaginal discharge, irritation, dyspareunia, urethral discharge or sexual exposure without a typical cystitis syndrome.

Stone or infected obstruction

Severe colicky or unilateral pain, hydronephrosis, recurrent infection, poor response or systemic illness with obstruction.

Complications

Pyelonephritis and renal abscess
Sepsis and septic shock
Acute kidney injury
Infected urinary obstruction and renal scarring in children
Pregnancy complications from upper-tract infection
Antibiotic adverse effects, resistance and recurrent ineffective treatment

Prognosis

Most uncomplicated lower UTIs respond to appropriate treatment, but pyelonephritis can progress to sepsis, renal injury or abscess. Prompt culture-led treatment, recognition of obstruction and safety-netting reduce avoidable complications and recurrent ineffective prescribing.

Guidelines

Urinary tract infection in under 16s: diagnosis and management (NG224) (NICE, 2025)
Urinary tract infection (lower): antimicrobial prescribing (NG109) (NICE, 2018)
Pyelonephritis (acute): antimicrobial prescribing (NG111) (NICE, 2018)