Urinary tract infections: diagnosis & antibiotics

Urinary tract infections: diagnosis and antibiotics

Urinary tract infection (UTI) management is not one algorithm for every patient. The correct decision depends on the site of infection, severity, anatomy, pregnancy, age, renal function, recent cultures and antimicrobial exposure. The key pharmacy skill is to distinguish uncomplicated lower UTI from infection that needs culture, a different antibiotic pathway or urgent escalation.

Quick answer: diagnose from symptoms and clinical context, not a positive dipstick alone. Screen every patient for pyelonephritis and sepsis. Follow the local antimicrobial guideline, use previous culture results, and review non-response within 48 hours.

Lower versus upper UTI

Lower UTI (cystitis)

Typical features include:

• dysuria;
• new urinary frequency or urgency;
• nocturia;
• suprapubic discomfort; and
• visible haematuria in some patients.

Vaginal discharge or irritation makes alternative diagnoses such as vaginitis, vulvovaginal candidiasis or sexually transmitted infection more likely. Dysuria can also arise from genitourinary syndrome of menopause, urethritis, stones or local irritation.

Upper UTI (acute pyelonephritis)

Think beyond cystitis when there is:

• fever, rigors or systemic illness;
• flank or loin pain or renal-angle tenderness;
• nausea or vomiting;
• marked malaise;
• hypotension, tachycardia, confusion or other sepsis features; or
• failure to improve on suitable lower-UTI treatment.

Nitrofurantoin and fosfomycin do not produce suitable renal-tissue concentrations for pyelonephritis. Do not “upgrade the duration” of a bladder-only antibiotic when the diagnosis has become an upper UTI; use the appropriate pyelonephritis pathway.

Start with the patient group

Patient group	Key approach
Non-pregnant woman with uncomplicated lower UTI	An immediate or back-up prescription may be appropriate depending on symptom severity, risk and preference. A 3-day course is usual when an antibiotic is used.
Pregnant patient	Give an immediate antibiotic, obtain a midstream urine sample before treatment and review the culture. Treat asymptomatic bacteriuria as well as symptomatic infection.
Man	Give an immediate antibiotic and send a pre-treatment culture. Consider prostatitis, obstruction and other complicated causes. A 7-day lower-UTI course is usual.
Child or young person	Obtain urine before antibiotics where possible and follow the age-specific NICE/BNFC pathway; infants under 3 months need urgent paediatric specialist assessment.
Older or frail adult	Diagnose clinically and consider other causes of delirium or deterioration. Avoid using a dipstick to label asymptomatic bacteriuria as infection.
Catheterised patient	Bacteriuria is expected with longer catheter use. Treat symptoms and systemic infection, not the dipstick or cloudy urine alone; follow the catheter-associated UTI pathway.

Urine dipsticks and cultures

Women under 65

In a woman with convincing uncomplicated symptoms, diagnosis may be clinical. Dipsticks can support decision-making when symptoms are less clear, but no single result overrides the history and red-flag assessment.

Adults over 65 and catheterised patients

UKHSA advises not using urine dipsticks to identify UTI in older adults or catheterised patients. Asymptomatic bacteriuria and pyuria are common, so leucocytes or nitrites may be positive without infection. By age 80, around half of care-home residents may have bacteria in the urine without a UTI.

New dysuria is useful, but nonspecific deterioration, falls, poor appetite or delirium requires assessment for multiple causes: dehydration, constipation, pain, medicine effects, respiratory infection, metabolic disturbance and others. A positive urine test must not close the diagnostic process.

When culture is especially important

Send urine for culture and susceptibility when recommended, particularly for:

• pregnancy;
• men;
• children and young people;
• suspected pyelonephritis;
• recurrent, complicated or catheter-associated infection;
• treatment failure;
• recent resistant organisms or repeated antibiotics; and
• diagnostic uncertainty in a higher-risk patient.

Collect the sample before antibiotics where possible, but do not delay urgent treatment in a severely unwell patient. Review the result and narrow, change or stop treatment according to susceptibility and the clinical response.

Antibiotic choices for uncomplicated lower UTI

Local resistance data and local policy take priority. The following reflects NICE NG109 for adults.

Non-pregnant women aged 16 and over

First choices are:

• nitrofurantoin 100 mg modified release twice daily for 3 days when eGFR is at least 45 mL/minute; or
• trimethoprim 200 mg twice daily for 3 days when resistance risk is low.

If the first choice is unsuitable or there is no improvement after at least 48 hours, options include nitrofurantoin if not already used, pivmecillinam or fosfomycin according to NICE and local policy.

Men aged 16 and over

NICE first choices are trimethoprim 200 mg twice daily or nitrofurantoin 100 mg modified release twice daily, both for 7 days, while culture guides therapy. Nitrofurantoin is inappropriate when prostate involvement is suspected because it does not achieve therapeutic prostatic concentrations.

Pregnancy

Take a culture before treatment and prescribe immediately. NICE lists nitrofurantoin for 7 days as first choice when renal function is suitable, but it should be avoided at term because of possible neonatal haemolysis. Alternatives include cefalexin or amoxicillin only when culture confirms susceptibility. Trimethoprim is a folate antagonist and is generally avoided in the first trimester; use the current BNF and local pregnancy guideline.

The existing article's broad statement that amoxicillin is a routine pregnancy option was unsafe: high resistance means it should be selected only with susceptible culture results.

Nitrofurantoin: the safety details

Nitrofurantoin is useful for lower UTI because it concentrates in urine, but it is not suitable for every infection.

• NICE normally uses it when eGFR is 45 mL/minute or more.
• It may be used cautiously at eGFR 30–44 only for selected uncomplicated lower infections caused by suspected or proven multidrug-resistant bacteria when benefit outweighs risk.
• It is not for pyelonephritis or suspected prostatitis.
• Avoid at term in pregnancy.

MHRA warns that pulmonary reactions can occur with short or long courses, with particular vigilance needed during the first week. Advise patients to seek help for new cough, breathlessness, chest pain or coughing blood. Stop and investigate suspected lung injury. Hepatic reactions are also possible; jaundice, dark urine, pale stools, itching or right-upper-quadrant pain need prompt review. Long-term prophylaxis requires periodic clinical and biochemical monitoring.

Trimethoprim: resistance and interactions

Use trimethoprim empirically only when resistance risk is low. Risk is higher after recent trimethoprim use, with resistant previous cultures and in some older or care-home populations.

Important cautions include:

• first-trimester pregnancy because of folate antagonism;
• renal impairment and hyperkalaemia;
• additive hyperkalaemia with ACE inhibitors, ARBs or potassium-sparing medicines; and
• enhanced antifolate or marrow toxicity with medicines such as methotrexate.

Check the BNF and patient-specific factors rather than treating “three days of trimethoprim” as universally benign.

Asymptomatic bacteriuria

Asymptomatic bacteriuria means significant bacteria in urine without UTI symptoms. It is not routinely screened for or treated in non-pregnant women, men, children, older adults or catheterised people because antibiotics do not provide benefit and can cause adverse effects, Clostridioides difficile infection and antimicrobial resistance.

Pregnancy is the important common exception: asymptomatic bacteriuria is treated because it increases the risk of pyelonephritis and premature delivery. Other specialist exceptions, such as some urological procedures, are managed under the relevant protocol.

Pyelonephritis and escalation

Obtain a pre-treatment urine sample and use the NICE/local upper-UTI antibiotic pathway. Refer to hospital for sepsis or another serious illness. NICE also advises considering referral or specialist input when the person:

• is significantly dehydrated or cannot take oral fluids or medicines;
• is pregnant; or
• has higher complication risk, such as a urinary tract abnormality, diabetes or immunosuppression.

Reassess if symptoms worsen at any time or fail to start improving within 48 hours. Obstruction plus infection is an emergency: severe colicky pain, anuria or known stones with systemic infection needs urgent assessment.

Recurrent UTI

In adults, recurrent UTI is usually defined as at least two episodes in 6 months or three in 12 months. Confirm that episodes are genuine infections, review cultures and consider triggers and underlying causes.

NICE's updated pathway includes:

1. behavioural and personal-hygiene advice;
2. vaginal oestrogen for appropriate peri- or postmenopausal patients when initial measures are ineffective or unsuitable;
3. trigger-based single-dose prophylaxis in selected non-pregnant patients;
4. methenamine hippurate as an alternative to daily antibiotics for selected non-pregnant people with a female urinary system after current infection is treated; and
5. daily antibiotic prophylaxis when earlier options are unsuitable or ineffective, with at least 6-monthly review.

Urinary alkalinising sachets reduce methenamine's effectiveness and should not be used with it. Seek specialist advice for recurrent upper UTI, complicated infection, men, pregnancy, children or an unknown underlying cause.

Self-care and safety-netting

Advise adequate fluid intake to avoid dehydration and appropriate analgesia. Evidence does not support cranberry products or urine-alkalinising agents as treatment for an active lower UTI.

Tell the patient to seek review if symptoms worsen or do not begin improving within 48 hours. Urgent assessment is needed for fever, rigors, flank pain, persistent vomiting, confusion, severe weakness, reduced urine output, hypotension, pregnancy with systemic symptoms or any concern about sepsis.

A structured OSCE approach

1. Characterise symptoms

Ask about dysuria, frequency, urgency, suprapubic pain, haematuria, vaginal or urethral symptoms and symptom duration.

2. Exclude upper or complicated infection

Check fever, rigors, loin pain, vomiting, sepsis features, pregnancy, male anatomy, catheter, stones, renal impairment, diabetes, immunosuppression and structural disease.

3. Review medicines and microbiology

Check allergies, renal function, pregnancy status, recent antibiotics, previous cultures and interactions.

4. Choose testing and treatment

Decide whether culture is needed, use the patient-group-specific local guideline and select the narrowest suitable antibiotic.

5. Close the loop

Explain duration, adherence, adverse effects, expected improvement, culture follow-up and explicit escalation signs.

Common mistakes

• Calling any positive dipstick a UTI in an older or catheterised patient.
• Treating asymptomatic bacteriuria outside a recognised indication.
• Using nitrofurantoin for pyelonephritis or suspected prostatitis.
• Ignoring renal function before nitrofurantoin.
• Giving amoxicillin empirically in pregnancy without susceptibility.
• Missing trimethoprim's pregnancy, potassium and methotrexate cautions.
• Failing to review treatment when symptoms have not started improving within 48 hours.

Active recall

1. Which symptoms move the diagnosis from lower UTI toward pyelonephritis?
2. Why are dipsticks unreliable in many older and catheterised patients?
3. When should asymptomatic bacteriuria be treated?
4. Why is nitrofurantoin unsuitable for pyelonephritis and prostatitis?
5. What changed in NICE's recurrent-UTI pathway in 2024?

Authoritative sources

• NICE NG109: Lower UTI antimicrobial prescribing
• NICE NG111: Acute pyelonephritis antimicrobial prescribing
• NICE NG112: Recurrent UTI antimicrobial prescribing
• UKHSA: UTI diagnostic tools for primary care
• MHRA: Nitrofurantoin pulmonary and hepatic reactions

Educational material for UK pharmacy learners. Follow the current BNF, local antimicrobial guideline, culture results and individual clinical circumstances.