Acute pyelonephritis

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Acute pyelonephritis is infection involving the kidney, commonly arising through ascent from the bladder. The clinical task is to recognise the systemic or upper tract pattern and then decide whether an antibiotic can be safely absorbed at home. Severity is not determined by the word pyelonephritis alone: a stable person drinking normally differs from someone with hypotension, vomiting, acute kidney injury or an obstructed collecting system.

Antibiotics and source assessment run together. A susceptible organism can persist behind obstruction or within an abscess, so failure to improve does not automatically mean that a broader drug is the missing intervention. Imaging is selective at presentation but urgent when the history or physiology suggests a complication. In pregnancy, renal transplantation or substantial immunosuppression, the assessment threshold and specialist pathway require additional attention.

Key points

Fever with flank pain, renal angle tenderness, rigors or vomiting suggests pyelonephritis, even when bladder symptoms are absent.
Send midstream urine for culture before antibiotics; assess observations, hydration, renal function and the possibility of obstruction at the same time.
Vomiting that prevents oral treatment, severe illness or an unsafe home situation favours hospital assessment and an intravenous regimen.
For a nonpregnant adult suitable for oral treatment, cefalexin 500 mg twice or three times daily for seven to ten days is a NICE option.
Deterioration requires immediate investigation; persistent symptoms after forty eight to seventy two hours of appropriate treatment raise concern for obstruction, abscess or resistance.
Nitrofurantoin, oral fosfomycin and pivmecillinam should not be used to treat renal parenchymal infection.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Ascending enteric infection

Bacteria commonly ascend from the lower urinary tract to the renal pelvis and parenchyma. Urinary stasis and altered tract anatomy increase the opportunity for upper tract infection.

Host and structural risks

Stones, obstruction, pregnancy and impaired host defences can increase complication risk. These factors influence investigation and disposition even when initial urinary symptoms appear similar.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Renal inflammatory response
Infection within renal tissue produces local inflammation and flank tenderness. The systemic inflammatory response can cause fever, rigors, nausea and malaise without prominent bladder symptoms.
2
Impaired drainage
Obstruction raises pressure and prevents normal clearance of infected urine. Antibiotics may not overcome the undrained source, while ongoing pressure and inflammation can worsen kidney function.
3
Systemic spread
Bacterial invasion and a dysregulated host response can impair circulation and organ function. Hypotension, altered cognition, oliguria and lactate elevation identify a different urgency from isolated lower symptoms.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Typical upper tract syndrome

Ask about fever, chills, unilateral or bilateral flank pain, nausea and vomiting, then examine for renal angle tenderness. Dysuria may coexist, but its absence does not exclude kidney infection.

Physiological instabilityRed flag

Low blood pressure, altered consciousness, tachypnoea, oliguria or rising lactate requires a sepsis assessment and urgent senior involvement. A seemingly modest temperature does not make these findings benign.

Obstruction cluesRed flag

Prior stones, severe colicky pain, a solitary functioning kidney, renal impairment or reduced urine output increases concern for an obstructed infected system. Infection plus obstruction requires urgent urological source control.

Safe outpatient conditions

Confirm that the person can retain fluids and medicines, has manageable pain, no concerning instability and access to prompt reassessment. Pregnancy, diabetes, immunosuppression or structural disease may lower the threshold for hospital advice.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Midstream urine culture and susceptibilityFirst step
Why
Establish a bacterial diagnosis and enable directed antibiotic review.
Interpretation and limitations
Collect before treatment when feasible, but do not delay urgent therapy in a deteriorating person. Review the organism and susceptibility against the clinical response; previous antibiotics may reduce culture yield without excluding infection.
02
Blood tests and selected blood cultures
Why
Assess organ effects and identify invasive infection when indicated.
Interpretation and limitations
Measure renal function, electrolytes and inflammatory markers according to severity. Add lactate and blood cultures for sepsis or severe illness; interpret trends with perfusion and urine output rather than waiting for inflammatory markers to normalise.
03
Indicated renal ultrasound
Why
Look for hydronephrosis when stones or impaired drainage are suspected.
Interpretation and limitations
Ultrasound is an initial anatomical investigation in relevant risk settings such as stone history or renal disturbance. Suspected infected obstruction with acute kidney injury requires immediate imaging and urological contact; a reassuring bladder urine result does not remove that concern.
04
Cross sectional imaging after nonresponse
Why
Find a complication that an antibiotic change alone cannot correct.
Interpretation and limitations
Discuss CT when there is immediate deterioration, suspected obstruction, or persistent symptoms despite forty eight to seventy two hours of appropriate treatment. Contrast decisions depend on the diagnostic question and renal risk. Ultrasound or MRI is preferred when imaging a pregnant patient. In a nonpregnant adult still febrile at seventy two hours, an available contrast-enhanced CT can assess renal and perinephric suppuration as well as drainage anatomy when renal and contrast assessment permits. A stone-focused unenhanced examination or a repeat ultrasound may answer a narrower question. MRI is an alternative when it is suitable and timely; selected prostate imaging instead requires a clinical reason to suspect that compartment.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Ureteric colic

A ureteric stone can produce flank pain and haematuria without infection. Fever or systemic illness alongside suspected stone disease should raise concern for coexisting infected obstruction.

Renal or perinephric abscess

A collection may cause persistent fever and focal pain despite an apparently appropriate antibiotic. Cross sectional imaging becomes important when the expected clinical improvement does not occur.

Nonurinary abdominal disease

Appendicitis, biliary disease and other abdominal disorders can mimic urinary pain or coexist with incidental bacteriuria. A discordant history or examination requires a broader diagnostic assessment.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Worked caseVomiting determines the initial routeFirst stepA forty two year old woman has fever and right flank tenderness with repeated vomiting.

1She is not pregnant, has no beta lactam allergy and has stable circulation, but cannot retain tablets. Creatinine is mildly above her documented baseline and she previously passed a ureteric stone, so urine culture and renal ultrasound are arranged during hospital assessment.
2She starts ceftriaxone 2 g intravenously once daily, infused over at least thirty minutes, with hydration tailored to examination. Ultrasound shows no hydronephrosis. A calcium-containing infusion is not given simultaneously with ceftriaxone; shared lines are appropriately flushed between sequential adult administrations.
3At forty eight hours she is afebrile, drinking and no longer vomiting. Urine grows cefalexin-susceptible Escherichia coli; blood cultures remain negative. Treatment changes to cefalexin 500 mg orally three times daily, with eight further days prescribed to complete ten days in total, including the two intravenous days.
4Renal function returns to baseline before discharge, and telephone follow-up confirms resolving flank discomfort without recurrent fever. She knows to return promptly if vomiting or systemic symptoms recur; routine post-treatment imaging is unnecessary after this uncomplicated recovery.

02Oral treatmentManage a stable community presentationThe adult can take oral medicines and has no admission indication.

1Obtain urine culture, review previous resistance and allergies, and choose an oral agent with renal tissue activity. Cefalexin is a NICE option; other agents require their own susceptibility and safety conditions.
2Agree a seven to ten day cefalexin course according to the clinical circumstances, explain hydration and provide explicit advice about deterioration or absent early improvement. Reconsider home management if reliable follow-up cannot be arranged.
3Review the culture and revise the antibiotic when resistance requires it. A fluoroquinolone should be considered only when other commonly recommended options are inappropriate and its specific risks have been assessed.

03Complication searchAct on an unexpected courseFever or pain persists despite appropriate treatment, or physiology worsens earlier.

1Reassess vital signs, perfusion, renal function, adherence and susceptibility immediately. Look for another source of fever as well as a urinary complication; the elapsed time should not replace clinical judgement.
2Arrange cross sectional imaging for obstruction, renal or perinephric abscess and other complications, with urgent urology and radiology involvement when drainage may be needed. Do not defer urgent action until a forty eight hour checkpoint.
3If obstruction is infected, combine immediate systemic antibiotics with decompression. If an abscess is found, the team selects drainage or another source intervention using anatomy and the clinical course, then documents a tailored duration and reassessment plan.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

An intravenous NICE option when vomiting or severity prevents suitable oral management, with subsequent narrowing and oral switch when appropriate.

Ceftriaxone 2 g powder for intravenous infusion

For the admitted nonpregnant adult example, give 2 g intravenously every twenty four hours over at least thirty minutes; review intravenous need at forty eight hours.

Do not administer with ceftriaxone or other cephalosporin hypersensitivity, or a history of severe hypersensitivity to another beta lactam. Never give a lidocaine-containing preparation intravenously. Do not mix with or administer simultaneously with calcium-containing intravenous fluids; adults can receive sequential treatment after line flushing. Monitor diarrhoea, rash and renal or neurological changes; combined severe liver and kidney dysfunction needs close review.

A NICE oral option for pyelonephritis, including a susceptible oral continuation after intravenous therapy has achieved clinical improvement.

Cefalexin 500 mg oral capsules

For an adult who can take oral treatment, use 500 mg twice or three times daily for seven to ten days; the worked switch completes ten total treatment days.

Marked renal impairment requires dose adjustment using the selected product and pharmacy advice; do not continue a normal schedule into evolving severe kidney injury. Known cefalexin, excipient or cephalosporin-group allergy excludes this product; assess penicillin reactions carefully, with specialist selection after a severe reaction. Monitor severe skin reactions and antibiotic-associated diarrhoea. The selected capsule product has a usual adult maximum of 4 g daily, so higher guideline ranges must not be transferred uncritically to this formulation.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Urosepsis

Upper urinary infection can cause organ dysfunction and circulatory compromise. Recognition of deteriorating physiology should trigger urgent sepsis management while clinicians continue to identify and control the source.

Acute kidney injury

Dehydration, sepsis and obstruction can each reduce renal function. More than one mechanism may coexist, making repeat assessment of perfusion, drainage and medicine dosing necessary.

Local suppuration

Renal or perinephric collections can sustain infection after initial treatment. Their anatomy and clinical impact determine whether antimicrobial treatment alone is sufficient or a source intervention is needed.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Review an intravenous prescription within forty eight hours, considering temperature, oral absorption, haemodynamic recovery, susceptibility and whether a source complication has been controlled.
Record urine output and renal function during significant illness, then reassess dose suitability as kidney function changes rather than relying on the admission estimate throughout.
Give a named route back to urgent care for rigors, worsening pain, faintness, reduced urine or inability to retain medicines; the patient should not wait for a routine appointment.
For persistent symptoms or recurrence within weeks, repeat culture and consider imaging or another diagnosis. Routine test-of-cure culture is not required after uncomplicated symptomatic recovery in a nonpregnant adult.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Total course arithmetic

When switching routes, count active intravenous treatment within the intended total course. Writing a fresh full oral course without reviewing earlier doses can unintentionally prolong antibiotic exposure.

Male systemic infection

A short course used for selected female pyelonephritis should not automatically be copied to a man with systemic UTI. EAU evidence indicates that seven days is inferior to fourteen in male systemic infection; assess possible prostate involvement and individualise treatment.

Imaging answers a question

An ultrasound is useful for collecting-system dilation, while CT can better define an abscess or complicated anatomy. The initial test and contrast choice should reflect the suspected lesion and the urgency of source control.

Urine susceptibility is insufficient

A bladder-concentrating drug can look active on a culture report yet fail to provide reliable renal tissue treatment. The site of infection remains part of prescribing after the organism is known.

11Common pitfallsFrequent interpretation and management errors.

01
Persisting with oral treatment despite repeated vomiting leaves effective drug exposure uncertain and can delay hospital assessment.
02
Waiting for a fixed forty eight hour interval despite new hypotension or oliguria misses an immediate sepsis or obstruction emergency.
03
Calling all persistent fever antibiotic resistance overlooks an infected stone, abscess, incorrect diagnosis or an uncontrolled nonurinary source.
04
Adding a full oral course after intravenous treatment without recording the intended total duration makes a safe switch harder to audit.

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