Chronic tubulointerstitial nephritis and analgesic nephropathy

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

The tubulointerstitium supports solute transport, urine concentration, acid secretion and endocrine kidney function. Repeated injury recruits inflammation and fibroblasts, leading to tubular dropout, interstitial scarring and progressive loss of filtration. Unlike a primarily glomerular process, protein excretion is often low or moderate and may include low-molecular-weight proteins rather than predominantly albumin. Polyuria, nocturia, salt wasting, renal tubular acidosis or potassium disturbance can therefore precede advanced loss of eGFR.

Causes overlap. Chronic obstruction and reflux produce patchy scars; lithium commonly causes a concentrating defect and chronic tubulointerstitial change; calcific, urate or oxalate deposition can injure tubules; autoimmune and granulomatous disease can sustain inflammation; and drugs or toxins may cause acute episodes that evolve into fibrosis. Analgesic nephropathy is a specific exposure-associated phenotype. Older teaching centred on phenacetin-containing mixtures, which are no longer used in the UK, while current practice focuses on heavy cumulative analgesic exposure, high-dose or prolonged NSAID use and individual susceptibility.

The clinical task is to establish chronicity, identify reversible contributors and decide whether biopsy will change management. Small echogenic kidneys and a long stable decline may indicate irreversible scarring, whereas systemic symptoms, active urine findings, unexplained rapid deterioration or preserved kidney size may justify nephrology-led serology and biopsy. Management is cause-directed and then follows current CKD guidance, with careful attention to medicine safety and avoidance of further tubular insults.

Key points

Chronic tubulointerstitial nephritis describes tubular atrophy and interstitial fibrosis caused by sustained toxic, metabolic, immune, infective, obstructive or inherited injury.
Typical clues are slowly declining eGFR, relatively modest albuminuria, a bland or low-grade urinary sediment, concentrating impairment and tubular electrolyte or acid-base abnormalities.
Review prescribed medicines, over-the-counter analgesics, combination products, supplements, occupational exposures and previous chemotherapy; a standard medication list may miss the cause.
Analgesic nephropathy classically combines chronic interstitial damage with papillary necrosis after substantial cumulative exposure, historically especially combination analgesics; ordinary occasional paracetamol use is not the same diagnosis.
NSAIDs can also produce haemodynamic acute kidney injury, acute interstitial nephritis, hyperkalaemia, sodium retention and papillary injury, especially during hypovolaemia or alongside renin-angiotensin system blockers and diuretics.
Exclude reflux, recurrent pyelonephritis, chronic obstruction, lithium exposure, hypercalcaemia, hypokalaemia, Sjogren syndrome, sarcoidosis, monoclonal disease and environmental toxins when the history suggests them.
Stopping an offending exposure may stabilise function but established interstitial fibrosis is often irreversible, so standard CKD cardiovascular and kidney-protection measures remain essential.
Visible haematuria, papillary calcification or filling defects must not be attributed automatically to analgesics because stones, infection, obstruction and urothelial malignancy require consideration.

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

Chronic medicine or toxin exposure

Substantial cumulative analgesic exposure, lithium, selected chemotherapy and occupational or environmental toxins can produce sustained tubular injury and interstitial scarring.

Obstruction, reflux and infection

Chronic urinary obstruction, vesicoureteric reflux and recurrent upper-tract infection repeatedly injure tubules through pressure, inflammation and focal scarring.

Metabolic and immune disease

Hypercalcaemia, chronic hypokalaemia, Sjögren syndrome, sarcoidosis and monoclonal protein disorders can damage tubular cells or maintain interstitial inflammation.

Inherited tubulointerstitial disease

Inherited tubular or interstitial disease can cause slowly progressive bland kidney dysfunction and should be considered when common acquired explanations are absent.

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

1
Repeated tubular injury
Persistent toxic, pressure, metabolic or inflammatory stress damages tubular epithelium and disrupts normal transport and repair.
2
Interstitial inflammation
Injured tubules release mediators that recruit inflammatory cells and activate fibroblasts within the surrounding interstitium over time.
3
Tubular atrophy and fibrosis
Repeated injury causes tubular dropout and interstitial scarring, producing concentrating, electrolyte and acid-base defects before severe filtration loss.
4
Papillary injury
Analgesic-associated ischaemic and toxic injury can necrose renal papillae, causing calcification, haematuria, sloughed tissue and intermittent obstruction.

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

Tubular pattern

Nocturia, polyuria, impaired urinary concentration, non-anion-gap acidosis or unusual potassium and phosphate losses point beyond a purely glomerular lesion.

Analgesic exposure

Ask about dose, duration, combined preparations, migraine remedies and non-prescription purchases; cumulative exposure matters more than a single remembered brand.

Papillary complicationRed flag

Flank pain, visible haematuria, sterile pyuria or passage of tissue may indicate papillary necrosis, with sloughed material capable of obstructing the ureter.

Hidden obstructionRed flag

Hydronephrosis, recurrent infection, retention symptoms or discrepant kidney size may reveal a structural cause that needs prompt decompression rather than immunosuppression.

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
Creatinine trend and electrolytesFirst step
Why
Confirm chronicity and detect tubular or dangerous metabolic complications.
Interpretation and limitations
Compare several prior values; bicarbonate, potassium, calcium, phosphate, magnesium and urate patterns can suggest a specific segment or systemic cause.
02
Urinalysis, ACR and PCR
Why
Characterise haematuria, pyuria and the quantity and type of protein loss.
Interpretation and limitations
A relatively low ACR despite a higher PCR suggests non-albumin tubular protein; culture pyuria and investigate persistent haematuria on its own merits.
03
Renal tract ultrasound
Why
Assess size, symmetry, scarring, stones, retention and hydronephrosis.
Interpretation and limitations
Small echogenic kidneys support chronic parenchymal damage, while dilatation demands correlation with bladder volume and an obstruction pathway.
04
Non-contrast CT or urography
Why
Define suspected papillary calcification, stone, sloughed papilla or urothelial lesion.
Interpretation and limitations
Choose modality with radiology or urology according to renal function and haematuria risk; imaging does not replace cystoscopy when indicated.
05
Targeted aetiology screen
Why
Investigate immune, granulomatous, monoclonal, metabolic or toxic explanations.
Interpretation and limitations
Select ANA and ENA, calcium, immunoglobulins and free light chains, lead testing or other studies from the phenotype rather than ordering an indiscriminate panel.
06
Kidney biopsy
Why
Confirm an uncertain potentially treatable interstitial process when tissue will alter care.
Interpretation and limitations
Interstitial fibrosis and tubular atrophy estimate chronic damage; active inflammation or deposits may identify a specific therapy, but biopsy risk and scarred small kidneys limit yield.

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

Diabetic or vascular CKD

Longstanding diabetes, retinopathy, vascular disease and albumin-predominant proteinuria support common glomerular or vascular causes, while tubular losses and exposure history favour interstitial disease.

Primary glomerular disease

Substantial albuminuria, dysmorphic haematuria, red-cell casts or nephrotic features point towards glomerular pathology rather than a predominantly bland tubulointerstitial process.

Ongoing urinary obstruction

Hydronephrosis, retention, recurrent infection or an anatomical lesion suggests continuing post-renal damage that may still be reversible, rather than fixed interstitial scarring alone.

Monoclonal renal disease

Anaemia, bone symptoms, discordant protein measurements or proximal solute wasting should prompt paraprotein assessment and possible biopsy for light-chain-mediated injury.

Additional chapter-specific clues

Systemic alternative

Dry eyes and mouth, hypercalcaemia, pulmonary or skin features, paraprotein symptoms and occupational metal exposure should redirect the aetiological work-up.

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

01IdentifyBuild an exposure timelineFirst stepChronic kidney impairment has a tubular or bland-sediment pattern.

1Reconstruct prescribed and non-prescribed analgesic, lithium, antimicrobial, chemotherapy, supplement and occupational exposures with approximate dose and duration.
2Map creatinine, urine and electrolyte changes against exposure, dehydration, infection and obstruction events rather than relying on the latest result.
3Discuss suspected essential-medicine toxicity with the prescribing specialty and pharmacy before withdrawal or substitution, documenting a safe shared plan.

02ExcludeFind reversible structural diseasePain, infection, haematuria, retention symptoms or an unexplained decline accompanies the chronic picture.

1EscalationObtain urine culture, bladder assessment and renal tract imaging, escalating urgently for infected obstruction, oliguria or rapidly worsening function.
2Use haematuria and stone pathways to investigate papillary injury, calculus and urothelial disease instead of accepting an exposure label as sufficient.
3Relieve obstruction or eradicate infection with urology and microbiology input, then reassess the residual chronic trajectory.

03ProtectSlow established CKDThe cause has been removed or irreversible fibrosis remains.

1Optimise blood pressure, albuminuria treatment, diabetes care, cardiovascular risk and smoking cessation using current NICE CKD recommendations.
2Adjust medicine doses to current kidney function and avoid repeated nephrotoxic combinations, especially during hypovolaemic illness.
3Monitor progression and refer for nephrology review when decline, ACR, complications or diagnostic uncertainty meets current thresholds.

04EscalateRespond to acute deteriorationEscalationA person with chronic interstitial disease develops AKI, hyperkalaemia, sepsis or colic.

1Assess haemodynamic state, urine output, ECG where indicated, infection and obstruction while withholding plausible acute nephrotoxins under the local protocol.
2Treat time-critical potassium, sepsis or obstruction through the relevant emergency pathway and seek early renal or urological advice.
3After recovery, reconcile medicines, document the precipitant and arrange repeat creatinine and electrolytes so unresolved AKI is not mislabelled as baseline CKD.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Removes a continuing haemodynamic and tubular insult and reduces recurrence during dehydration or interacting therapy.

NSAID withdrawal or avoidance

Do not prescribe a replacement NSAID dose for suspected analgesic nephropathy; review indication and stop or minimise exposure with the responsible clinician.

Abruptly removing analgesia without an alternative can cause harm. Create a multimodal pain plan, consider opioid and paracetamol limits, and account for gastrointestinal, cardiovascular and renal risks.

May provide non-NSAID analgesia as part of an individual pain strategy; therapeutic use is not synonymous with classic analgesic nephropathy.

Paracetamol

If appropriate, use the lowest effective licensed dose within the current BNF maximum and reduce or avoid it where liver disease, low body weight or interacting factors require.

Check combination products to prevent duplicate dosing. Overdose is an emergency, and prolonged high exposure still needs indication and safety review.

Provides blood-pressure and proteinuria reduction when the person meets CKD indications, despite not reversing interstitial fibrosis.

ACE inhibitor or angiotensin-receptor blocker

Use one class for albuminuric CKD when indicated, titrating through the local formulary with creatinine and potassium checks after initiation and dose changes.

Do not combine ACE inhibitor and ARB. Review hyperkalaemia, hypotension, renal-artery disease, pregnancy and acute volume depletion using current guidance.

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

Progressive chronic kidney disease

Established tubular atrophy and interstitial fibrosis are often irreversible and can progress to kidney failure despite removal of the initiating exposure.

Concentrating failure

Impaired tubular water conservation causes polyuria and nocturia, increasing vulnerability to dehydration and acute-on-chronic kidney injury during illness.

Electrolyte and acid-base disorders

Tubular transport failure may produce potassium, bicarbonate, sodium or phosphate abnormalities that contribute to weakness, arrhythmia and bone disease.

Papillary obstruction and bleeding

Sloughed or calcified papillae can cause colic, visible haematuria, infection and post-renal obstruction; malignancy and stones still require independent exclusion.

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

Trend eGFR, creatinine, potassium and bicarbonate at a frequency based on CKD stage, recent exposure withdrawal and rate of change.
Quantify urine ACR and, where tubular protein is suspected, compare with PCR rather than relying on dipstick protein alone.
Review all analgesics and nephrotoxic medicines at every transition of care, including over-the-counter and combination products.
Ask about nocturia, volume depletion, pain, visible haematuria, passage of tissue, fever and voiding symptoms that may signal a complication.
Track blood pressure and cardiovascular risk while documenting an agreed sick-day and repeat-testing plan for intercurrent illness.

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

Protein pattern matters

Tubular proteinuria may produce a higher PCR than ACR because low-molecular-weight proteins are not captured as albumin.

Papillae can obstruct

Necrotic papillary tissue may detach, mimic a calculus and cause acute post-renal injury even in chronic bilateral disease.

Lithium needs coordination

Suspected lithium nephrotoxicity requires joint renal and mental-health planning because unplanned cessation can destabilise serious illness.

Old terminology can mislead

Classic phenacetin-associated nephropathy remains examinable, but phenacetin history should not be projected onto occasional modern paracetamol use.

Scarring limits recovery

Removing the cause is still worthwhile, although eGFR may not improve when tubular atrophy and interstitial fibrosis are established.

11Common pitfallsFrequent interpretation and management errors.

01
Calling every CKD case with analgesic use analgesic nephropathy without demonstrating a plausible cumulative exposure and phenotype.
02
Equating a bland urine sediment with harmless disease and failing to look for a concentrating or electrolyte defect.
03
Missing obstructed infected papillary necrosis because the chronic diagnosis appears to explain the creatinine.
04
Stopping lithium or another essential treatment without the prescribing specialist and a safe replacement plan.
05
Ignoring visible haematuria or weight loss once analgesic exposure has been identified.
06
Continuing NSAIDs during dehydration while renin-angiotensin blockade and diuretics compound perfusion risk.