Acute interstitial nephritis is the AKI that hides. It rarely announces itself with the textbook rash and fever, its urine findings are suggestive rather than diagnostic, and the test most clinicians reach for — urinary eosinophils — is close to useless. Yet it is common, frequently caused by a drug on the chart, and often reversible if the drug is stopped early and inflammation is treated before it scars. The skill is to suspect it without the triad, confirm it when it matters, and act while recovery is still possible.
What AIN is, and what now causes it
AIN is an immune-mediated inflammation of the renal interstitium and the tubules running through it, producing intrinsic AKI. Its epidemiology has shifted: where infection once dominated, drugs now account for the large majority of cases. That shift matters clinically, because it means the cause is usually something you can stop. The interstitial oedema and inflammatory infiltrate impair tubular function, which is why AIN presents not only with a falling GFR but with tubular signatures — sterile pyuria, white-cell casts, and sometimes frank tubular defects such as a Fanconi syndrome or a concentrating defect.
A hypersensitivity, not a poisoning
The mechanism is a cell-mediated, type IV delayed hypersensitivity reaction. T cells infiltrate the interstitium, accompanied by eosinophils, plasma cells, and macrophages, and in some cases granulomas form. Three consequences follow directly from this immunology. First, the reaction is idiosyncratic, not dose-dependent — it is not about how much drug, but about an immune response to it, so a standard dose can trigger it. Second, it can occur after a drug has been taken for a long time, which is why proton-pump-inhibitor disease so often surfaces months in, with no obvious temporal clue. Third, it can recur, faster and harder, on re-exposure to the same or a related agent — a fact that should be flagged prominently in the record. This is a sensitised immune system reacting, not a tubule being directly poisoned.
The culprits, and their quirks
Three drug groups lead. Antibiotics — penicillins and cephalosporins, ciprofloxacin, rifampicin, sulfonamides, and vancomycin — are the historical archetype. NSAIDs cause AIN with two quirks: the onset can be delayed by months, and, uniquely, the disease can carry nephrotic-range proteinuria because a minimal-change glomerular lesion accompanies the interstitial one. Proton-pump inhibitors have become a leading cause precisely because they are so widely used and their AIN is so insidious, often presenting as unexplained CKD discovered late rather than an obvious acute event. Beyond these, 5-aminosalicylates, allopurinol, diuretics, anticonvulsants, and — increasingly — the immune checkpoint inhibitors round out the list. Infections and systemic diseases such as sarcoidosis, Sjögren syndrome, and IgG4-related disease cause the remainder, with the tubulointerstitial-nephritis-and-uveitis syndrome a memorable named entity.
Why the classic triad fails you
Every student learns the triad of rash, fever, and eosinophilia, and every experienced clinician knows it is usually absent. The full triad appears in only a minority of patients, and any one component can be missing; relying on it to diagnose AIN means missing most cases. The lesson is to invert the reasoning: do not wait for the triad, but suspect AIN whenever AKI appears in a patient on a known culprit, especially with sterile pyuria or white-cell casts and no better explanation. The triad, when present, is a useful clue; when absent, it tells you nothing.
The eosinophiluria trap
The natural next move — sending the urine for eosinophils — is a trap. Urinary eosinophils are neither sensitive nor specific for AIN: they are frequently absent in true AIN and frequently present in ATN, glomerulonephritis, urinary infection, and atheroembolic disease. A positive result does not confirm AIN and a negative result does not exclude it, so the test changes nothing and should not drive decisions. It is one of those investigations whose chief effect is to create false confidence in either direction, and the modern position is to stop ordering it for this purpose.
When only a biopsy will do
Because the clinical and urinary features are suggestive rather than diagnostic, the kidney biopsy is the gold standard — showing an interstitial inflammatory infiltrate rich in eosinophils, tubulitis, and oedema, and crucially quantifying the degree of fibrosis, which predicts recovery. Biopsy is not needed in every case: when a clear culprit drug is stopped and the patient recovers promptly, the diagnosis is made retrospectively. But biopsy earns its place when the diagnosis is uncertain, when an alternative such as glomerulonephritis or vasculitis is plausible, when the AKI is significant or progressive, and — importantly — when you are about to commit a patient to corticosteroids, since that decision deserves a tissue diagnosis where feasible.
Treatment: stop the drug, and the timing of steroids
The first and most important treatment is to stop the offending drug. Many patients recover on withdrawal alone, and no other intervention substitutes for removing the antigen. The second question is corticosteroids, and here the evidence is honest about its limits: it is observational, not from large randomised trials, but it consistently suggests that early steroids — begun within roughly one to two weeks of the injury — improve renal recovery and reduce the progression to fibrosis, while steroids started late, after fibrosis is established, do not help. So steroids are offered for biopsy-proven or strongly suspected AIN that is significant or failing to improve after drug withdrawal, typically as oral prednisolone around 0.5 to 1 mg/kg/day for several weeks with a taper, sometimes preceded by intravenous methylprednisolone. The actionable principle is that timing, not just the decision to treat, determines benefit — a reason not to dawdle between suspicion and action.
Checkpoint inhibitors, recurrence, and the road to CKD
Immune checkpoint inhibitors deserve special mention because they are now a frequent and distinctive cause. By releasing the brakes on T cells, they can break renal tolerance and produce an AIN that is often delayed by weeks to months and may accompany other immune-related adverse events. Management is to hold the drug, give corticosteroids, and decide on rechallenge jointly with oncology, weighing the cancer against the kidney. Across all causes, two themes close the chapter: re-exposure to a culprit drug can provoke a faster, more severe recurrence, so the offending agent and its relatives must be recorded and avoided; and delayed diagnosis or treatment lets interstitial inflammation harden into fibrosis, turning a reversible AKI into permanent CKD. The whole strategy — suspect early, confirm when it matters, stop the drug, treat the inflammation before it scars — is built around getting ahead of that fibrosis.
