Chronic Tubulointerstitial Nephritis and Analgesic Nephropathy
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Direct answer
Chronic tubulointerstitial nephritis is progressive scarring of the renal interstitium and tubules, with glomeruli relatively spared until late. It produces small contracted kidneys, tubular rather than nephrotic-range proteinuria and a concentrating defect. Analgesic nephropathy is its classic example, caused by long-term combination analgesics containing phenacetin, which produce papillary necrosis.
What you must remember
- Definition: interstitial inflammation and fibrosis with tubular atrophy, while the glomeruli are relatively preserved until late.
- Analgesic nephropathy: long-term combination analgesics containing phenacetin are the classic cause, with heavy NSAID use also implicated.
- Papillary necrosis: a hallmark of analgesic injury, which can cause renal colic and sloughed papillae in the urine.
- Imaging: small, contracted and often asymmetric kidneys, with papillary calcification in some cases.
- Proteinuria: tubular in type and usually well below the nephrotic range, made of low-molecular-weight proteins such as beta-2 microglobulin.
- Urine findings: sterile pyuria, a concentrating defect and sometimes renal tubular acidosis.
- Urothelial risk: analgesic and aristolochic acid nephropathy both raise the risk of urothelial carcinoma of the upper urinary tract.
- Other causes: lead, lithium, reflux and recurrent pyelonephritis, sickle cell disease and sarcoidosis.
A woman with polyuria and a long analgesic history
A 58-year-old woman who has taken combination pain relievers for many years has polyuria and a mildly raised creatinine. Her kidneys look small on ultrasound, and her urine shows white cells without a bacterial growth.
| Feature | Tubulointerstitial disease | Glomerular disease |
|---|---|---|
| Proteinuria | Tubular, usually non-nephrotic | Often nephrotic range |
| Urine sediment | Sterile pyuria, few red cells | Red cells and red cell casts |
| Kidney size | Small and contracted | Normal or enlarged |
| Concentrating ability | Impaired early | Often preserved early |
| Papillae | Necrosis or calcification | Normal |
The table separates the two patterns on the features that matter most in an exam. Tubulointerstitial disease gives small kidneys with low-level proteinuria, whereas glomerular disease gives heavy proteinuria with red cell casts. Here the drug history points to analgesic nephropathy, which is the diagnosis the examiner expects.
Urine microscopy is the quickest bedside clue, since sterile pyuria without casts is hard to explain by glomerular injury. Renal ultrasound then checks kidney size and papillary calcification, and a biopsy is reserved for cases where the picture stays unclear.
Where the examiner tests the wrong pattern
Most errors come from reading heavy proteinuria or a long drug history the wrong way.
- Trap: assuming heavy proteinuria means analgesic nephropathy Correct: tubular disease gives low-level, non-nephrotic proteinuria, so heavy proteinuria points to a glomerular cause.
- Trap: dismissing years of combination analgesic use as harmless Correct: these drugs cause papillary necrosis and chronic kidney disease, and they carry a urothelial cancer risk.
- Trap: accepting small kidneys as end-stage without an exposure history Correct: a careful history of analgesics and herbal remedies is the key, since stopping the offending drug may halt progression in early disease.
Frequently asked questions
What causes analgesic nephropathy?
Analgesic nephropathy is caused by long-term use of combination analgesics containing phenacetin, and it is also linked with heavy long-term NSAID use. The drug-induced injury produces papillary necrosis, tubular damage and chronic interstitial fibrosis.
Why does analgesic nephropathy increase the risk of urothelial cancer?
Analgesic nephropathy raises the risk of urothelial carcinoma of the renal pelvis and ureter, and the association is well recognised. The exact mechanism is incompletely understood, but chronic exposure of the upper urinary tract to the offending drug and its metabolites is thought to contribute.
What type of proteinuria does tubulointerstitial disease produce?
Tubulointerstitial disease produces tubular proteinuria, which is usually non-nephrotic and well below the range seen in glomerular disease. The proteins are low-molecular-weight species such as beta-2 microglobulin, which the damaged tubules fail to reabsorb.
Which urinary findings point to tubulointerstitial nephritis?
Sterile pyuria, with white cells but no organisms on culture, and a defective urinary concentration suggest tubulointerstitial nephritis. Red cell casts are more typical of glomerular disease, so their absence supports an interstitial cause.
Which herbal remedy causes aristolochic acid nephropathy?
Aristolochic acid nephropathy is caused by plants of the Aristolochia genus, which appear in some herbal and traditional remedies and in slimming products. It causes rapidly progressive interstitial fibrosis and carries a high risk of urothelial cancer.