Explain simply
Acute Interstitial Nephritis (AIN) — Simple Explanation
What is it?
Inflammation of the kidney's interstitium (the tissue between the tubules). Also called tubulointerstitial nephritis.
What causes it?
Mostly drug hypersensitivity — your immune system treats the drug as a foreign invader (hapten), triggering an allergic reaction in the kidney.
Use the mnemonic DRAINS to remember the main drug causes:
| Letter | Drug |
|---|---|
| D | Diuretics |
| R | Rifampin |
| A | Antibiotics (penicillins, cephalosporins) |
| I | proton pump Inhibitors (PPIs) |
| N | NSAIDs |
| S | Sulfa drugs |
Less common causes: infections (e.g., Mycoplasma) or autoimmune diseases (Sjögren's, SLE, sarcoidosis).
What are the symptoms?
Classic triad:
- 🔴 Fever
- 🔴 Rash
- 🔴 Eosinophils in urine (pyuria) + blood in urine (hematuria)
- Flank/back pain (costovertebral angle tenderness)
- Elevated creatinine (azotemia = kidney not filtering well)
⚠️ Can also be completely asymptomatic — found only on lab work.
One-line summary: AIN = kidney inflammation from a drug allergy (usually), causing fever, rash, and abnormal urine — remember DRAINS for the culprit drugs.
Do the same
Acute Kidney Injury (AKI) — Simple Explanation
What is it?
A sudden drop in kidney function. There are 3 types based on where the problem is.
Dialysis Indications — mnemonic AEIOU
When AKI is severe, you need dialysis for:
| Letter | Meaning |
|---|---|
| A | Acidosis |
| E | Electrolyte abnormalities (e.g., high potassium) |
| I | Intoxication (e.g., lithium overdose) |
| O | Overload (fluid overload) |
| U | Uremia |
The 3 Types of AKI
1️⃣ Prerenal — "Not enough blood reaching the kidney"
- Cause: Low blood volume, low cardiac output, or low effective circulation (e.g., heart failure, liver failure)
- Mechanism: ↓ blood flow → ↓ GFR → kidney tries to save fluid by reabsorbing Na⁺ and water
- Labs: Urine is concentrated (kidney working hard to retain fluid)
- Urine osmolality >500, Urine Na⁺ <20, FENa <1%, BUN/Cr >20
2️⃣ Intrinsic Renal — "The kidney itself is damaged"
- Causes:
- Tubules: ATN (ischemia or toxins), AIN (drug allergy)
- Glomerulus: Acute glomerulonephritis
- Vessels: Vasculitis, hypertensive emergency, TTP-HUS
- Mechanism (ATN): Patchy necrosis → debris clogs tubules → fluid backs up → ↓ GFR
- Labs: Urine is dilute (kidney can't concentrate anymore)
- Urine osmolality <350, Urine Na⁺ >40, FENa >2%, BUN/Cr <15
3️⃣ Postrenal — "Obstruction below the kidney"
- Causes: Kidney stones, BPH, tumor, congenital anomalies
- Mechanism: Outflow blocked bilaterally → pressure backs up → ↓ GFR
- Labs: Variable (depends on duration)
Quick Comparison Table
| Lab | Prerenal | Intrinsic | Postrenal |
|---|---|---|---|
| Urine Osmolality | >500 | <350 | Varies |
| Urine Na⁺ | <20 | >40 | Varies |
| FENa | <1% | >2% | Varies |
| BUN/Cr | >20 | <15 | Varies |
💡 Key trick: In prerenal, the kidney is healthy but underperfused — so it tries hard to retain Na⁺ and water (concentrated urine). In intrinsic, the kidney is broken — it can't concentrate anything (dilute urine).
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