Home RTA Portal Deep Dive Assembly Line
Renal · Acid-Base

RTA Deep Dive

Three tubular acidoses, one reflex: read the potassium first, then split on urine pH and the urine anion gap. Work the diagram, flip the villains, drill the vignettes.

⚙
The Mechanism Unfolds · Watch the H+ Move

Tap a type to watch the defect happen. The broken segment lights up. The ions pile up in blood instead of draining into urine.

GLOM Filter PROXIMAL Reabsorb HCO₃⁻ OK LOOP Conc. COLLECTING Secrete H⁺ + K⁺ OK URINE pH 4.5-6 HCO₃⁻ H⁺ K⁺ All segments working normally

🧬
The Nephron · Where Each RTA Breaks

Tap a type to see which segment breaks. Follow the flow left to right.

GLOM Filter PROXIMAL Reabsorb HCO₃⁻ Make NH₃ OK LOOP Concentrate COLLECTING DUCT Secrete H⁺ & K⁺ OK URINE pH 4.5-6 All segments working normally
1
The Big Picture: What Are RTAs?

All RTAs cause the same lab pattern:

HyperchloremicToo much chloride (Cl⁻) in the blood. When bicarb drops, chloride rises to keep charges balanced. · Non-Anion GapAnion gap = Na⁺ - (Cl⁻ + HCO₃⁻). Normal is 8-12. In RTA, the gap stays normal because Cl⁻ rises as HCO₃⁻ drops. · Metabolic AcidosisBlood is too acidic (low pH) due to a kidney problem, not a lung problem.

Then use two questions to figure out which type:

1. Is the patient hyperkalemic? If YES = Type 4. Done.
2. If hypokalemic: Is urine pH > 5.5 (Type 1) or < 5.5 (Type 2)?

Two different "anion gaps"

🧬 Serum Anion Gap = Na⁺ - (Cl⁻ + HCO₃⁻) in blood. Normal is 8-12. In ALL RTAs, this stays normal because when HCO₃⁻ drops, Cl⁻ rises. That's why it's called "non-anion gap" acidosis. If the serum AG is HIGH, it's NOT an RTA.
🧬 Urine Anion Gap = (Na⁺ + K⁺ - Cl⁻) in urine. This tells you: is the kidney dumping enough NH4⁺ (acid)?

Negative UAG = kidney IS dumping NH4⁺. Type 2 (or diarrhea): the collecting duct works fine, problem is upstream.

Positive UAG = kidney is NOT dumping NH4⁺. Types 1 and 4: collecting duct is broken.
From the Attending

Here is how I teach this to every student: start with the K. Always. Hyperkalemia in a NAGMA patient is Type 4 until proven otherwise. Eighty percent of the time it is diabetes with hyporeninemic hypoaldosteronism. You don't need the urine anion gap, you don't need the urine pH. You already have your answer. The UAG and pH split Type 1 from Type 2 - that's the second branch, not the first. Do not skip the K shortcut.

2
Type 4 RTA · The Banana Backup

AldosteroneA hormone made by the adrenal glands. It tells the collecting duct to: (1) reabsorb Na⁺, (2) secrete K⁺, and (3) secrete H⁺. Without it, K⁺ and H⁺ get trapped in blood. deficiency or resistance. No aldosterone means the collecting ductThe final part of the nephron (kidney tube). Last chance to fine-tune urine: dump acid, dump potassium, concentrate urine. can't secrete K+ or H+.

🍌 HyperkalemiaToo much potassium in the blood (normal: 3.5-5.0). Dangerous because it can cause fatal heart arrhythmias. Classic ECG: peaked T waves. is THE hallmark. This is the ONLY RTA with high K+.

The hyperkalemia itself makes things worse: high K+ suppresses NH3 synthesisThe proximal tubule makes ammonia (NH3) to help carry acid out. NH3 grabs an H⁺ to become NH4⁺, which gets excreted in urine. High K⁺ blocks this process: acid has no ride out. in the proximal tubule, which means less NH4+ excretion = less acid dumped.

FeatureType 4
Serum K+HIGH
Urine pH< 5.5
Acidosis severityMild
Urine anion gapPositive
Stones?No

Common causes and WHY they cause Type 4:

🍌 DiabetesLong-standing diabetes damages the juxtaglomerular cells that produce renin. No renin = no angiotensin II = no signal to the adrenals to make aldosterone. This is called hyporeninemic hypoaldosteronism. · #1 cause. Diabetes slowly destroys the kidney cells that make renin. No renin means the whole renin-angiotensin-aldosterone chain never fires. So aldosterone never gets made.
💊 ACE inhibitors and ARBsACE inhibitors (lisinopril, enalapril) block the enzyme that converts angiotensin I to II. ARBs (losartan, valsartan) block the receptor. Either way: no angiotensin II signal = adrenals don't make aldosterone. · These drugs INTENTIONALLY block the RAAS pathway. Either way, the signal to "make aldosterone" never arrives. Less aldosterone = K⁺ builds up. That's why you check K⁺ when starting these drugs.
💊 NSAIDsIbuprofen, naproxen, etc. They block prostaglandins, which normally stimulate renin release. No prostaglandins = no renin = no aldosterone. · NSAIDs block prostaglandins. Prostaglandins normally help trigger renin release. Block prostaglandins = less renin = less aldosterone. Same domino chain, different entry point.
🧬 Adrenal insufficiencyThe adrenal glands (which sit on top of the kidneys) are damaged or destroyed. Since aldosterone is made in the adrenal cortex (zona glomerulosa), no adrenals = no aldosterone directly. · If the adrenals are destroyed (Addison's, steroid withdrawal), the factory is gone. Simple.

Treatment:

💊 FludrocortisoneA synthetic version of aldosterone. You're literally replacing what's missing: giving the collecting duct the hormone it needs to start dumping K⁺ and H⁺ into urine again. · The problem is "no aldosterone." The fix is "give them aldosterone." Fludrocortisone IS synthetic aldosterone. Collecting duct gets the signal, starts dumping K⁺ and H⁺ again.
💊 FurosemideLoop diuretic. Blocks Na⁺/K⁺/2Cl⁻ reabsorption in the loop of Henle. This floods the collecting duct with Na⁺, and Na⁺ reabsorption there is coupled to K⁺ secretion. More Na⁺ in = more K⁺ out. · It blocks reabsorption in the loop, which floods the collecting duct with sodium. More flow = more K⁺ washout.
3
Type 1 RTA · The Acid Trap

Alpha-intercalated cellsSpecial acid-pumping cells in the collecting duct. Their only job: grab H⁺ from blood and pump it into urine. In Type 1, these cells are broken. in the collecting duct can't secrete H+. Acid is trapped in the blood.

🔎 Urine pH is ALWAYS > 5.5. The collecting duct CANNOT acidify. This is the board differentiator.
FeatureType 1
Serum K+Low-normal
Urine pH> 5.5 ALWAYS
Acidosis severitySevere
Stones?YES (calcium phosphate)
Bone effectsDemineralization, osteopenia

Why stones?

Basic urine pH causes calcium phosphate precipitationWhen urine is too basic (alkaline), calcium and phosphate can't stay dissolved: they crystallize into stones. That's why only Type 1 (basic urine) gets stones.. Type 1 is the ONLY RTA with kidney stones.

Causes:

🧬 Sjogren syndromeAutoimmune disease where the immune system attacks moisture glands (dry eyes, dry mouth). The same autoimmune attack can target the kidney's intercalated cells. · Autoimmune attack targets the alpha-intercalated cells directly. Your own immune system destroys the acid pumps.
💊 Amphotericin BA powerful antifungal drug. Works by poking holes in fungal cell membranes. Problem: it also pokes holes in human kidney tubule cells. · It pokes holes in collecting duct cells. Leaky cells can't maintain the H⁺ gradient needed to pump acid out.
💊 Analgesic nephropathyKidney damage from chronic use of painkillers (NSAIDs, acetaminophen). Damages the medulla and papillae where the collecting duct lives. · Chronic painkillers damage the medulla where the collecting duct lives. Destroy the neighborhood, the acid-pumping cells can't do their job.

Treatment:

💊 Alkali therapyGiving sodium bicarbonate (NaHCO₃) or potassium citrate by mouth. You're directly replacing the base the body can't keep, and the citrate binds calcium to prevent stones. · Potassium citrate is preferred because citrate also binds calcium in urine, preventing kidney stones.
4
Type 2 RTA · The Proximal Leak

PCT cellsProximal Convoluted Tubule cells. The workhorses of the nephron: they reabsorb ~85% of filtered bicarbonate, plus glucose, amino acids, and phosphate. can't reabsorb HCO₃⁻. Base leaks into the urine early, but the collecting duct still works fine.

💡 Urine pH < 5.5 · because the collecting duct CAN still acidify. The defect is upstream.
FeatureType 2
Serum K+Low-normal
Urine pH< 5.5
Acidosis severityModerate
Stones?No
Bone effectsRickets/osteomalacia (Vit D-resistant)

Key associations:

Fanconi syndromeThe proximal tubule loses its ability to reabsorb EVERYTHING: glucose, amino acids, phosphate, uric acid, AND bicarbonate all spill into urine. Like a broken shopping bag. (proximal tubule dysfunction loses everything: glucose, amino acids, phosphate, uric acid, HCO₃⁻). Also: acetazolamideA carbonic anhydrase inhibitor (used for glaucoma, altitude sickness). Blocks the enzyme that helps the proximal tubule reabsorb HCO₃⁻: intentionally causes Type 2 RTA as a side effect. (carbonic anhydrase inhibitor), tenofovir, multiple myeloma, Wilson disease.

Treatment:

💊 Alkali therapy · Same idea as Type 1 (give bicarb), but here's the catch: the proximal tubule is the broken part, and that's WHERE bicarb gets reabsorbed. So you give bicarb and it leaks right back out into urine. You need much higher doses to overwhelm the leak. It's like filling a bucket with a hole: you just pour faster.
🎭
The RTA Villains · Flip to Expose Each Type

Each RTA type breaks a different part of the nephron. Know their signature moves.

🔐

Type 1 RTA

The Acid Trap
Distal · Collecting Duct
Tap to reveal stats

Type 1 · Distal RTA

Problem: Alpha-intercalated cells can't secrete H+
Urine pH: Always > 5.5 (can't acidify)
Serum K+: Low (potassium wasting)
Stones: Calcium phosphate (alkaline urine)
UAG: Positive
Causes: Sjogren, amphotericin B, analgesic nephropathy
Rx: Alkali therapy (K-citrate)
🐷

Type 2 RTA

The Proximal Leak
Proximal Tubule
Tap to reveal stats

Type 2 · Proximal RTA

Problem: PCT can't reabsorb HCO₃⁻
Urine pH: < 5.5 (CD still acidifies)
Serum K+: Low (bicarbonaturia drags K+)
Stones: No
UAG: Negative
Causes: Fanconi, acetazolamide, tenofovir, myeloma
Rx: Alkali (high dose)
🍌

Type 4 RTA

The Banana Backup
Collecting Duct · Aldosterone
Tap to reveal stats

Type 4 · Hyporeninemic

Problem: No aldosterone signal
Urine pH: < 5.5
Serum K+: HIGH (the only one!)
Stones: No
UAG: Positive
Causes: Diabetes (#1), ACEi/ARBs, NSAIDs, adrenal insufficiency
Rx: Fludrocortisone + furosemide
🧬

Healthy Nephron

The Reference
All systems go
Tap for what normal looks like

Normal Function

Proximal: Reabsorbs 85% of HCO₃⁻
Collecting duct: Secretes H+ and K+ freely
Urine pH: 4.5 to 6.0 (appropriately acidic)
Serum K+: 3.5 to 5.0
Serum HCO₃⁻: 22 to 26
Anion gap: 8 to 12 (normal)
UAG: Negative (dumping NH4+ normally)

Tap any card to flip it and see the full board stats.

5
Speed Run · 4 Questions
1. Hyperkalemia? = Type 4. Done. Move on.
2. Urine pH > 5.5 = Type 1 (distal, can't acidify).
Urine pH < 5.5 = Type 2 (proximal, can acidify).
3. Kidney stones? = Type 1 only.
4. Fanconi syndrome? = Type 2.
FeatureType 1Type 2Type 4
Where?Distal CDProximal PTCD + Aldo
What breaks?H+ secretionHCO₃⁻ reabsorptionAldosterone
K+LowLowHIGH
Urine pH> 5.5< 5.5< 5.5
UAGPositiveNegativePositive
Stones?YesNoNo
RxAlkaliAlkali (high dose)Fludrocortisone + furosemide
💡
Memory Hooks · Tap Each to Lock It In
🎯 If K+ is high, it's always Type 4The BANANA BACKUP. Bananas are full of K+. Type 4 = too much K+. Aldosterone is gone, K+ has nowhere to go. Banana backup = K+ piling up.. No other RTA raises potassium. Hyperkalemia + NAGMA = Type 4. Done.
🔎 Type 1 urine is permanently basicType 1 cannot acidify urine EVER. Even when blood is screaming acid. The pH stays above 5.5 no matter what. Think: the drain is clogged. H+ can't escape. Basic urine = Type 1 = stones.. The board tests this: acidosis with urine pH > 5.5 = collecting duct can't pump H+.
🧱 Type 2 has a leaky bucketThe proximal tubule is a bucket that holds bicarbonate. In Type 2, there's a hole. You pour bicarb in (therapy) and it leaks right back out. That's why Type 2 needs MUCH higher doses of alkali therapy than Type 1.. Bicarb leaks out proximally. Collecting duct is fine. That's why UAG is negative: the distal nephron is working perfectly.
⛰ The UAG splits 1 from 2Urine Anion Gap = Na+ + K+ - Cl- (in urine). POSITIVE = Types 1 and 4 (collecting duct broken, no NH4+ out, Cl- low). NEGATIVE = Type 2 (collecting duct fine, lots of NH4+ exits, Cl- high). UAG tells you: is the collecting duct working?. Both types 1 and 2 have hypokalemia. You can't split them by K+ alone. Urine pH and UAG together give the diagnosis.
📷
Clinical Context · RTA-Associated Findings

Visual anchors for conditions that cause or complicate RTAs. Tap any image for detail.

Nephrocalcinosis X-ray showing bilateral renal calcification
Nephrocalcinosis · Type 1 RTA complication
Diagram of renal bicarbonate excretion in acidosis
Renal acid-base handling · HCO₃⁻ excretion
Histology of kidney tubules showing proximal and distal segments
Kidney tubule histology · Proximal vs distal
MUDPILES mnemonic diagram for high anion gap metabolic acidosis
MUDPILES mnemonic · NOT RTA (high AG)
🏠
RTA Decision Tree · Work Through It

The board gives you labs. You follow the branches. Start at the top every time.

Step 1: Is the serum anion gap elevated?

Anion gap = Na+ minus (Cl- + HCO₃⁻). Normal is 8 to 12. All RTAs have a normal anion gap with high Cl-.

This is NOT an RTA.

High anion gap metabolic acidosis = think MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. RTAs are always non-anion gap (hyperchloremic).

Step 2: What is the serum potassium?

This is the fastest single question in all of RTA decision-making.

Diagnosis: Type 4 RTA

Type 4 is the ONLY RTA with hyperkalemia. Mechanism: aldosterone deficiency or resistance. No aldosterone = collecting duct can't secrete K+ or H+. Urine pH is < 5.5 (the collecting duct still makes acidic urine, just can't handle K+). Most common cause: diabetes with hyporeninemic hypoaldosteronism. Other causes: ACEi/ARBs, NSAIDs, adrenal insufficiency. Treatment: fludrocortisone, furosemide, low-K+ diet.

Step 3: What is the urine pH?

You have hypokalemia + NAGMA. Now check whether the collecting duct can acidify urine.

Diagnosis: Type 1 RTA (Distal)

Type 1: urine pH persistently > 5.5 even with systemic acidosis. The alpha-intercalated cells in the collecting duct are broken. They can't pump H+ out no matter how acidic the blood gets. Consequences: alkaline urine precipitates calcium phosphate = kidney stones + nephrocalcinosis. Bone is buffered = demineralization. Associations: Sjogren syndrome, amphotericin B, analgesic nephropathy. UAG is positive. Treatment: potassium citrate (replaces base AND prevents stones).

Diagnosis: Type 2 RTA (Proximal)

Type 2: urine pH < 5.5 because the collecting duct still works. The defect is upstream: proximal tubule can't reabsorb bicarbonate. Key: UAG is NEGATIVE (collecting duct is healthy, dumping NH4+ fine). Associated with Fanconi syndrome (all proximal transport fails). Causes: tenofovir, acetazolamide, multiple myeloma, Wilson disease. Treatment: high-dose alkali (the bucket has a hole, you pour faster).
Board Drill

RTA Clinical Cases

Five clinical stems. One RTA type each. Use the hyperkalemia shortcut and urine pH to nail them all. Clue highlights glow after you answer.

Five clinical stems. Underlined words are hidden clues that glow after you answer correctly. Use them to find the diagnosis.

Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.

Bone Wizardry · RTA Deep Dive