Acid–base questions love to look scary, but they’re usually a repeatable pattern-recognition game: identify the primary disorder, check compensation, then decide if there’s a mixed problem—and only then hunt the cause. Here’s a step-by-step flowchart you can run in under a minute on USMLE.
The 60-second acid–base flowchart (renal-focused)
Step 1) Look at the pH
- pH < 7.35 → acidemia
- pH > 7.45 → alkalemia
One-liner: The pH tells you the direction; the next steps tell you the driver.
Step 2) Identify the primary process (HCO₃⁻ vs PaCO₂)
Use the rule:
- HCO₃⁻ moves pH in the same direction
- ↓HCO₃⁻ → metabolic acidosis
- ↑HCO₃⁻ → metabolic alkalosis
- PaCO₂ moves pH in the opposite direction
- ↑PaCO₂ → respiratory acidosis
- ↓PaCO₂ → respiratory alkalosis
Quick mnemonic:
“ROME”
- Respiratory Opposite (PaCO₂ opposite pH)
- Metabolic Equal (HCO₃⁻ equals pH direction)
Step 3) Check if compensation is appropriate (to catch mixed disorders)
If primary metabolic acidosis → use Winter’s formula
- Measured PaCO₂ higher than expected → additional respiratory acidosis
- Measured PaCO₂ lower than expected → additional respiratory alkalosis
If primary metabolic alkalosis → expected respiratory compensation
Rule of thumb:
- PaCO₂ rises about 0.7 mmHg per 1 mEq/L rise in HCO₃⁻
(Or memorize: “met alkalosis → hypoventilate, but not perfectly”)
If primary respiratory disorder → kidneys compensate (this is the renal gold)
Use these high-yield deltas:
| Primary disorder | Acute compensation (HCO₃⁻ change) | Chronic compensation (HCO₃⁻ change) |
|---|---|---|
| Respiratory acidosis (↑PaCO₂) | +1 mEq/L per +10 PaCO₂ | +3–4 mEq/L per +10 PaCO₂ |
| Respiratory alkalosis (↓PaCO₂) | −2 mEq/L per −10 PaCO₂ | −4–5 mEq/L per −10 PaCO₂ |
One-liner: Acute = small renal change, chronic = big renal change (because kidneys need time to reprogram transporters and ammoniagenesis).
Step 4) If metabolic acidosis → calculate the anion gap (AG)
- “Normal” AG is roughly 8–12 (depends on lab)
- Correct for albumin (super high-yield):
One-liner: Low albumin hides an anion gap acidosis unless you correct it.
Step 5) If AG metabolic acidosis → do the delta gap (mixed metabolic disorders)
- If → concurrent metabolic alkalosis
- If → concurrent non–anion gap metabolic acidosis
One-liner: The delta gap tells you if another metabolic process is “pulling” HCO₃⁻ beyond what the AG explains.
Visual: the “Acid–Base Ladder” (shareable mental image)
Picture a ladder with pH at the top, and two rungs underneath: PaCO₂ (lungs) and HCO₃⁻ (kidneys).
- Top rung: pH says acidemia vs alkalemia
- Middle rung: PaCO₂ points to respiratory driver
- Bottom rung: HCO₃⁻ points to metabolic/renal driver
- Side rails: compensation rules keep you from falling for a mixed disorder
One-liner: “pH chooses the lane, PaCO₂/HCO₃⁻ chooses the driver, compensation checks if someone else is in the car.”
Renal physiology that actually shows up in acid–base questions
How kidneys fix acid: “Reclaim bicarb + create new bicarb”
You have two jobs:
- Reabsorb filtered HCO₃⁻ (mostly proximal tubule)
- Generate new HCO₃⁻ by excreting acid (distal nephron)
Proximal tubule (bulk work)
- Carbonic anhydrase enables:
Filtered HCO₃⁻ → CO₂ (diffuses in) → re-forms HCO₃⁻ inside cell → returned to blood - NHE3 (Na⁺/H⁺ exchanger) secretes H⁺ into lumen to allow HCO₃⁻ reclamation
High-yield tie-in: Acetazolamide (CA inhibitor) → proximal RTA (type 2)-like effect early: ↑HCO₃⁻ loss → metabolic acidosis, alkaline urine initially.
Alpha-intercalated cells (distal acid secretion)
- Secrete H⁺ via H⁺-ATPase and H⁺/K⁺-ATPase
- Generate new HCO₃⁻ returned to blood
- Trap H⁺ as:
- H₂PO₄⁻ (titratable acid)
- NH₄⁺ (ammonium) from proximal glutamine metabolism
High-yield tie-in: Chronic acidosis → ↑ammoniagenesis → more NH₄⁺ excretion (major adaptive mechanism).
Beta-intercalated cells (base secretion)
- Secrete HCO₃⁻ (pendrin, Cl⁻/HCO₃⁻ exchanger)
- Used in alkalosis states
The “3 metabolic acidosis buckets” (with renal hooks)
1) Anion gap metabolic acidosis (AG ↑): “Add acid”
Classic mnemonic: GOLD MARK
- Glycols (ethylene/propylene)
- Oxoproline (chronic acetaminophen)
- L-lactate
- D-lactate
- Methanol
- Aspirin (late: AG metabolic acidosis; early: resp alkalosis)
- Renal failure (uremia)
- Ketoacidosis (DKA, alcoholic, starvation)
Renal pearl: Uremia = failure to excrete fixed acids + reduced ammoniagenesis.
2) Non–anion gap (hyperchloremic) metabolic acidosis: “Lose HCO₃⁻ or can’t excrete H⁺”
Think GI loss vs renal tubular acidosis (RTA).
RTA cheat table (very USMLE)
| RTA type | Primary defect | Urine pH | Serum K⁺ | Classic associations |
|---|---|---|---|---|
| Type 1 (distal) | Can’t secrete H⁺ (α-intercalated) → no new HCO₃⁻ | > 5.5 | Low | Kidney stones (↑pH), autoimmune (Sjogren), amphotericin B |
| Type 2 (proximal) | Can’t reabsorb HCO₃⁻ (prox tubule) | < 5.5 (after steady state) | Low | Fanconi, acetazolamide, multiple myeloma |
| Type 4 (hypoaldosteronism) | ↓Aldo or resistance → ↓NH₄⁺ excretion | < 5.5 (often) | High | Diabetic nephropathy, ACEi/ARB, heparin, adrenal insufficiency |
One-liner:
- Type 1: can’t dump acid → urine stays basic
- Type 2: dump bicarb early → later urine can acidify
- Type 4: can’t make NH₄⁺ → hyperkalemic acidosis
Quick renal test tip:
- Urine pH > 5.5 in metabolic acidosis screams distal (type 1) RTA.
3) Metabolic alkalosis: “Gain HCO₃⁻ or lose H⁺” and kidneys choose whether to keep it
A metabolic alkalosis persists only if kidneys are forced to hold onto HCO₃⁻.
Two big categories (chloride helps you decide):
Chloride-responsive (urine Cl⁻ low): volume depletion
- Vomiting/NG suction (loss of HCl)
- Post-diuretics (“contraction alkalosis”)
- Responds to normal saline
Renal mechanism: Low volume → ↑RAAS → ↑Na⁺ reabsorption → ↑H⁺ secretion + ↑HCO₃⁻ reabsorption.
Chloride-resistant (urine Cl⁻ high): mineralocorticoid effect
- Hyperaldosteronism, Cushing, Liddle
- Severe hypokalemia (drives H⁺ into cells)
Renal mechanism: Aldosterone → ↑ENaC activity → lumen negative → ↑H⁺ secretion (α-intercalated) + ↑K⁺ secretion.
Rapid-fire high-yield one-liners (test-day glue)
- Compensation never overshoots: if pH is normal but PaCO₂/HCO₃⁻ are abnormal → think mixed disorder.
- Albumin correction prevents missing AG acidosis in sick patients.
- Chronic respiratory acidosis (COPD) → high HCO₃⁻ via renal retention/new generation.
- Chronic respiratory alkalosis (pregnancy, cirrhosis) → low HCO₃⁻ via renal loss.
- Type 4 RTA is the hyperkalemic RTA (most commonly tested differentiator).
Pocket “Flowchart in 10 words” (shareable mnemonic)
“pH → driver → compensation → AG → delta → renal cause.”
Frequently Asked Questions
How do you tell whether an acid-base disorder is metabolic or respiratory on ABGs?
Use pH first, then compare HCO3− and PaCO2 to the pH direction. HCO3− moves pH in the same direction, so low HCO3− means metabolic acidosis and high HCO3− means metabolic alkalosis. PaCO2 moves pH in the opposite direction, so high PaCO2 means respiratory acidosis and low PaCO2 means respiratory alkalosis. The mnemonic ROME summarizes this pattern: Respiratory Opposite, Metabolic Equal.
How do you know if compensation is appropriate or if there is a mixed acid-base disorder?
Check the expected compensation for the primary disorder and compare it with the measured value. In metabolic acidosis, use Winter’s formula to calculate expected PaCO2; a higher measured PaCO2 suggests an additional respiratory acidosis, and a lower one suggests an additional respiratory alkalosis. For respiratory disorders, acute processes cause small bicarbonate changes, while chronic processes cause larger renal bicarbonate changes because the kidneys need time to adapt.
Why do you correct the anion gap for albumin?
You correct the anion gap for albumin because low albumin can mask a true anion gap metabolic acidosis. The corrected anion gap is calculated as AG + 2.5(4 − albumin). This matters because a patient may appear to have a normal anion gap at first glance, but once albumin is accounted for, the hidden gap becomes apparent.
What does the delta gap tell you in an anion gap metabolic acidosis?
The delta gap helps detect a second metabolic process on top of an anion gap metabolic acidosis. Compare the rise in anion gap, calculated as AG − 12, with the fall in bicarbonate, calculated as 24 − HCO3−. If the delta AG is greater than the delta bicarbonate, there is a concurrent metabolic alkalosis. If the delta AG is smaller, there is a concurrent non-anion gap metabolic acidosis.
How do the kidneys fix acidosis in renal physiology questions?
The kidneys correct acidosis by reclaiming filtered bicarbonate and generating new bicarbonate. In the proximal tubule, carbonic anhydrase and the Na+/H+ exchanger help reabsorb filtered HCO3−. In the distal nephron, alpha-intercalated cells secrete H+ through H+-ATPase and H+/K+-ATPase, while generating new HCO3− for the blood. During chronic acidosis, ammoniagenesis increases, leading to more NH4+ excretion as a major adaptive response.