# Acid-Base Disorders: A Student Guide to Blood Gas Interpretation

## Learning Objectives

- Interpret arterial blood gases systematically
- Distinguish primary disorders from compensation
- Calculate anion gap and identify mixed disorders
- Apply Winter’s formula and other compensation rules
- Approach complex acid-base cases confidently

## Fundamental Concepts

### pH Basics

- **Acidemia:** pH \<7.40
- **Alkalemia:** pH \>7.40
- **Acidosis:** The underlying PROCESS causing acidemia
- **Alkalosis:** The underlying PROCESS causing alkalemia

**Critical:** You can have acidosis WITHOUT acidemia (if something else compensates)

### The Bicarbonate System (Foundation of Everything)

    CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

**Key insight:** This is an OPEN system - Lungs eliminate CO₂ (respiratory component) - Kidneys excrete/retain HCO₃⁻ (metabolic component) - When system becomes CLOSED (respiratory failure), severe acidosis develops rapidly

### Normal Values

- **pH:** 7.35-7.45
- **PCO₂:** 35-45 mmHg (respiratory control)
- **HCO₃⁻:** 22-26 mEq/L (metabolic control)

## The Systematic ABC Approach

### Step A: Assess pH

- Is it \<7.40 (acidemia) or \>7.40 (alkalemia)?
- This tells you the DIRECTION of the primary problem

### Step B: Identify Primary Disorder

Check **HCO₃⁻ and PCO₂** together to determine what matches the pH direction:

**If ACIDEMIA (pH \<7.40), look for:** - ↓ HCO₃⁻ (\<22) = **Metabolic acidosis** (primary) - ↑ PCO₂ (\>45) = **Respiratory acidosis** (primary)

**If ALKALEMIA (pH \>7.40), look for:** - ↑ HCO₃⁻ (\>26) = **Metabolic alkalosis** (primary) - ↓ PCO₂ (\<35) = **Respiratory alkalosis** (primary)

**Critical rule:** Compensation is ALWAYS INCOMPLETE. If compensation appears complete or excessive, suspect a SECOND disorder.

### Step C: Assess Compensation (Use Formulas!)

For **Metabolic Acidosis** → Respiratory Compensation (Winter’s Formula):

    Expected PCO₂ = 1.5 × [HCO₃⁻] + 8 ± 2

**Example:** HCO₃⁻ = 10 - Expected PCO₂ = 1.5(10) + 8 ± 2 = 23 ± 2 → 21-25 mmHg - If actual PCO₂ is 28 → inadequate respiratory compensation (concurrent respiratory problem) - If actual PCO₂ is 18 → appropriate compensation ✓

For **Metabolic Alkalosis** → Respiratory Compensation:

    PCO₂ increases by 0.7 mmHg per 1 mEq/L rise in HCO₃⁻

For **Respiratory Acidosis** (↑PCO₂) → Metabolic Compensation: - **Acute:** HCO₃⁻ rises by 1 mEq/L per 10 mmHg rise in PCO₂ - **Chronic:** HCO₃⁻ rises by 3.5 mEq/L per 10 mmHg rise in PCO₂

For **Respiratory Alkalosis** (↓PCO₂) → Metabolic Compensation: - **Acute:** HCO₃⁻ drops by 2 mEq/L per 10 mmHg drop in PCO₂ - **Chronic:** HCO₃⁻ drops by 5 mEq/L per 10 mmHg drop in PCO₂

**Remember:** Respiratory changes happen in MINUTES. Renal compensation takes DAYS.

### Step D: Calculate Anion Gap

**Formula:**

    AG = Na⁺ - (Cl⁻ + HCO₃⁻)

**Normal:** 8-12 mEq/L (varies by lab; check YOUR lab’s normal)

**Elevated AG (\>12):** Means unmeasured anions present (organic acids accumulating)

**Narrow AG (\<8):** Unusual; suggests hypoalbuminemia, myeloma, or specific toxins

**Clinical pearl:** Albumin correction - Each 1 g/dL drop in albumin below 4 g/dL → AG decreases by ~2.5 mEq/L - In ICU (hypoalbumemic patients) → ALWAYS correct AG for albumin

## High-Yield Disorders: Anion Gap Metabolic Acidosis

### MUDPILES Mnemonic

- **M:** Methanol
- **U:** Uremia (kidney failure)
- **D:** DKA (Diabetic ketoacidosis)
- **P:** Propylene glycol (from IV Ativan!)
- **I:** Isoniazid, Iron
- **L:** Lactic acidosis
- **E:** Ethylene glycol
- **S:** Salicylates (aspirin overdose)

### Most Common in Practice

1.  **Lactic acidosis** (shock, sepsis, hypoxemia)
2.  **DKA** (diabetes out of control)
3.  **Uremia** (kidney failure)
4.  **Methanol/ethylene glycol** (toxic ingestions)

## Non-Anion Gap Metabolic Acidosis

**When AG is normal but HCO₃⁻ is low → the body is losing bicarbonate or retaining acid**

**Diagnosis:** Check URINARY ANION GAP (UAG)

    UAG = (Urine Na⁺ + Urine K⁺) - Urine Cl⁻

**Interpretation:** - **Negative UAG:** Kidney producing ammonia appropriately (losing ammonium) → extrarenal cause - **Positive UAG:** Kidney NOT producing ammonia (renal cause)

### USED CARP Mnemonic

**Negative UAG (kidney works fine, GI problem):** - **U:** Ureterosigmoidostomy - **S:** Small bowel fistula - **E:** Extra chloride (NS administration) - **D:** Diarrhea (most common)

**Positive UAG (kidney problem):** - **C:** Chronic renal failure - **A:** Acetazolamide, Addison’s disease - **R:** Renal tubular acidosis (RTA) - **P:** Protein overfeeding

## Detecting Mixed Disorders

### The Delta/Delta Ratio

Used when you suspect MULTIPLE metabolic problems in anion gap acidosis:

    Δ AG = Measured AG - Normal AG (10)
    Δ HCO₃⁻ = Normal HCO₃⁻ (24) - Measured HCO₃⁻
    Ratio = Δ AG / Δ HCO₃⁻

**Interpretation:** - **Ratio \<1:** HCO₃⁻ dropped MORE than expected → concurrent non-AG acidosis - **Ratio 1-2:** Pure AG acidosis (appropriate) - **Ratio \>2:** HCO₃⁻ didn’t drop as much as expected → concurrent metabolic alkalosis

### Potential Bicarbonate Method

Alternative approach:

    Potential HCO₃⁻ = Measured HCO₃⁻ + (Measured AG - 10)

**Interpretation:** - **\<22:** Suggests concurrent NAGMA - **22-26:** Pure AG acidosis - **\>26:** Suggests concurrent metabolic alkalosis

## SGLT2 Inhibitor-Associated DKA (Emerging!)

**Know this for exams:** - SGLT2 inhibitors can cause DKA at mild-moderate hyperglycemia - DKA without severe hyperglycemia (glucose \<200) - High anion gap with normal HCO₃⁻ initially - Risk factors: Type 1 diabetes, surgery, illness - Management: Hold SGLT2i, treat with IV fluids + insulin

## Clinical Cases: Putting It Together

### Case 1: Severe Sepsis with Shock

**ABG:** pH 7.15, PCO₂ 22, HCO₃⁻ 8, Lactate 8

**Analysis:** 1. pH 7.15 → **Acidemia** 2. HCO₃⁻ 8 (low) → **Metabolic acidosis** (primary) 3. Winter’s formula: Expected PCO₂ = 1.5(8) + 8 ± 2 = 20 ± 2 → 18-22 mmHg - Actual 22 → **appropriate respiratory compensation** ✓ 4. AG = 140 - (110 + 8) = 22 (**elevated**) 5. **Diagnosis:** High AG metabolic acidosis with appropriate respiratory response 6. **Cause:** Likely lactic acidosis from shock 7. **Management:** Aggressive fluid resuscitation, antibiotics, vasopressors

### Case 2: DKA with Concurrent Infection

**ABG:** pH 7.10, PCO₂ 28, HCO₃⁻ 9, Glucose 450, β-hydroxybutyrate 4.2

**Analysis:** 1. **Acidemia** (pH 7.10) 2. **Metabolic acidosis** (HCO₃⁻ 9) 3. Winter’s formula: Expected PCO₂ = 1.5(9) + 8 ± 2 = 21.5 ± 2 → 19-24 mmHg - Actual 28 → **inadequate respiratory compensation** (higher than expected!) 4. **Interpretation:** There’s a **concurrent respiratory acidosis** or the respiratory center is depressed 5. **Possible causes:** Concurrent pneumonia, pulmonary edema, CNS depression 6. **Management:** Aggressive insulin, fluids, monitor respiratory status closely

## Quick Reference: What Changes With Each Disorder

| Disorder           | pH  | HCO₃⁻          | PCO₂           |
|--------------------|-----|----------------|----------------|
| **Met Acidosis**   | ↓   | ↓              | ↓ (compensate) |
| **Met Alkalosis**  | ↑   | ↑              | ↑ (compensate) |
| **Resp Acidosis**  | ↓   | ↑ (compensate) | ↑              |
| **Resp Alkalosis** | ↑   | ↓ (compensate) | ↓              |

## Practice Questions

**Q1:** ABG shows pH 7.32, HCO₃⁻ 15, PCO₂ 38. Calculate if compensation is appropriate.

Answer

Using Winter’s formula: Expected PCO₂ = 1.5(15) + 8 ± 2 = 30.5 ± 2 → 28-33 mmHg. Actual PCO₂ is 38, which is HIGHER than expected. This indicates inadequate respiratory compensation, suggesting a concurrent respiratory acidosis or respiratory problem. The patient has metabolic acidosis PLUS respiratory acidosis = mixed disorder.

**Q2:** A patient with advanced COPD has pH 7.25, HCO₃⁻ 28, PCO₂ 65. What’s happening?

Answer

Respiratory acidosis (PCO₂ high). HCO₃⁻ is 28 (elevated), which is appropriate renal compensation for chronic hypercapnia. However, with pH still 7.25, the patient is acidemic—the compensation isn’t enough. This is chronic respiratory acidosis from COPD exacerbation. Treatment: oxygen, bronchodilators, consider BiPAP, treat infection.

**Q3:** ABG: pH 7.28, HCO₃⁻ 12, PCO₂ 20, AG 18. What’s the diagnosis?

Answer

AG metabolic acidosis with appropriate respiratory compensation. Delta/delta ratio: (18-10) / (24-12) = 8/12 = 0.67, which is \<1. This suggests a CONCURRENT non-AG acidosis (like diarrhea or saline administration). So the patient has HIGH AG + NON-AG acidosis = mixed metabolic acidosis. Look for both DKA/lactic acidosis AND diarrhea/renal tubular disease.

## Key Takeaways

- **Always follow the ABC approach** systematically
- **Winter’s formula** predicts respiratory response to metabolic acidosis
- **Compensation is incomplete** → if appears complete, suspect mixed disorder
- **Anion gap** separates high-AG from non-AG acidosis
- **Albumin matters** → correct AG for hypoalbuminemia
- **Respiratory changes happen fast, renal slow** → temporal clue to acuity
- **Delta/delta ratio** identifies mixed metabolic disorders
- **MUDPILES for high AG** causes
- **USED CARP for non-AG** causes

------------------------------------------------------------------------

**Memory aid:** “ABC = Always Be Calculating” — Use formulas, don’t guess at compensation!

**Exam strategy:** If ABG doesn’t make sense, calculate Winter’s formula. It catches mixed disorders most students miss.

------------------------------------------------------------------------

## See Also

### Related Student Handouts

- Hyperkalemia Management
- Hypokalemia Management
- Hyponatremia Management
- Hypernatremia and SIADH
- AKI Workup and Diagnosis

### Clinical Content (01-Clinical-Medicine/Nephrology)

- Electrolyte Disorders Hub
- Acid-Base Disorders Clinical Reference
- Essential Renal Laboratory Tests

### Atomic Notes (ZK)

- RAAS System and Electrolyte Regulation

### Butler-COM Resources

- Butler COM - Nephrology Deep Dive

## Clinical Resources

- Clinical Review: Acid Base Guide — Comprehensive clinical review with PubMed references
- Clinical Review: Acid Base Formal Review — Comprehensive clinical review with PubMed references

### Also on this topic

The same subject at other levels of depth.

- **Primer chapter:** [Chapter 16: Acid-Base Disorders In-Depth](https://urinenephrology.org/nephrology-textbook/chapters/chapter-13-acid-base-review.html)
- **Clinical Mastery deep dive:** [Acid-Base Review: A Comprehensive Student Guide](https://urinenephrology.org/mastery/acid-base/student-guide.html)


---

[Website version](https://urinenephrology.org/student-resources/electrolyte-acid-base/acid-base-student-handout.html) · Markdown synchronized October 3, 2026.