Acid-Base Calculator
Interpret arterial blood gas (ABG) results and calculate the anion gap. Identifies acid-base disorders, evaluates compensation, and computes the delta-delta ratio.
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Acid-Base Diagnosis
Table of Contents
What is Acid-Base Balance?
The body maintains blood pH within a remarkably narrow range of 7.35 to 7.45. This precise regulation is essential because even small deviations can impair enzyme function, alter protein structure, and disrupt cellular processes. The body uses three main systems to maintain acid-base balance:
- Chemical buffer systems: Bicarbonate (HCO₃⁻/CO₂), phosphate, and protein buffers provide immediate but limited buffering capacity
- Respiratory system: The lungs regulate CO₂ elimination within minutes. Increasing ventilation lowers CO₂ (raises pH), while decreasing ventilation raises CO₂ (lowers pH)
- Renal system: The kidneys regulate bicarbonate reabsorption and hydrogen ion excretion over hours to days, providing the most powerful but slowest compensation
The relationship between these components is described by the Henderson-Hasselbalch equation:
This equation shows that pH depends on the ratio of bicarbonate (metabolic component) to dissolved CO₂ (respiratory component). Disturbances in either component lead to acid-base disorders.
Arterial Blood Gas (ABG) Basics
An arterial blood gas is a laboratory test performed on blood drawn from an artery (usually the radial artery at the wrist). It provides several critical measurements:
| Parameter | Normal Range | What It Measures |
|---|---|---|
| pH | 7.35 – 7.45 | Overall acid-base status of the blood |
| PaCO₂ | 35 – 45 mmHg | Partial pressure of CO₂ — respiratory component |
| HCO₃⁻ | 22 – 26 mEq/L | Bicarbonate concentration — metabolic component |
| PaO₂ | 75 – 100 mmHg | Partial pressure of oxygen (oxygenation status) |
| SaO₂ | 95 – 100% | Oxygen saturation of hemoglobin |
| Base Excess | -2 to +2 mEq/L | Amount of acid/base needed to normalize pH |
The Four Primary Acid-Base Disorders
| Disorder | pH | Primary Change | Compensation |
|---|---|---|---|
| Metabolic Acidosis | < 7.35 | HCO₃⁻ decreased | PaCO₂ decreases (hyperventilation) |
| Metabolic Alkalosis | > 7.45 | HCO₃⁻ increased | PaCO₂ increases (hypoventilation) |
| Respiratory Acidosis | < 7.35 | PaCO₂ increased | HCO₃⁻ increases (renal retention) |
| Respiratory Alkalosis | > 7.45 | PaCO₂ decreased | HCO₃⁻ decreases (renal excretion) |
Figure 1: Systematic approach to ABG interpretation — start with pH, identify the primary disorder, then assess compensation and anion gap.
The Anion Gap
The anion gap (AG) represents the difference between measured cations and measured anions in the blood. It exists because not all ions are routinely measured — the "gap" is filled by unmeasured anions (albumin, phosphate, sulfate, organic acids).
Normal range: 8 – 12 mEq/L
An elevated anion gap indicates the presence of excess unmeasured anions, typically from an acid that has been added to the blood (such as lactic acid, ketoacids, or toxic alcohols). The mnemonic MUDPILES helps remember causes of elevated AG metabolic acidosis:
- M — Methanol
- U — Uremia (renal failure)
- D — Diabetic ketoacidosis (DKA)
- P — Propylene glycol
- I — Isoniazid, Iron
- L — Lactic acidosis
- E — Ethylene glycol
- S — Salicylates (aspirin)
A normal AG metabolic acidosis (also called hyperchloremic metabolic acidosis) is caused by bicarbonate loss or impaired renal acid excretion. The mnemonic HARDUPS helps:
- H — Hyperalimentation
- A — Acetazolamide / Addison's disease
- R — Renal tubular acidosis
- D — Diarrhea
- U — Ureteral diversions
- P — Pancreatic fistula
- S — Saline infusion (dilutional)
Corrected Anion Gap
Albumin is the major unmeasured anion contributing to the normal anion gap. In hypoalbuminemic patients (common in critically ill, liver disease, nephrotic syndrome), the AG may be falsely normal even when excess unmeasured acids are present. The corrected AG adjusts for this:
(Where albumin is in g/dL)
For every 1 g/dL decrease in albumin below 4.0, the expected anion gap decreases by about 2.5 mEq/L. The corrected AG "adds back" this hidden gap.
Compensation Rules
When a primary acid-base disorder occurs, the body attempts to compensate to minimize the pH change. Compensation never fully corrects the pH back to normal (if pH is exactly 7.40, consider a mixed disorder).
| Primary Disorder | Compensation | Expected Response |
|---|---|---|
| Metabolic Acidosis | Respiratory (hyperventilation) | PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 (Winter's) |
| Metabolic Alkalosis | Respiratory (hypoventilation) | PaCO₂ = 0.7 × HCO₃⁻ + 21 ± 2 |
| Respiratory Acidosis (acute) | Metabolic (renal) | HCO₃⁻ rises 1 mEq/L per 10 mmHg rise in CO₂ |
| Respiratory Acidosis (chronic) | Metabolic (renal) | HCO₃⁻ rises 3.5 mEq/L per 10 mmHg rise in CO₂ |
| Respiratory Alkalosis (acute) | Metabolic (renal) | HCO₃⁻ drops 2 mEq/L per 10 mmHg drop in CO₂ |
| Respiratory Alkalosis (chronic) | Metabolic (renal) | HCO₃⁻ drops 5 mEq/L per 10 mmHg drop in CO₂ |
Winter's Formula
Winter's formula predicts the expected PaCO₂ in the setting of a primary metabolic acidosis:
- If measured PaCO₂ is within the expected range: appropriate respiratory compensation
- If measured PaCO₂ is higher than expected: concurrent respiratory acidosis (the patient is not ventilating adequately)
- If measured PaCO₂ is lower than expected: concurrent respiratory alkalosis (the patient is hyperventilating beyond what compensation requires)
Delta-Delta Ratio
The delta-delta ratio (also called the delta gap or delta ratio) is used in AG metabolic acidosis to detect a concurrent non-AG metabolic acidosis or metabolic alkalosis:
| Delta-Delta Value | Interpretation |
|---|---|
| < 1 | Concurrent non-AG metabolic acidosis (excess HCO₃⁻ loss beyond what AG acids explain) |
| 1 – 2 | Pure AG metabolic acidosis (the AG increase fully accounts for the HCO₃⁻ decrease) |
| > 2 | Concurrent metabolic alkalosis (HCO₃⁻ is higher than expected — a metabolic alkalosis is "hiding") |
Systematic ABG Interpretation
Follow these steps to systematically interpret any ABG:
- Assess the pH: Is the patient acidemic (< 7.35) or alkalemic (> 7.45)?
- Identify the primary disorder: Which component (respiratory or metabolic) explains the pH change?
- Evaluate compensation: Is the other component changing in the expected direction and magnitude?
- Calculate the anion gap: If metabolic acidosis is present, is the AG elevated?
- Apply Winter's formula: In metabolic acidosis, is the respiratory compensation appropriate?
- Calculate the delta-delta: In AG metabolic acidosis, is there a concurrent metabolic disorder?
- Consider mixed disorders: If compensation is inadequate or excessive, suspect a mixed acid-base disturbance.
Common Causes of Acid-Base Disorders
Metabolic Acidosis
Elevated AG: Lactic acidosis (sepsis, shock, seizures), diabetic ketoacidosis, renal failure (uremia), toxic ingestion (methanol, ethylene glycol, salicylates)
Normal AG: Diarrhea, renal tubular acidosis (RTA), saline resuscitation (dilutional), acetazolamide use, ureteral diversion
Metabolic Alkalosis
Chloride-responsive (urine Cl < 25): Vomiting, NG suction, diuretic use (post-effect), post-hypercapnia
Chloride-resistant (urine Cl > 40): Hyperaldosteronism, Cushing syndrome, Bartter/Gitelman syndrome, severe hypokalemia
Respiratory Acidosis
Acute: Airway obstruction, pneumothorax, pulmonary edema, drug overdose (opioids, benzodiazepines), neuromuscular paralysis
Chronic: COPD, obesity hypoventilation syndrome, neuromuscular disease, chest wall deformity
Respiratory Alkalosis
Common causes: Anxiety/hyperventilation, pain, fever, sepsis, pulmonary embolism, pregnancy, salicylate toxicity (early), high altitude, mechanical ventilation (overventilation)
Frequently Asked Questions
What does a normal pH with abnormal PaCO₂ and HCO₃⁻ mean?
A normal pH (7.35–7.45) with abnormal PaCO₂ and HCO₃⁻ typically indicates either: (1) a fully compensated acid-base disorder, or (2) a mixed acid-base disorder where opposing disturbances cancel out. Look at which value is most abnormal and consider the clinical context. Chronic disorders have more time for compensation and may achieve near-normal pH.
Can a patient have multiple acid-base disorders simultaneously?
Yes, mixed acid-base disorders are common, especially in critically ill patients. For example, a patient with sepsis may have both a lactic acidosis (metabolic acidosis) and a respiratory alkalosis from hyperventilation. The anion gap, delta-delta ratio, and comparison of measured vs. expected compensation help identify mixed disorders.
What is the normal anion gap?
The normal anion gap is typically 8–12 mEq/L. Some labs use a normal range of 10–12 mEq/L. The exact normal value depends on the lab's methodology and whether potassium is included in the calculation. Always compare to your laboratory's reference range.
Why is albumin important for the anion gap?
Albumin is a negatively charged protein that constitutes a significant portion of the unmeasured anions making up the normal anion gap. In critically ill patients, albumin levels are frequently low (hypoalbuminemia). This can mask an elevated anion gap acidosis — the AG may appear "normal" even though excess acids are present. The corrected anion gap accounts for this by adding 2.5 mEq/L to the AG for every 1 g/dL decrease in albumin below 4.0.
What is base excess?
Base excess (BE) is the amount of strong acid (in mEq/L) needed to return blood pH to 7.40 at a PaCO₂ of 40 mmHg and 37°C. A positive BE indicates excess base (metabolic alkalosis), while a negative BE (also called base deficit) indicates excess acid (metabolic acidosis). Normal BE is -2 to +2 mEq/L. It provides a pure measure of the metabolic component independent of respiratory effects.