Assess pH
Healthy individual with normal acid-base status
Why This Health Metric Matters
Why: This calculation helps assess important health parameters for clinical and personal wellness tracking.
How: Enter your values above and the calculator will apply validated formulas to compute your results.
- โEvidence-based calculations
- โUsed in clinical settings worldwide
- โRegular monitoring recommended
Clinical Scenarios
Normal ABG
Healthy individual with normal acid-base status
Metabolic Acidosis - DKA
Diabetic ketoacidosis with respiratory compensation
Respiratory Acidosis - COPD
Acute on chronic respiratory acidosis in COPD exacerbation
Metabolic Alkalosis - Vomiting
Severe vomiting causing hypochloremic metabolic alkalosis
Mixed Disorder - Sepsis
Septic patient with lactic acidosis and respiratory alkalosis
Blood Gas Values
Electrolytes
Additional Parameters
Analysis Results
Normal acid-base status
Normal acid-base status. Compensation is appropriate. Normal oxygenation.
Corrected AG (mEq/L)
Delta Ratio
P/F Ratio
Severity
Compensation Analysis
Status: appropriate
Differential Diagnosis
Recommendations
Visualizations
ABG Components
pH vs HCO3 Map
Step-by-Step Analysis
Step 1: Assess pH
Formula: ext{pH} < 7.35 = ext{acidemia}, ext{pH} > 7.45 = ext{alkalemia}
Calculation: pH = 7.4
Result: Normal
First step is to determine if the patient is acidemic, alkalemic, or has normal pH.
Step 2: Identify Primary Disorder
Formula: ext{Match} ext{pH} ext{direction} ext{with} pCO2 ext{or} HCO3 ext{abnormality}
Calculation: pH: 7.4, pCO2: 40, HCO3: 24
Result: Normal acid-base status
The primary disorder causes the pH change; the other parameter compensates.
Step 3: Calculate Anion Gap
Formula: ext{AG} = ext{Na} - ( ext{Cl} + HCO3)
Calculation: AG = 140 - (104 + 24)
Result: 12.00 mEq/L (Corrected: 12.00)
Elevated AG (>12) indicates high anion gap metabolic acidosis.
Step 4: Check Compensation
Formula: HCO3 change per 10 mmHg pCO2
Calculation: Expected HCO3: 22.00 - 26.00
Result: Actual: 24
Compare actual values to expected compensation to identify mixed disorders.
ABG Normal Reference Ranges
| Parameter | Normal Range | Critical Low | Critical High | Clinical Notes |
|---|---|---|---|---|
| pH | 7.35-7.45 | <7.20 | >7.60 | Primary determinant of acidemia/alkalemia |
| pCO2 | 35-45 mmHg | <20 | >70 | Respiratory component |
| HCO3 | 22-26 mEq/L | <10 | >40 | Metabolic component |
| PaO2 | 80-100 mmHg | <60 | N/A | Hypoxemia if <60 on room air |
| Anion Gap | 8-12 mEq/L | N/A | >20 | Elevated in HAGMA (MUDPILES) |
| P/F Ratio | >400 | <200 (ARDS) | N/A | <300 = Acute Lung Injury |
Acid-Base Disorder Classification
| Disorder | pH | Primary Change | Compensation | Common Causes |
|---|---|---|---|---|
| Metabolic Acidosis | <7.35 | โ HCO3 | โ pCO2 | DKA, lactic acidosis, renal failure, diarrhea |
| Metabolic Alkalosis | >7.45 | โ HCO3 | โ pCO2 | Vomiting, diuretics, hypokalemia, contraction |
| Respiratory Acidosis | <7.35 | โ pCO2 | โ HCO3 | COPD, hypoventilation, sedation, neuromuscular |
| Respiratory Alkalosis | >7.45 | โ pCO2 | โ HCO3 | Hyperventilation, anxiety, PE, sepsis, pregnancy |
How to Interpret an ABG
Systematic 6-Step Approach
- 1
Assess pH - Acidemia or Alkalemia?
<7.35 = Acidemia, >7.45 = Alkalemia, 7.35-7.45 = Normal (or mixed/compensated)
- 2
Identify Primary Disorder
Look at pCO2 and HCO3 - which explains the pH change? That's the primary disorder.
- 3
Assess Compensation
Use formulas to determine if compensation is appropriate. Inappropriate compensation = mixed disorder.
- 4
Calculate Anion Gap
AG = Na - (Cl + HCO3). Normal 8-12. Elevated = HAGMA (use MUDPILES differential)
- 5
Calculate Delta Ratio (if HAGMA)
Delta Ratio = ฮAG / ฮHCO3. <1 = mixed HAGMA+NAGMA, 1-2 = pure HAGMA, >2 = HAGMA + met alkalosis
- 6
Assess Oxygenation
Calculate P/F ratio and A-a gradient. P/F <300 = ALI, <200 = ARDS. Elevated A-a = V/Q mismatch.
When to Order an ABG
Respiratory Failure
Dyspnea, hypoxia, altered mental status, respiratory distress requiring ventilator management.
ICU Management
Critically ill patients, ventilator settings optimization, shock assessment.
Metabolic Emergencies
DKA, HHS, toxic ingestions, sepsis, lactic acidosis evaluation.
Drug Overdose
Salicylate, methanol, ethylene glycol, metformin toxicity assessment.
Unexplained Symptoms
Altered mental status, unexplained tachypnea, suspected acid-base disorder.
Treatment Monitoring
Response to bicarbonate therapy, ventilator weaning, COPD exacerbation management.
What is Acid-Base Analysis?
pH Regulation
The body maintains pH between 7.35-7.45 through respiratory (CO2) and metabolic (HCO3) buffering systems. The Henderson-Hasselbalch equation relates pH to these components.
Compensation
When one system is disturbed, the other compensates to minimize pH change. Respiratory compensation is rapid (minutes), metabolic compensation takes 3-5 days.
Clinical Importance
ABG analysis is crucial for managing respiratory failure, metabolic emergencies, ventilator management, and critically ill patients.
Key Formulas
Winter's Formula (Metabolic Acidosis)
Expected pCO2 = (1.5 ร HCO3) + 8 ยฑ 2
Metabolic Alkalosis Compensation
Expected pCO2 = (0.7 ร HCO3) + 21 ยฑ 2
Respiratory Acidosis (Acute)
HCO3 rises 1 mEq/L per 10 mmHg pCO2 increase
Respiratory Acidosis (Chronic)
HCO3 rises 3.5 mEq/L per 10 mmHg pCO2 increase
Frequently Asked Questions
What's the difference between acidemia and acidosis?
Acidemia refers to blood pH <7.35 (the measured state). Acidosis refers to the process causing acid accumulation. You can have acidosis without acidemia if compensation is adequate (e.g., chronic respiratory acidosis with renal compensation may have normal pH).
Can compensation normalize pH completely?
No. Physiological compensation minimizes pH change but never normalizes it completely. If pH is exactly 7.40 with abnormal pCO2 and HCO3, suspect a mixed disorder where two opposite processes are occurring simultaneously.
When should I use VBG vs ABG?
VBG is acceptable for pH and HCO3 (very close to arterial). For oxygenation assessment (PaO2, A-a gradient, P/F ratio) an ABG is required. In emergencies, VBG can screen for acid-base disorders while avoiding arterial puncture risks.
What is the significance of the delta ratio?
In high anion gap acidosis, delta ratio (ฮAG/ฮHCO3) reveals hidden disorders: <1 suggests concurrent non-anion gap acidosis, 1-2 is pure HAGMA, >2 suggests pre-existing metabolic alkalosis. It's essential for detecting mixed disorders.
How do I know if compensation is acute or chronic?
Clinical history is key. Metabolic compensation for respiratory disorders takes 3-5 days. If HCO3 change matches acute rules (ยฑ1 per 10 pCO2), it's acute. If it matches chronic rules (ยฑ3.5 per 10 pCO2), it's chronic. Intermediate values suggest subacute or mixed.
Clinical Pearls
Triple Acid-Base Disorder
Triple disorders are rare but real. Example: Alcoholic with DKA (HAGMA) + diarrhea (NAGMA) + vomiting (metabolic alkalosis). Delta ratio and careful analysis are essential.
Salicylate Pattern
Salicylate toxicity classically causes both HAGMA (uncoupling) AND respiratory alkalosis (direct respiratory center stimulation). This mixed disorder is nearly pathognomonic.
Contraction Alkalosis
Volume depletion concentrates bicarbonate and activates RAAS, perpetuating alkalosis. Saline-responsive alkalosis (urine Cl <20) responds to volume expansion.
Minimum pCO2
Maximum hyperventilation can only lower pCO2 to ~10-12 mmHg. If Winter's formula predicts lower, the patient has reached maximum respiratory compensation.
Related Calculators
Anion Gap
Calculate and correct anion gap
Winters Formula
Expected pCO2 calculation
Serum Osmolality
Calculate osmolar gap
Bicarbonate Deficit
Calculate replacement needs
Urine Anion Gap
NAGMA workup tool
Arterial Blood pH
Comprehensive ABG analysis
Corrected Calcium
Albumin-corrected calcium
Sodium Correction
Glucose correction
โ ๏ธFor informational purposes only โ not medical advice. Consult a healthcare professional before acting on results.
๐ฅ Health Facts
โ WHO
โ CDC