Acid–base imbalances can disrupt critical bodily functions. To identify these disorders, healthcare professionals rely on arterial blood gas (ABG) analysis. This test provides essential data for diagnosing whether the body is in a state of acidosis (pH too low) or alkalosis (pH too high). Specifically, there are three blood variables considered when making a diagnosis of acidosis or alkalosis: pH, partial pressure of carbon dioxide (PaCO₂), and bicarbonate (HCO₃⁻).
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1. Blood pH
Blood pH indicates whether the blood is acidic or alkaline. The normal range is 7.35 to 7.45.
- Below 7.35 = Acidosis
- Above 7.45 = Alkalosis
A deviation from this range signifies a disruption in the body’s acid–base balance, which can be life-threatening if not corrected.
2. Partial Pressure of Carbon Dioxide (PaCO₂)
PaCO₂ reflects the amount of carbon dioxide in the blood and indicates respiratory function. The normal range is 35–45 mm Hg.
- High PaCO₂ (>45 mm Hg) = Respiratory acidosis
- Low PaCO₂ (<35 mm Hg) = Respiratory alkalosis
This variable helps identify if the lungs are contributing to or compensating for an acid–base disorder.
📚 Cleveland Clinic – Arterial Blood Gases
3. Bicarbonate (HCO₃⁻)
Bicarbonate is a base that helps neutralize acids. It reflects metabolic function. The normal range is 22–26 mEq/L.
- Low HCO₃⁻ (<22 mEq/L) = Metabolic acidosis
- High HCO₃⁻ (>26 mEq/L) = Metabolic alkalosis
HCO₃⁻ values indicate whether the kidneys are involved in the imbalance or compensating for it.
🔍 Learn more from NIH – Acid–Base Disorders
Summary Table: Blood Variables in Acid–Base Diagnosis
| Variable | Normal Range | Indicates Abnormality When… |
|---|---|---|
| pH | 7.35–7.45 | pH < 7.35 = acidosis, pH > 7.45 = alkalosis |
| PaCO₂ | 35–45 mm Hg | ↑ = respiratory acidosis, ↓ = alkalosis |
| HCO₃⁻ | 22–26 mEq/L | ↓ = metabolic acidosis, ↑ = alkalosis |
Conclusion
The three blood variables considered when making a diagnosis of acidosis or alkalosis—pH, PaCO₂, and HCO₃⁻—provide a full picture of a patient’s acid–base status. Together, these values help determine whether an imbalance is respiratory or metabolic in origin and guide treatment decisions accordingly. Regular monitoring of these parameters is crucial in critical care and clinical diagnosis.