Metabolic Acidosis

Metabolic acidosis is a common acid–base disorder that occurs when there is either too much acid in the body or too little bicarbonate, causing blood pH to fall. It is often first detected on routine blood tests showing low bicarbonate and changes in the anion gap, sometimes before symptoms become severe.

This condition develops when acid production increases, acid excretion by the kidneys is impaired, or bicarbonate is lost from the body. The result is a disruption in the body’s normal acid–base balance, which is essential for proper cellular function.

Types and Mechanisms

Metabolic acidosis is typically classified based on the anion gap, a key lab calculation used to identify the underlying cause:

High anion gap metabolic acidosis:
Caused by the accumulation of unmeasured acids in the blood. Common causes include:
- Lactic acidosis
- Ketoacidosis (diabetic, alcoholic, or starvation)
- Kidney failure (uremia)
- Toxins (e.g., methanol, ethylene glycol, salicylates)
Normal anion gap metabolic acidosis (hyperchloremic):
Occurs when bicarbonate is lost and replaced by chloride, keeping the anion gap normal. Common causes include:
- Diarrhea (loss of bicarbonate from the gastrointestinal tract)
- Renal tubular acidosis
- Certain medications

This distinction—high vs normal anion gap metabolic acidosis—is central to determining the cause.

What Causes Low Bicarbonate?

Metabolic acidosis can result from several underlying mechanisms:

Increased acid production:
- Lactic acidosis (e.g., sepsis, shock)
- Ketoacidosis (diabetic, alcoholic, starvation)
Reduced acid excretion:
- Chronic kidney disease
- Acute kidney injury
Loss of bicarbonate:
- Diarrhea
- Renal tubular acidosis
Toxin exposure:
- Methanol
- Ethylene glycol
- Salicylates

In many cases, more than one mechanism contributes to metabolic acidosis.

Key Biomarkers & Tests

Metabolic acidosis is diagnosed and evaluated through blood testing:

Serum bicarbonate (HCO₃⁻) (low levels)
Blood pH (acidic)
Anion gap (elevated or normal, depending on cause)
Electrolytes (sodium, potassium, chloride)
Lactate
Ketones
Creatinine and eGFR

Arterial or venous blood gas testing is often used to confirm pH changes and assess respiratory compensation.

Low bicarbonate is the hallmark finding. The anion gap helps determine whether unmeasured acids are present.

Low albumin lowers the calculated anion gap and can partially mask a high anion gap metabolic acidosis, so corrected anion gap calculations are sometimes needed. Mixed acid–base disorders may also occur and require careful interpretation.

Symptoms of Metabolic Acidosis

Symptoms of metabolic acidosis vary depending on severity and cause but may include:

Rapid or deep breathing (compensatory response, known as Kussmaul respirations)
Fatigue and weakness
Nausea and vomiting
Confusion or decreased alertness
Headache

In more severe cases, metabolic acidosis can impair heart and brain function.

Complications

If untreated, metabolic acidosis can lead to:

Cardiovascular instability
Arrhythmias
Reduced organ function
Bone loss in chronic acidosis (chronic acid buffering draws calcium from bones)
Increased risk of mortality in severe cases

Many of these complications can be detected early through lab monitoring—declining bicarbonate, worsening acidosis, and rising underlying markers (such as lactate, ketones, or creatinine) are key indicators.

Treatment & Management

Metabolic acidosis treatment focuses on identifying and correcting the underlying cause:

Treat the underlying condition (e.g., sepsis, kidney disease, ketoacidosis)
Restore fluid and electrolyte balance
Address acid buildup or bicarbonate loss
Use bicarbonate therapy in selected cases
Monitor with repeat blood tests (bicarbonate, pH, anion gap)

With early diagnosis and appropriate treatment, many cases of metabolic acidosis can be corrected. However, persistent or severe acidosis requires close monitoring, as it often reflects ongoing disease processes.