Low bicarbonate levels on a blood test usually mean your body is dealing with too much acid, losing bicarbonate through the gut or kidneys, or compensating for a breathing pattern that has shifted your blood chemistry. The normal range for bicarbonate is 22 to 26 mEq/L. A result below 22 suggests one of several underlying processes is pulling that number down, and the cause matters because it shapes what happens next.
Bicarbonate is your blood’s main acid buffer. Your kidneys filter roughly 4,500 mEq of it every day, reabsorbing most of it and generating new bicarbonate to replace what gets used up neutralizing the acids your body constantly produces. When something disrupts that balance, bicarbonate drops and blood becomes more acidic, a state called metabolic acidosis.
How Doctors Sort Out the Cause
If your bicarbonate is low, the next step is usually calculating something called the anion gap, a simple formula using sodium, potassium, chloride, and bicarbonate from the same blood panel. The result splits low bicarbonate into two broad categories: high anion gap (meaning unmeasured acids are accumulating in your blood) and normal anion gap (meaning bicarbonate itself is being lost or not produced). That distinction points the diagnosis in very different directions.
Acid Buildup in the Blood
The most common reasons for a high anion gap, where acids pile up faster than bicarbonate can neutralize them, fall into four groups: ketoacidosis, lactic acidosis, kidney failure, and toxic ingestions.
Ketoacidosis happens when the body burns fat for fuel and produces acidic byproducts called ketones. It’s a well-known complication of type 1 diabetes, but it also occurs with heavy alcohol use, prolonged fasting, and severe malnutrition. Diabetic ketoacidosis can drop bicarbonate dramatically and is a medical emergency.
Lactic acidosis develops when tissues don’t get enough oxygen and switch to a backup energy pathway that generates lactic acid. The most dangerous form occurs during shock, when blood pressure drops so low that organs are starved of oxygen. Seizures, severe lung disease, and carbon monoxide poisoning can trigger it as well. Certain medications, including metformin in the setting of acute kidney injury, are rare but recognized causes.
Kidney failure lowers bicarbonate through a different mechanism. As kidney function declines, the kidneys lose their ability to produce ammonia, which is essential for excreting acid into the urine. At the same time, each remaining kidney unit gets overloaded with more bicarbonate than it can reabsorb, so some spills into the urine. The net effect is that acid accumulates and bicarbonate falls. This is one reason people with chronic kidney disease are routinely monitored for acidosis.
Toxic ingestions round out the list. Ethylene glycol (antifreeze), methanol, and large doses of aspirin all produce acidic metabolites. Aspirin poisoning is particularly tricky because it initially causes fast breathing and respiratory alkalosis, then shifts to a worsening metabolic acidosis as lactic acid builds up from disrupted energy production in cells. By the time severe acidosis sets in, the person may be too fatigued to keep hyperventilating, and the situation deteriorates rapidly.
Direct Loss of Bicarbonate
In normal anion gap acidosis, the problem isn’t extra acid. It’s that bicarbonate is literally leaving the body, either through the intestines or the kidneys.
Diarrhea is the most common intestinal cause. The fluid your gut secretes is rich in bicarbonate, and severe or prolonged diarrhea flushes it out before it can be reabsorbed. Chronic laxative abuse, pancreatic fistulas, and surgical diversions of the intestine do the same thing. The body compensates by holding onto chloride, so blood tests typically show elevated chloride alongside the low bicarbonate.
Kidney tubular problems are the other major route. In certain conditions, the kidney’s tubules fail to reclaim bicarbonate from filtered fluid or fail to secrete acid properly. These are grouped under the term renal tubular acidosis. They can be inherited, develop alongside autoimmune diseases, or be triggered by medications.
Medications That Lower Bicarbonate
Several widely prescribed drugs can push bicarbonate down. Acetazolamide, used for glaucoma and altitude sickness, directly blocks an enzyme the kidneys need to reabsorb bicarbonate. The result is that bicarbonate spills into the urine, producing a mild but measurable acidosis.
Topiramate, prescribed for seizures, migraines, and sometimes weight loss, works through the same enzyme pathway. It causes a mild metabolic acidosis and also sharply reduces citrate in the urine, which raises the risk of kidney stones. If you’re taking topiramate and your blood work shows low bicarbonate, the drug is a likely explanation.
Sulfonamide antibiotics have similar enzyme-blocking activity, though this is mainly a concern when large amounts are absorbed, such as through burn wounds treated with topical formulations.
Compensation for Breathing Changes
Not every case of low bicarbonate means something is wrong with your metabolism. When you chronically hyperventilate, you blow off too much carbon dioxide, making your blood more alkaline. Your kidneys respond by dumping bicarbonate into the urine to bring pH back toward normal. This is a healthy, expected compensation, not a disease.
The classic example is altitude acclimatization. At high elevations, low oxygen triggers faster and deeper breathing, which lowers CO2 and creates a respiratory alkalosis. Over days, the kidneys reduce acid excretion and increase bicarbonate excretion to rebalance blood pH. A blood test taken at altitude would show low bicarbonate even though the body is functioning as it should. Anxiety-driven hyperventilation and certain lung conditions can produce the same pattern.
What Low Bicarbonate Feels Like
Mild drops in bicarbonate often produce no symptoms at all and are caught only on routine lab work. As acidosis worsens, the body tries to compensate by breathing more deeply. This pattern, sometimes called Kussmaul breathing, involves slow, deliberate, unusually deep breaths rather than rapid panting. It’s the body’s attempt to blow off CO2 and reduce acidity.
Other symptoms depend on the underlying cause. Fatigue, confusion, and drowsiness are common. In kidney failure, you might notice dry, itchy skin and a general sense of feeling unwell. Severe acute acidosis can cause dangerously low blood pressure and, in extreme cases, loss of consciousness.
When Bicarbonate Stays Low Long-Term
Chronic low bicarbonate is most often seen in people with kidney disease. It’s not just a lab number to monitor. Sustained acidosis accelerates the decline of remaining kidney function, breaks down muscle protein, and weakens bones as the body pulls alkaline minerals from the skeleton to buffer the excess acid.
Current kidney disease guidelines recommend starting treatment when bicarbonate falls below 18 mEq/L. Evidence from clinical trials supports this threshold: among people with chronic kidney disease and bicarbonate below 18, those who achieved higher bicarbonate levels experienced slower progression of their kidney disease. Treatment typically involves oral alkali supplements, essentially pills that supply the bicarbonate the kidneys can no longer produce on their own. Dietary changes that reduce the body’s daily acid load, such as increasing fruit and vegetable intake, can also help.