Causes of increased anion gap include:

- Lactic acidosis

- Ketoacidosis

- Chronic renal failure (accumulation of sulfates, phosphates, urea)

- Intoxication:

- Organic acids, salicylates, ethanol, methanol, formaldehyde, ethylene glycol, paraldehyde, isoniazid

- Sulfates, metformin

- Massive rhabdomyolysis

A mnemonic can also be used - MUDPILES

- M-Methanol

- U-Uremia (chronic kidney failure)

- D-Diabetic ketoacidosis

- P-Paraldehyde

- I-Infection, Iron, Isoniazid, Inborn errors of metabolism

- L-Lactic acidosis

- E-Ethylene glycol (Note: Ethanol is sometimes included in this mnemonic, as well, although the acidosis caused by ethanol is actually primarily due to the increased production of lactic acid found in such intoxication.)

- S-Salicylates

The several different causes of lactic acidosis include:

- Genetic conditions

- Biotinidase deficiency, multiple carboxylase deficiency, or nongenetic deficiencies of biotin

- Diabetes mellitus and deafness

- Fructose 1,6-bisphosphatase deficiency

- Glucose-6-phosphatase deficiency

- GRACILE syndrome

- Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes

- Pyruvate dehydrogenase deficiency

- Pyruvate carboxylase deficiency

- Drugs

- Linezolid

- Phenformin

- Metformin

- Isoniazid toxicity

- Propofol

- Propylene glycol (D-lactic acidosis)

- Nucleoside reverse transcriptase inhibitors

- Abacavir/dolutegravir/lamivudine

- Emtricitabine/tenofovir

- Potassium cyanide (cyanide poisoning)

- Fialuridine

- Other

- Impaired delivery of oxygen to cells in the tissues (e.g., from impaired blood flow (hypoperfusion))

- Bleeding

- Polymyositis

- Ethanol toxicity

- Sepsis

- Shock

- Advanced liver disease

- Diabetic ketoacidosis

- Excessive exercise (overtraining)

- Regional hypoperfusion (e.g., bowel ischemia or marked cellulitis)

- Cancers such as Non-Hodgkin's and Burkitt lymphomas

- Pheochromocytoma

The underlying cause determines the prognosis of lactic acidosis. In sepsis, elevated lactate levels correlate with mortality. The mortality of lactic acidosis in people taking metformin was previously reported to be 50%, but in more recent reports this was closer to 25%.

Causes include:

The newest mnemonic was proposed in "The Lancet" reflecting current causes of anion gap metabolic acidosis:

- G — glycols (ethylene glycol & propylene glycol)

- O — oxoproline, a metabolite of paracetamol

- L — L-lactate, the chemical responsible for lactic acidosis

- D — D-lactate

- M — methanol

- A — aspirin

- R — renal failure

- K — ketoacidosis, ketones generated from starvation, alcohol, and diabetic ketoacidosis

The mnemonic MUDPILES is commonly used to remember the causes of increased anion gap metabolic acidosis.

- M — Methanol

- U — Uremia (chronic kidney failure)

- D — Diabetic ketoacidosis

- P — Paracetamol, Propylene glycol (used as an inactive stabilizer in many medications; historically, the "P" also stood for Paraldehyde, though this substance is not commonly used today)

- I — Infection, Iron, Isoniazid (which can cause lactic acidosis in overdose), Inborn errors of metabolism (an especially important consideration in pediatric patients)

- L — Lactic acidosis

- E — Ethylene glycol (Note: Ethanol is sometimes included in this mnemonic as well, although the acidosis caused by ethanol is actually primarily due to the increased production of lactic acid found in such intoxication.)

- S — Salicylates

Another frequently used mnemonic is KARMEL.

- K — Ketoacidosis

- A — aspirin

- R — Renal failure

- M — Methanol

- E — Ethylene glycol

- L — Lactic acidosis

Another frequently used mnemonic is KULT.

- K — Ketoacidosis (DKA, AKA)

- U — Uremia

- L — Lactic acidosis

- T — Toxins (Ethylene glycol, methanol, as well as drugs, such as aspirin, Metformin)

The preferred mnemonic of D. Robert Dufour, the chief of the Pathology and Laboratory Medicine Service, Veterans Affairs Medical Center, is DUMPSALE, which omits the I of MUDPILES as the proposed values of *I* are exceedingly rare in clinical practice.

- D — Diabetic ketoacidosis

- U — Uremia

- M — Methanol

- P — Paraldehyde

- S — Salicylates

- A — Alcoholic ketoacidosis

- L — Lactic acidosis

- E — Ethylene Glycol

The mnemonic for the [rare, in comparison] toxins is ACE GIFTs: Aspirin, Cyanide, Ethanolic ketosis, Glycols [ ethylene and propylene ], Isoniazid, Ferrous iron, Toluene. Most of these cause a lactic acidosis.

Metabolic acidosis occurs when the body produces too much acid, or when the kidneys are not removing enough acid from the body. Several types of metabolic acidosis occur. The main causes are best grouped by their influence on the anion gap.

The anion gap can be spuriously normal in sampling errors of the sodium level, e.g. in extreme hypertriglyceridemia. The anion gap can be increased due to relatively low levels of cations other than sodium and potassium (e.g. calcium or magnesium).

When acidosis is present on blood tests, the first step in determining the cause is determining the anion gap. If the anion gap is high (>12 mEq/L), there are several potential causes.

High anion gap metabolic acidosis is a form of metabolic acidosis characterized by a high anion gap (a medical value based on the concentrations of ions in a patient's serum). An anion gap is usually considered to be high if it is over 12 mEq/L.

High anion gap metabolic acidosis is caused generally by acid produced by the body. More rarely, high anion gap metabolic acidosis may be caused by ingesting methanol or overdosing on aspirin. The Delta Ratio is a formula that can be used to assess elevated anion gap metabolic acidosis and to evaluate whether mixed acid base disorder (metabolic acidosis) is present.

The list of agents that cause high anion gap metabolic acidosis is similar to but broader than the list of agents that cause a serum osmolal gap.

In the fetus, the normal range differs based on which umbilical vessel is sampled (umbilical vein pH is normally 7.25 to 7.45; umbilical artery pH is normally 7.20 to 7.38). In the fetus, the lungs are not used for ventilation. Instead, the placenta performs ventilatory functions (gas exchange). Fetal respiratory acidemia is defined as an umbilical vessel pH of less than 7.20 and an umbilical artery PCO of 66 or higher or umbilical vein PCO of 50 or higher.

Metabolic acidosis may result from either increased production of metabolic acids, such as lactic acid, or disturbances in the ability to excrete acid via the kidneys, such as either renal tubular acidosis or the renal acidosis of renal failure, which is associated with an accumulation of urea and creatinine as well as metabolic acid residues of protein catabolism.

An increase in the production of other acids may also produce metabolic acidosis. For example, lactic acidosis may occur from:

1. severe (PaO <36mm Hg) hypoxemia causing a fall in the rate of oxygen diffusion from arterial blood to tissues

2. hypoperfusion (e.g., hypovolemic shock) causing an inadequate blood delivery of oxygen to tissues.

A rise in lactate out of proportion to the level of pyruvate, e.g., in mixed venous blood, is termed "excess lactate", and may also be an indicator of fermentation due to anaerobic metabolism occurring in muscle cells, as seen during strenuous exercise. Once oxygenation is restored, the acidosis clears quickly. Another example of increased production of acids occurs in starvation and diabetic ketoacidosis. It is due to the accumulation of ketoacids (via excessive ketosis) and reflects a severe shift from glycolysis to lipolysis for energy needs.

Acid consumption from poisoning such as methanol ingestion, elevated levels of iron in the blood, and chronically decreased production of bicarbonate may also produce metabolic acidosis.

Metabolic acidosis is compensated for in the lungs, as increased exhalation of carbon dioxide promptly shifts the buffering equation to reduce metabolic acid. This is a result of stimulation to chemoreceptors, which increases alveolar ventilation, leading to respiratory compensation, otherwise known as Kussmaul breathing (a specific type of hyperventilation). Should this situation persist, the patient is at risk for exhaustion leading to respiratory failure.

Mutations to the V-ATPase 'a4' or 'B1' isoforms result in distal renal tubular acidosis, a condition that leads to metabolic acidosis, in some cases with sensorineural deafness.

Arterial blood gases will indicate low pH, low blood HCO, and normal or low PaCO.

In addition to arterial blood gas, an anion gap can also differentiate between possible causes.

The Henderson-Hasselbalch equation is useful for calculating blood pH, because blood is a buffer solution. In the clinical setting, this equation is usually used to calculate HCO from measurements of pH and PaCO2 in arterial blood gases. The amount of metabolic acid accumulating can also be quantitated by using buffer base deviation, a derivative estimate of the metabolic as opposed to the respiratory component. In hypovolemic shock for example, approximately 50% of the metabolic acid accumulation is lactic acid, which disappears as blood flow and oxygen debt are corrected.

In general, the cause of a hyperchloremic metabolic acidosis is a "loss of base", either a gastrointestinal loss or a renal loss.

- Gastrointestinal loss of bicarbonate ()

- Severe diarrhea (vomiting will tend to cause hypochloraemic alkalosis)

- Pancreatic fistula with loss of bicarbonate rich pancreatic fluid

- Nasojejunal tube losses in the context of small bowel obstruction and loss of alkaline proximal small bowel secretions

- Chronic laxative abuse

- Renal causes

- Proximal renal tubular acidosis with failure of resorption

- Distal renal tubular acidosis with failure of secretion

- Long-term use of a carbonic anhydrase inhibitor such as acetazolamide

- Other causes

- Ingestion of ammonium chloride, hydrochloric acid, or other acidifying salts

- The treatment and recovery phases of diabetic ketoacidosis

- Volume resuscitation with 0.9% normal saline provides a chloride load, so that infusing more than 3-4L can cause acidosis

- Hyperalimentation ("i.e.", total parenteral nutrition)

Metabolic alkalosis is usually accompanied by low blood potassium concentration, causing, e.g., muscular weakness, muscle pain, and muscle cramps (from disturbed function of the skeletal muscles), and muscle spasms (from disturbed function of smooth muscles).

It may also cause low blood calcium concentration. As the blood pH increases, blood transport proteins, such as albumin, become more ionized into anions. This causes the free calcium present in blood to bind more strongly with albumin. If severe, it may cause tetany.

In the United States, hyperkalemia is induced by lethal injection in capital punishment cases. Potassium chloride is the last of the three drugs administered and actually causes death. Injecting potassium chloride into the heart muscle disrupts the signal that causes the heart to beat. This same amount of potassium chloride would do no harm if taken orally and not injected directly into the blood.

Familial disorders

- Cystinosis

- Galactosemia

- Glycogen storage disease (type I)

- Hereditary fructose intolerance

- Lowe syndrome

- Tyrosinemia

- Wilson's disease

Acquired disorders

- Amyloidosis

- Multiple myeloma

- Paroxysmal nocturnal hemoglobinuria

- Toxins, such as HAART, ifosfamide, lead, and cadmium

Respiratory alkalosis is caused by hyperventilation, resulting in a loss of carbon dioxide. Compensatory mechanisms for this would include increased dissociation of the carbonic acid buffering intermediate into hydrogen ions, and the related excretion of bicarbonate, both of which lower blood pH. Hyperventilation-induced alkalosis can be seen in several deadly central nervous system diseases such as strokes or Rett syndrome.

Metabolic alkalosis can be caused by repeated vomiting, resulting in a loss of hydrochloric acid in the stomach contents. Severe dehydration, and the consumption of alkali are other causes. It can also be caused by administration of diuretics and endocrine disorders such as Cushing's syndrome. Compensatory mechanism for metabolic alkalosis involve slowed breathing by the lungs to increase serum carbon dioxide, a condition leaning toward respiratory acidosis. As respiratory acidosis often accompanies the compensation for metabolic alkalosis, and vice versa, a delicate balance is created between these two conditions.

Hyperchloremic acidosis is a form of metabolic acidosis associated with a normal anion gap, a decrease in plasma bicarbonate concentration, and an increase in plasma chloride concentration (see anion gap for a fuller explanation). Although plasma anion gap is normal, this condition is often associated with an "increased" urine anion gap, due to the kidney's inability to secrete ammonia.

Decreased kidney function is a major cause of hyperkalemia. This is especially pronounced in acute kidney injury where the glomerular filtration rate and tubular flow are markedly decreased, characterised by reduced urine output. This can be further intensified by active cellular breakdown which causes increase in serum potassium levels. In chronic kidney disease, hyperkalemia occurs as a result of reduced aldosterone responsiveness and reduced sodium and watery deliveries in distal tubules.

Medications that interferes with urinary excretion by inhibiting the renin–angiotensin system is one of the most common causes of hyperkalemia. Examples of medications that can cause hyperkalemia include ACE inhibitors, angiotensin receptor blockers, beta blockers, and calcineurin inhibitor immunosuppressants such as ciclosporin and tacrolimus. For potassium-sparing diuretics, such as amiloride and triamterene; both the drugs block epithelial sodium channels in the collecting tubules, thereby preventing potassium excretion into urine. Spironolactone acts by competitively inhibiting the action of aldosterone. NSAIDs such as ibuprofen, naproxen, or celecoxib inhibit prostaglandin synthesis, leading to reduced production of renin and aldosterone, causing potassium retention. The antibiotic trimethoprim and the antiparasitic medication pentamidine inhibits potassium excretion, which is similar to mechanism of action by amiloride and triamterene.

Mineralocorticoid (aldosterone) deficiency or resistance can also cause hyperkalemia. Primary adrenal insufficiency are: Addison's disease and congenital adrenal hyperplasia (CAH) (including enzyme deficiencies such as 21α hydroxylase, 17α hydroxylase, 11β hydroxylase, or 3β dehydrogenase).

- Type IV renal tubular acidosis (aldosterone resistance of the kidney's tubules)

- Gordon's syndrome (pseudohypoaldosteronism type II) ("familial hypertension with hyperkalemia"), a rare genetic disorder caused by defective modulators of salt transporters, including the thiazide-sensitive Na-Cl cotransporter.

A further effect of chronic lactic acidosis in GSD I is hyperuricemia, as lactic acid and uric acid compete for the same renal tubular transport mechanism. Increased purine catabolism is an additional contributing factor. Uric acid levels of 6–12 mg/dl are typical of GSD I. Allopurinol may be needed to prevent uric acid nephropathy and gout.

In Dalmatian dogs, a lack of uricase (a genetic trait fixed in this breed) contributes to hyperuricemia and corresponding hyperuricosuria.

Developmental delay is a potential secondary effect of chronic or recurrent hypoglycemia, but is at least theoretically preventable. Normal neuronal and muscle cells do not express glucose-6-phosphatase, so GSD I causes no other neuromuscular effects.

Increased levels predispose for gout and, if very high, kidney failure. The metabolic syndrome often presents with hyperuricemia. Prognosis is good with regular consumption of Allopurinol.

People with gout, and by inference hyperuricemia, are significantly less likely to develop Parkinson's disease, unless they also require diuretics.

The main causes of hypokalemic acidosis are systemic disorders that can be divided into:

- Carbonic anhydrase inhibitors such as acetazolamide

- Dialysis, in the post-treatment

- Diarrhea

- Renal tubular acidosis

- Treated DKA with insulin therapy

- VIPoma

Proximal renal tubular acidosis (pRTA) or Type 2 Renal tubular acidosis (RTA) is a type of RTA caused by a failure of the proximal tubular cells to reabsorb filtered bicarbonate from the urine, leading to urinary bicarbonate wasting and subsequent acidemia. The distal intercalated cells function normally, so the acidemia is less severe than dRTA and the urine can acidify to a pH of less than 5.3. pRTA also has several causes, and may occasionally be present as a solitary defect, but is usually associated with a more generalised dysfunction of the proximal tubular cells called Fanconi syndrome where there is also phosphaturia, glycosuria, aminoaciduria, uricosuria and tubular proteinuria.

Patients with type 2 RTA are also typically hypokalemic due to a combination of secondary hyperaldosteronism, and potassium urinary losses - though serum potassium levels may be falsely elevated because of acidosis. Administration of bicarbonate prior to potassium supplementation might lead to worsened hypokalemia, as potassium shifts intracellularly with alkanization.

The principal feature of Fanconi syndrome is bone demineralization (osteomalacia or rickets) due to phosphate and vitamin D wasting.

No sexual predilection is observed because the deficiency of glycogen synthetase activity is inherited as an autosomal recessive trait.

The overall frequency of glycogen-storage disease is approximately 1 case per 20,000–25,000 people. Glycogen-storage disease type 0 is a rare form, representing less than 1% of all cases. The identification of asymptomatic and oligosymptomatic siblings in several glycogen-storage disease type 0 families has suggested that glycogen-storage disease type 0 is underdiagnosed.

Hypokalemic acidosis is a normal anion gap metabolic acidosis that has various direct and associated symptoms. Symptoms are associated with hypokalemia instead of hyperkalemia.

The differential diagnosis of normal anion gap acidosis is relatively short (when compared to the differential diagnosis of "acidosis"):

- Hyperalimentation

- Acetazolamide and other carbonic anhydrase inhibitors

- Renal tubular acidosis

- Diarrhea: due to a loss of bicarbonate. This is compensated by an increase in chloride concentration, thus leading to a normal anion gap, or hyperchloremic, metabolic acidosis. The pathophysiology of increased chloride concentration is the following: fluid secreted into the gut lumen contains higher amounts of Na than Cl; large losses of these fluids, particularly if volume is replaced with fluids containing equal amounts of Na and Cl, results in a decrease in the plasma Na concentration relative to the Clconcentration. This scenario can be avoided if formulations such as lactated Ringer’s solution are used instead of normal saline to replace GI losses.

- Ureteroenteric fistula - an abnormal connection (fistula) between a ureter and the gastrointestinal tract

- Pancreaticoduodenal fistula - an abnormal connection between the pancreas and duodenum

- Spironolactone

As opposed to high anion gap acidosis (which involves increased organic acid production), normal anion gap acidosis involves either increased production of chloride (hyperchloremic acidosis) or increased excretion of bicarbonate.