The anion gap is a medical diagnostic measure used to assess the balance between positively charged ions (cations) and negatively charged ions (anions) in the blood. It is primarily used to evaluate acid-base imbalances, which can indicate if the blood is too acidic (acidosis) or not acidic enough (alkalosis). The anion gap is calculated using the formula:
Anion Gap=(Na++K+)−(Cl−+HCO3−)\text{Anion Gap}=(\text{Na}^++\text{K}^+)-(\text{Cl}^-+\text{HCO}_3^-)Anion Gap=(Na++K+)−(Cl−+HCO3−​)
where Na+ is sodium, K+ is potassium (sometimes omitted in practice for simplicity), Cl- is chloride, and HCO3- is bicarbonate. Normally, the anion gap ranges from about 4 to 12 mmol/L, reflecting unmeasured ions such as proteins (mainly albumin), phosphate, and other anions or cations not included in the calculation.

• A high anion gap usually indicates metabolic acidosis, caused by excess acids in the blood such as in diabetic ketoacidosis, lactic acidosis, ingestion of toxins (like antifreeze or aspirin), kidney failure, or prolonged diarrhea.
• A low anion gap is rare and may be seen in conditions like hypoalbuminemia (low albumin levels), often due to malnutrition or kidney disease.

Overall, the anion gap is a useful clinical tool that helps healthcare providers identify and diagnose underlying health problems related to electrolyte and acid-base imbalances in the body, guiding appropriate treatment. It is derived from standard blood electrolyte tests called an electrolyte panel. In summary, the anion gap is the numerical difference between major measured cations and anions in the blood that helps indicate acid-base balance and potential health issues related to this balance. If you want, I can explain how various conditions affect the anion gap or how it is interpreted in specific clinical scenarios. Let me know!