The Urine Anion Gap (UAG) is a clinical calculator primarily used in the evaluation of patients with metabolic acidosis, specifically to differentiate between renal and extrarenal causes. It is calculated using the formula: UAG = (urine sodium + urine potassium) - urine chloride. A positive UAG suggests a renal cause of metabolic acidosis, such as renal tubular acidosis, while a negative UAG is indicative of an extrarenal cause, such as diarrhea. It is important to note that the UAG is not applicable in patients with mixed acid-base disorders or those with significant proteinuria or ketoacidosis, as these conditions can interfere with the accuracy of the calculation.