Which specific scoring system is universally applied to adult liver transplant candidates to objectively prioritize allocation based on their mortality risk?

The specific scoring system universally applied to adult liver transplant candidates to objectively prioritize allocation based on their mortality risk is the Model for End-Stage Liver Disease, commonly known as the MELD score. The MELD score is a numerical scale ranging from 6 (least ill) to 40 (gravely ill) that estimates the severity of a patient's liver disease and their risk of dying within 90 days without a liver transplant. It provides an objective measure for organ allocation by quantifying a patient's medical urgency. The MELD score is calculated using a mathematical formula that incorporates three routine laboratory values: bilirubin, international normalized ratio (INR), and creatinine. Bilirubin is a yellowish pigment produced during the breakdown of red blood cells, and elevated levels indicate that the liver is not adequately processing and removing waste from the body. The INR measures how long it takes for a patient's blood to clot, reflecting the liver's ability to produce essential clotting factors; a higher INR indicates poorer liver function. Creatinine is a waste product filtered by the kidneys, and elevated levels can signify kidney dysfunction, which is often associated with advanced liver disease. To further enhance accuracy in predicting mortality and thus improve allocation, the MELD score was later refined to include serum sodium levels, becoming the MELD-Na score. Sodium is an important electrolyte, and lower-than-normal serum sodium (hyponatremia) in patients with liver disease is an independent predictor of increased mortality risk. Patients with higher MELD-Na scores are considered to be at a greater risk of death without a transplant and are therefore given higher priority on the national liver transplant waiting list. This objective system ensures that the sickest patients, who are most likely to benefit immediately from a liver transplant, are prioritized for organ allocation, reducing subjective bias and promoting fairness in the distribution of a scarce and life-saving resource.