Arterial ammonia levels as a predictor of mortality in acute liver failure

Bhatia V et al. (2006) Predictive value of arterial ammonia for complications and outcome in acute liver failure. Gut 55: 98–104

In patients with acute liver failure (ALF), the brain is exposed to high levels of ammonia, which has a neurotoxic effect. Researchers in India investigated the relationship between ammonia levels at admission and patient outcome by following 80 patients with ALF to either recovery or death.

The median arterial ammonia concentration at admission was 128.6 µmol/l. The median time from admission to death (42/80 patients) was 4 days. Median arterial ammonia concentration was significantly higher in patients who died (174.7 µmol/l) than in survivors (105.0 µmol/l, P <0.001). By regression analysis, an arterial ammonia level of ≥124 µmol/l was found to have a sensitivity of 78.6% and specificity of 76.3% as a predictor of death (P <0.001). Ammonia levels above this value had an odds ratio of 10.9 as a predictor of death (95% CI 5.9–284.0). Other factors found to be highly predictive of death were cerebral edema (odds ratio 12.6; 95% CI 1.5–108.5) and blood pH of 7.4 or below (odds ratio 6.6; 95% CI 0.8–57.5). These factors were incorporated into an equation predicting mortality risk, scoring risk factors as 0 if absent, or 1 if present: z = 2.53 + 2.91(ammonia) + 2.41(edema) + 1.40(pH). The probability of death, P_x, could then be calculated as P_x = 1/(1 + e^−z), where e is the Euler number, approximating to 2.718.

The authors conclude that arterial ammonia levels are predictive of patient outcome and can be used for risk stratification, and that their results provide a rationale for the use of ammonia-lowering treatment in patients presenting with ALF.