A Reassessment of the Optimal Sampling Time for the MEGX Liver Function Test

Abstract

The measurement of a single plasma concentration of the lidocaine (lignocaine) metabolite monoethylglycinexylidide (MEGX), after a standard intravenous dose of lidocaine, is now widely used as a test of liver function. However, apart from the dose of lidocaine, the test has not been rigorously standardised, and various postinjection times, ranging from 15 to 60 minutes, have been claimed to be optimal for sampling. The purpose of this study was to re-evaluate the optimal sampling time for the MEGX test. Plasma MEGX concentrations were measured 15, 30, 45 and 60 minutes after intravenous lidocaine administration in 10 healthy volunteers and 20 age-matched patients with either grade A or grade C liver cirrhosis according to Child’s classification. Differences in MEGX concentrations between controls and patients with cirrhosis or between the 2 groups of patients with cirrhosis were statistically significant at all sampling times, but the level of significance markedly increased with increased sampling time as a result of a progressive decrease in interindividual variability within each group. Discriminant analysis showed that the MEGX test was far superior to conventional liver function tests in distinguishing between the 3 study groups. The specificity, sensitivity, diagnostic accuracy and predictive values of the MEGX test increased with the sampling time and, overall, were maximal at 60 minutes. All measured MEGX concentrations were significantly related to serum albumin, prothrombin time and Pugh’s score. The 60-minute MEGX concentration showed the strongest relationship with all 3 variables and the most significant correlation was observed with the Pugh’s score (r = −0.71, p = 0.00001). These results indicate that MEGX concentrations determined 60 minutes after lidocaine administration provide the most accurate data for discriminating between either healthy subjects and patients with cirrhosis or between patients with different degrees of liver dysfunction.