Objective To explore the value of blood lactic acid and its 6 h lactic clearance in the prognostic evaluation of severe patients.Methods 58 cases were chosen in critically ill patients,their arterial blood lactic acid level was tested on admission,and re-tested 6 h after treatment.The 6h blood lactic clearance and corresponding APACHE Ⅱ score were calculated.According to the morbid outcome,patients were divided into the survival group(n=43) and the death group(n=15).According to the patients' blood lactic acid concentration on admission,patients were divided into hyperlactacidemia group(n=38) and lactic acidosis group(n=20).According to the 6h blood lactic clearance,patients were divided into low 6h lactic clearance group(n=23) and high 6 h lactic clearance group(n=35).Results Compared with the death group,the 6h blood lactic clearance of the patients in the survival group was higher(P0.01),and there was no significant difference in the blood lactic acid and APACHE Ⅱ score between two groups on admission(P0.05).The APACHE Ⅱscores and fatality rate of patients in hyperlactacidemia group were lower than those in lactic acidosis group(P0.05).The mortality rate in the low 6 h lactic clearance group was higher than the high 6 h lactic clearance group(P0.05),while the APACHE Ⅱ score of patients in two groups had no significant difference.In severe patients,the APACHE Ⅱ score was along with the blood lactic acid level increasing gradually(P0.05).Conclusion Blood lactic acid level could be used as the index to judge the severity degree of severe patients,and 6h lactic clearance should have more significance for prognosis evaluation.