Serum lactate on admission to intensive care and outcome: an observational cohort study

Introduction

Lactate is an important substrate for intermediary metabolism and allows movement of carbon and reducing power between cells [1]. Its use as a measure of end organ perfusion and as a marker of tissue hypoxia in the critically ill is well established. It is suggested that a rise in serum lactate is a sensitive indicator of poor prognosis in this group [2]. We wished to identify whether or not this was the case in our patient population.

Methods

A retrospective case note review of 504 consecutive admissions to Glasgow Royal Infirmary ICU was undertaken over an 18-month period. Serum lactate on initial blood gas analysis following admission to intensive care was retrieved from the clinical information system. Demographic, APACHE II score and outcome data were retrieved from the Ward Watcher system in the ICU.

Results

Complete data were available for 454 patients. Demographics of the patient group were as follows: 65% were male with a mean age of 53.8 ± 1.5 years. Average length of stay was 6.1 ± 0.46 days with a median APACHE II score of 18 (IQR 12 to 24). Crude ICU and hospital mortalities were 28% and 34.7%, respectively. Mean lactate was 2.46 ± 0.24 mmol/l. We constructed an ROC curve for admission lactate vs ICU mortality with an area under the curve (AUC) = 0.66, P < 0.001. ROC curve for APACHE II vs ICU mortality gives AUC = 0.80, P < 0.001. Using univariate analysis and chi-square testing we found that, using different thresholds for lactate level, we were able to establish a relative risk of death for those exceeding this level as follows (RR, P value): >2 mmol/l (RR 1.29, P < 0.001), >4 mmol/l (RR 1.63, P < 0.001), >6 mmol/l (RR 2.08, P < 0.001), >8 mmol/l (RR 3.43, P < 0.001), >10 mmol/l (RR 3.43, P < 0.001).

Conclusions

While using ROC analysis suggests serum lactate is of moderate predictive value for ICU mortality, applying serial increasing lactate cutoff points and comparing those above and below these points suggests a strong relationship between increasing lactate level and increased risk of ICU mortality.