USE OF DIFFERENT ANTICOAGULANTS IN TEST TUBES FOR ANALYSIS OF BLOOD LACTATE CONCENTRATIONS .2. IMPLICATIONS FOR THE PROPER HANDLING OF BLOOD SPECIMENS OBTAINED FROM CRITICALLY ILL PATIENTS

Objectives: a) To test the hypothesis that the measurement of the circ ulating lactate concentration is influenced by the anticoagulant in th e test tube that contains the blood sample; b) to test the hypothesis that the measurement of the circulating lactate concentration is influ enced by the tissue used for analysis. Design: A prospective, controll ed study. Setting: A critical care research laboratory, a 20-bed inten sive care unit (ICU), and the general wards. Subjects: Twenty-three IC U and ward patients with hyperlactatemia and 19 healthy volunteers. In terventions: Blood samples were collected for determination of blood l actate concentration. Measurements and Main Results: Venous blood samp les (12 mt) were obtained from each of the 19 normal subjects and each 12-mL specimen was evenly divided into six aliquot portions (six test tubes). Experiment 1: Of the six tubes, two tubes were set aside for experiment 2. The other four tubes were used to test four anticoagulan ts (one anticoagulant per tube). The anticoagulants tested were: sodiu m heparin; EDTA; lithium heparin; and sodium citrate. Lactate concentr ations were analyzed using an ion-selective, amperometric electrode th at we have previously validated. There were no statistically significa nt differences between the lactate concentrations derived from blood s amples stored in sodium heparin, EDTA, or lithium heparin (p >.05; n=1 9; Student-Newman-Keuls' multiple comparisons test). The lactate conce ntration of blood stored in sodium citrate, however, was lower than al l other anticoagulants (p <.001; n=19; Student-Newman-Keuls' multiple comparisons). Experiment 2: Of the remaining two test tube samples fro m each subject, one tube contained sodium heparin and the other tube d id not contain an anticoagulant. Each of these two tubes was centrifug ed at 50 degrees F (10 degrees C) for 15 mins to obtain plasma and ser um samples. Lactate concentrations were measured in the serum and plas ma and compared with those concentrations found in whole blood samples from the tube containing sodium heparin from experiment 1. The plasma and serum lactate concentrations were consistently higher than the wh ole blood lactate values from the same specimen (p <.05; n=42; Student -Newman-Keuls' multiple comparisons test). Since experiment 1 involved the collection of blood from healthy volunteers with normal lactate c oncentrations, we chose to investigate whether this discordance betwee n plasma or serum and whole blood was dependent on the lactate concent ration. To answer this question, we studied 23 patients with known hyp erlactatemia and found that in subjects with a lactate concentration o f <2.2 mmol/L, there was a difference of 0.11 mmol/L in the mean value s between plasma and whole blood concentrations (p <.0004; n=19; paire d t-test). In subjects with a lactate concentration of >2.2 mmol/L, th ere was a difference of 0.14 mmol/L (p <.0001; n=23; paired t-test) in the mean values between plasma and whole blood. In all samples at all concentrations, there was no significant difference between serum vs. plasma samples (p >.05; Student-Newman-Keuls' test). Conclusions: a) Sodium citrate, as an anticoagulant, caused lower lactate concentratio ns to be measured as compared with heparin or EDTA; b) the measurement of lactate concentrations in plasma or serum samples yields a higher value than the concentration found in the original whole blood specime n.