Comparison of paraoxonase 1 measurements in serum and in lithium-heparin-anticoagulated plasma samples.

To the Editor: Paraoxonase 1 (PON1) is a HDL-associated enzyme that catalyzes the hydrolysis of lipid peroxides in LDL and HDL and has been postulated as a member of the plasma antioxidant system. Decreased PON1 activity has been associated with atherosclerosis in persons with diabetes mellitus, familial hypercholesterolemia, and renal disease (1, 2). Serum is the preferred sample for PON1 measurement because this enzyme requires calcium for both activity and stability. The presence of calcium chelators such as EDTA or citrate as anticoagulants inhibits PON1 activity (3). This is a serious limitation in retrospective studies, in which serum is not always available. Moreover, in studies on experimental animals, in which the amount of blood collected is often minimal, it is more convenient to use anticoagu-lants because the recovery of plasma is generally higher than that of serum and there is no interference by the clotting process. Lithium heparin is an anticoagu-lant used extensively in laboratories around the world. Although it has been reported that lithium inhibits PON1 activity (4), several groups have reported studies on PON1 activity in lithium-heparin-treated samples, and the results obtained were consistent with those obtained in serum (5, 6). However, to the best of our knowledge, the reliability of lithium-heparin plasma samples has not been clearly demonstrated. The present study was designed to investigate the degree of agreement between measurements of PON1 activity and concentration in serum and in lithium-heparin-anticoagu-lated plasma samples. We used samples from 100 consecutive patients attending the outpatient facility of Hospital Universitari de Sant Joan for routine biochemical analysis. Blood was collected into two different tubes: BD Vacutainer ® (Becton Dickinson) with serum sepa-rator (SST TM II Plus, 13 ϫ 75 mm), and BD Vacutainer with lithium hep-arin (LH 68 IU Plus, 13 ϫ 75 mm). After the requested conventional tests were performed, the remaining portions were stored at Ϫ80 °C for PON1 measurements. The use of sample leftovers for methodologic assessments is in agreement with the European Law for Medical and Diagnostic Products. PON1 activity and concentration were measured as described previously (7, 8). Because neither PON1 activity nor concentration followed a gaussian distribution, we analyzed differences between groups by Wilcoxon rank-sum test. The results are reported as medians and 95% confidence intervals (95% CIs). The associations between measurements in serum and plasma were analyzed by Deming regression (9). The degree of agreement between both types of samples was estimated by …