INFLUENCE OF HEMATOCRIT ON THE MEASUREMENT OF LIPOPROTEINS DEMONSTRATED BY THE EXAMPLE OF LIPOPROTEIN(A)

Background. The measurement of many parameters of human blood is usual ly performed in plasma or serum. Since lipoproteins or apolipoproteins , for example, are found almost exclusively in the plasma fraction aft er low-speed centrifugation, these parameters can be expected to be di stributed in a different plasma volume depending on the hematocrit val ue. Therefore, the measured plasma levels might be relatively too low or too high in comparison to the whole blood concentrations in the cas e of abnormal hematocrit levels. The aim of our experiments was to eva luate the extent of differences between whole blood and plasma concent rations, taking as an example lipoprotein(a) [Lp(a)] in hemodialysis p atients with documented decreased hematocrit values. Methods. Lp(a) wa s measured ill plasma as well as whole blood of 15 hemodialysis patien ts with low hematocrit values (0.29 +/- 0.02) in comparison to 11 cont rol subjects (0.45 +/- 0.04). Results. Plasma concentrations were 27% higher in patients than in controls (19.7 vs. 15.5 mg/dl). The relativ e difference was twice as high (59%) when measured in whole blood (13. 5 vs. 8.5 mg/dl). Similar relative differences were observed when whol e blood concentrations of 125 hemodialysis patients and 256 controls w ere calculated with the formula [Lp(a)(pasma) (1-hematocrit)]. Concl usions. Our findings clearly demonstrate that hematocrit is a strong c onfounding variable of lipoprotein measurement in epidemiological stud ies when concentrations are measured in plasma, especially in cases of abnormal hematocrit values. Furthermore, studies investigating the lo ngitudinal changes of lipoproteins should consider potential hematocri t changes. Lipoproteins and apolipoproteins are usually measured in pl asma or serum [1]. This is convenient and can be done in frozen sample s. Whole blood measurements are impossible in many cases since especia lly colorimetric assays are disturbed by high levels of bilirubin or h emoglobin. Handling of whole blood samples using micropipettes or auto mated pipetting also often disturbs the measurement due to pipette clo gging. Despite: the uncontested advantages of measurement in plasma or serum, the possible influence of abnormal hematocrit values in case-c ontrol studies and of fluctuating hematocrit values in longitudinal st udies remains to be evaluated. During recent years several studies des cribed an association between high lipoprotein(a) [Lp(a)] plasma conce ntrations and coronary heart disease [2, 3]. Patients with renal disea se have an increased risk for coronary heart disease [4, 5] and high L p(a) plasma concentrations [6-13; reviewed in 4]. This patient group a lso suffers from renal anemia with low hematocrit values. The aim of t his study was, therefore, to investigate the influence of hematocrit v alues on the measurement of parameters distributed exclusively in the plasma fraction after low-speed centrifugation. We attempted to illust rate this question by measuring Lp(a) in hemodialysis patients. We dem onstrate that Lp(a) from the whole blood of hemodialysis patients is d istributed in a higher plasma volume after centrifugation due to the l ow hematocrit levels in these patients. Therefore, the amount and poss ibly also the clinical relevance of Lp(a) in these patients is underes timated in relative terms when measured as plasma instead of whole blo od concentration (Fig. 1).