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As leptin is the object of intensive clinical research, we compared the radio-immunological assay (RIA) and enzyme-linked immunosorbent assay (ELISA) commercially available for measuring its plasma concentration in humans (Study 1), and sought to determine the power of a single plasma leptin measurement to characterise adequately a subject within a population on the basis of its intra- and inter-individual variations (Study 2).Study 1--Plasma leptin concentrations were determined by means of RIA and ELISA in a sample of 80 males. The measurements obtained using the two methods were closely correlated (r = 0.942), but the bias of the means was 21.1 +/- 73.5\% (M +/- SD, p < 0.001) and indicated that the two assays were not in agreement with each other. As expected, there were strong statistical associations between plasma leptin and a number of anthropometric indices, but the slopes of the regression of leptin concentration was significantly steeper when measured by ELISA. Study 2--ELISA was used to measure plasma leptin concentrations in three different samples obtained from 12 males and 12 females at two-week intervals. The inter-individual variation in plasma leptin was much greater than its intra-individual variation (the ratio of intra-to inter-individual variance = 0.05 and 0.04 in males and females, respectively), thus suggesting that a single fasting measurement is sufficient to characterise an individual's plasma leptin level within a population.ELISA is at least as effective as RIA in measuring plasma leptin, and is fully suitable for epidemiological investigations. A single measurement made in the morning and under fasting conditions is sufficient to characterise an individual within a population.

The aim of this study was to analyze the relationships among impulsivity, intelligence and P300, a well-known component of the event-related potential widely studied in personality and intelligence research. Eighty-two males completed the Barratt Impulsiveness Scale and the two-subtest form of the Wechsler Abbreviated Scale of Intelligence. A subsample of 45 participants (mean age = 24.4, SD = 4.6) performed a visual oddball task, consisting of a two-letter recognition task, during which psychophysiological data were recorded. Although no significant relationships emerged for P300 latency, overall results suggest that the P300 amplitude was positively related to IQ and negatively related to impulsivity. Those who scored high on impulsivity (high impulsives) had lower P300 amplitudes than low impulsives, but this relationship was not significant when controlling for individual differences in mental ability. The results also showed an inverse relationship between mental ability and impulsivity. That is, high impulsives demonstrated reduced cognitive performance on intelligence testing and it is reflected in their reduced P300 amplitude. These findings are likely due to high impulsives' less efficient ability to inhibit task-irrelevant information or to ignore additional information intake. It was suggested that impulsivity exerts a disadvantageous influence on the performance of tasks (such as those used on intelligence tests) in which exclusive concentration and sustained attention are necessary.

A reduction in mean erythrocyte volume has been reported in some strains of genetically hypertensive rat, and more recently it has been suggested that a similar alteration might be found in human essential hypertension. The relationship between erythrocyte volume and blood pressure was therefore studied in a random sample of an untreated male working population (n = 317; age 45.1 +/- 6.4 years, mean +/- s.d.). Neither systolic nor diastolic blood pressures were found to be related to erythrocyte volume (r = 0.022 and r = -0.014, respectively); in fact, erythrocyte volume was not different across quintiles of blood pressure. Smokers (n = 171) had lower blood pressure and a greater erythrocyte volume than non-smokers or ex-smokers (n = 144; 91.6 +/- 4.7 versus 88.2 +/- 5.5 fl; P less than 0.001), and heavy drinkers (greater than 110 g ethanol/day) had higher blood pressure and a greater erythrocyte volume compared with the rest of the study population (P less than 0.01). However, after adjustment of erythrocyte volume for these two potentially confounding factors, again no statistical association was found with blood pressure. The present study, therefore, does not support the hypothesis of a negative association between erythrocyte volume and blood pressure, whereas it confirms that the smoking habit and habitual alcohol intake are strong determinants of erythrocyte volume.