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Seasonal changes in blood count (haematocrit and haemoglobin values) have been described in studies of small samples. Since Israel is characterized by high summer temperatures, we attempted to determine whether there are circannual changes in blood count, and how they are influenced by mean monthly temperature. In a cross-sectional study, venous blood was drawn from 4317 male employees in industry aged 20-64 years. Blood count values were analysed by month. To estimate possible changes in serum osmolality, serum electrolytes were also examined. Values of haemoglobin and haematocrit in August were significantly lower than during the rest of the year. Circannual rhythms explained between 10% and 57% of the variances of all the variables as assessed by cosinor rhythmometry, although they had wide confidence intervals and most were not statistically significant. The lower mean haematocrit in August reflected reduction in mean corpuscular volume (MCV) rather than a decrease in red blood cell count (RBC). The change observed in haematocrit and haemoglobin in August is possibly related to heat acclimatization rather than to endogenous sinusoidal circannual rhythms. Both plasma and red cell volume expansion appear to occur. Possible seasonal changes should be taken into account in evaluation of blood counts.
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