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CARDIFF, UNITED KINGDOM. Researchers at the University of Wales report that older men with high
blood viscosities have slower reaction times and poorer cognitive function than do men with less viscous
blood. Their study involved 2154 men aged 55 to 69 years at the time of blood sampling and testing of
cognitive function. The men were tested for reaction times and cognitive function using the AH4 test, which
involves a series of alternating verbal and mathematical questions of increasing difficulty. Men with the
lowest blood viscosity had the highest (best) scores. Fibrinogen level showed no association with cognitive
function or reaction time. Hematocrit showed a U-shaped relationship with men with a hematocrit level of
0.46 having the best performance. Hematocrit (packed cell volume) is the volume of red blood cells in
blood, expressed as a fraction of the total volume of the blood.
The researchers point out that hematocrit can be lowered by venesection (blood letting) and that several
studies have shown that people with mental confusion or reduced alertness can benefit substantially from
having their hematocrit lowered by venesection. Dr. Gordon Lowe of the Royal Infirmary in Glasgow points
out, in an accompanying editorial, that drinking plenty of water can also lower blood viscosity and
hematocrit.
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