Clinicians measuring total testosterone levels in men with erectile dysfunction (ED) who are younger than 45 years should have blood drawn as close to 7 AM as possible, according to researchers.

The reason is that these men experience a clinically relevant decline in total testosterone (TT) levels over the course of the day, R. Charles Welliver Jr., MD, of Southern Illinois University in Springfield, and colleagues reported online ahead of print in The Journal of Urology.

“Men older than 45 years with ED can have their TT drawn at any time before 2 PM without fear of misleading results,” they observed.

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Dr. Welliver’s team reviewed the charts of 2,569 men with ED, looking at TT and blood draw times and comparing results by age group. The mean TT levels for 7-9 AM versus 9 AM.-2 PM were both clinically and statistically different only for men younger than 40 and 40-44. The levels for the men in these age groups were 207 and 149 ng/dL lower, respectively, during 9 AM-2 PM compared with 7-9 AM.

The investigators noted that, based on studies showing the circadian rhythmicity of testosterone levels, the optimal time of day to draw blood to measure testosterone levels classically has been reported as between 8 and 11 AM. Additionally studies, however, have shown that the circadian rhythmicity of testosterone levels becomes blunted as men age.