Measurement of testosterone: how important is a morning blood draw?
Level 4 - case-series / case-control
Cross-sectional observational study comparing single blood draw measurements across different participants at different times of day.
PubMed 26360789 · doi:10.1185/03007995.2015.1082994
What was done
Total testosterone (TT) levels were measured in 671 men (mean age 61 years) participating in the 2013 Prostate Cancer Awareness Week. Researchers compared TT levels from blood drawn during the standard morning window (8 to 11 AM, n = 229) against levels drawn outside this window across four time windows: 11 AM to 2 PM, 2 to 5 PM, 5 to 8 PM, and 8 PM to 8 AM (total non-morning n = 442), adjusting for age and BMI.
What was found
TT levels drawn from 8 to 11 AM averaged 411.7 ng/dl compared to 368.3 ng/dl drawn outside this window (p = 0.0003). Significant differences existed across the five time windows (overall p < 0.0001). TT levels from 2 to 5 PM (344.3 ng/dl) and 5 to 8 PM (334.4 ng/dl) were significantly lower than 8 to 11 AM (p < 0.05). TT levels from 11 AM to 2 PM (396.5 ng/dl, p = 0.90) and 8 PM to 8 AM (373.4 ng/dl, p = 0.73) did not differ significantly from the 8 to 11 AM window.
Why it matters
Guidelines mandate early morning blood draws to diagnose hypogonadism, which can create clinical scheduling constraints. These findings suggest the diagnostic sampling window might be reasonably extended to 2 PM in older men.
Limits
The study was not prospectively designed to establish an optimal sampling window. Measurements were cross-sectional rather than repeated within the same individuals across different times, and day-to-day variability was not evaluated.
Cited by
- supports Testosterone is produced nocturnally and peaks in the morning.