Which physiological change can affect the results of lab tests if blood is collected at different times of the day?

Hormonal fluctuations are known to cause variations in laboratory test results depending on the time of day blood is collected. Many hormones, such as cortisol, insulin, and growth hormone, follow a diurnal rhythm, meaning their levels fluctuate throughout the day based on the body's circadian clock. For example, cortisol levels are typically higher in the morning and lower in the evening. If blood is collected at different times, the results may not accurately reflect the true underlying physiological state of the patient, leading to potentially misleading interpretations.

Other factors like blood volume changes, skin temperature variations, and venous pressure differences can influence test outcomes, but they do not display the same level of systematic diurnal variation as hormonal levels. Hence, hormonal fluctuations stand out as the most significant physiological change impacting lab results based on the time of blood collection.