warum kein kaffee vor blutabnahme — what does this mean exactly?

The advice to avoid coffee before a blood draw is based on the fact that caffeine and other coffee compounds can transiently alter several lab values. Most importantly, fasting glucose, triglycerides, and cortisol levels are affected. Caffeine stimulates adrenaline and cortisol release, which can raise blood sugar and blood pressure. The diterpenes in coffee (cafestol, kahweol) can influence lipid metabolism, especially in unfiltered coffee. Coffee also has a mild diuretic effect, potentially making veins harder to access. Standard clinical guidelines recommend a strict 8–12 hour fast with only water allowed. The evidence is strong and rooted in decades of clinical practice („practice“). However, individual responses vary; for some people the effects are minimal. For certain tests like genetic or microbiome analyses, coffee is usually not a problem because they do not depend on acute metabolic changes. Nevertheless, to ensure accurate and comparable results, it is safest to abstain from coffee before a blood draw. Always follow the specific instructions given by your healthcare provider.