wie viel wasser trinken vor blutabnahme — which blood markers matter?

The question of how much water to drink before a blood draw depends on which markers are being measured. For most routine blood tests (e.g., complete blood count, kidney function, thyroid hormones), clear water in moderate amounts (one to two glasses) is allowed and even recommended to improve vein filling. However, certain markers are affected by food or fluid intake: fasting glucose, triglycerides, LDL cholesterol, and oral glucose tolerance tests require strict fasting from food for 8–12 hours – water is usually permitted, but large volumes could dilute the blood and alter values like sodium or hemoglobin. HbA1c is not affected by short-term fluid intake. For specialized tests such as cortisol day profiles or renin/aldosterone, fluid intake must be carefully controlled. Too much water (over 1 liter shortly before the draw) can cause dilutional effects, falsely lowering creatinine, BUN, and electrolytes. Dehydration, conversely, raises hemoglobin and hematocrit. The evidence is based on clinical practice and guidelines, not on randomized trials. For individual recommendations, consult the lab or physician. In summary: one to two glasses of water are safe for most tests, but for specific fasting panels, follow the lab's exact instructions.