Why preparation matters
Many values change over the day and respond to food, exercise, sleep and stress. Glucose, insulin and triglycerides rise after eating, while cortisol and testosterone have a pronounced morning peak. Laboratories set reference ranges for standard conditions – usually a fasting morning draw – and these ranges differ between labs. If conditions are not met, a result can look worse or better than it really is. And even a value “within range” is not necessarily optimal from a prevention point of view, which is why it makes sense to follow your own trend under the same conditions.
What can distort results
- Food and drinks – breakfast, sweetened coffee or juice raise glucose and insulin; a heavy, fatty dinner can still raise triglycerides the next morning.
- Alcohol – even the day before, it can raise triglycerides, uric acid and liver enzymes.
- Intense training – strenuous exercise 24–48 hours before the test raises muscle enzymes and liver tests and can affect glucose and hormones.
- Sleep, stress and time of draw – a short night, rushing or a late-afternoon draw change cortisol and testosterone.
- Supplements and illness – high-dose biotin can distort hormone tests; an acute infection changes inflammatory markers, iron and hormones.
How to prepare
- Fast for 8–12 hours – a light dinner the evening before, only plain water in the morning; avoid fasting much longer than recommended.
- Morning between 7 and 9 a.m. – after normal sleep; for hormone tests in women, take into account the cycle phase your doctor specifies.
- No alcohol or heavy exercise – skip alcohol and intense training for 1–2 days before, do not smoke in the morning, and have your coffee after the draw.
- Supplements and medicines – pause biotin-containing supplements as the lab advises; ask your doctor about the timing of medicines – do not stop them on your own.
- Calm before the draw – drink water, arrive with time to spare and sit quietly for 5–10 minutes before blood is taken.
When to talk to a doctor
Talk to your doctor in advance if you have diabetes or take medicines that can cause low blood sugar when fasting, if you are pregnant, have an eating disorder or are acutely ill. Your doctor will tell you whether to postpone the test and when to take your medicines. If a result differs markedly from previous ones, ask for a repeat under standard conditions.