Fasting blood tests in Australia: what the rules actually are

How long. Eight to twelve hours is the standard window for fasting glucose, insulin and a lipid profile. Longer is not better — a fast beyond about fourteen hours starts to shift free fatty acids and can nudge glucose upward through the dawn response, which is the opposite of what you want.

What you can have. Water, freely, and you should — mild dehydration concentrates the sample and can push haematocrit, albumin and urea up. Black coffee is not neutral: caffeine acutely affects insulin sensitivity and can move glucose and lipids, so leave it until after the draw. No tea, no juice, no milk, no chewing gum with sugar. Take prescribed medicines as normal unless your own doctor has told you otherwise.

Which markers need it. Fasting glucose, fasting insulin, HOMA-IR and triglycerides genuinely need the fast. HbA1c does not — it reflects roughly three months of average glucose and can be drawn any time. Total cholesterol, HDL and LDL move only modestly with a meal, but because triglycerides feed the LDL calculation, most laboratories still want the profile fasted. Hormones are timed, not fasted: what matters for testosterone and cortisol is a morning draw, ideally 7am–10am.

What else moves the numbers. A hard training session in the 24 hours before a draw raises creatine kinase and can lift liver enzymes. Alcohol the night before affects triglycerides and liver markers. Acute illness suppresses testosterone and raises inflammatory markers. Biotin supplements — common in hair and nail products at high doses — interfere with several immunoassays including thyroid and can produce dramatically wrong results, so stop them for a few days beforehand and tell the collector you have taken them.