Most people arrive at a check-up having done the one thing they were told — skipped breakfast — and having done three or four other things that quietly change the numbers. A heavy gym session the day before raises muscle enzymes and can make kidney function look worse than it is. A high-dose biotin supplement, sold everywhere for hair and nails, can push a thyroid result far enough to suggest a disease that is not there. A long cycle ride can lift PSA. None of these are exotic problems; they are the ordinary reasons a panel gets repeated, and every one of them is avoidable with three days of notice. The list below is what a well-run programme sends you in advance. If nobody sends you one, ask.
What you'll read
Fasting: how long, and what you may drink
Eight to twelve hours is the standard, counted from the end of your last meal to the blood draw, which for a morning appointment usually means finishing dinner by about ten in the evening. Longer is not better: fasting beyond fourteen hours starts to change the results in its own right, raising bilirubin and free fatty acids. Plain water is not only allowed but wanted — being well hydrated makes the vein easier to find and is necessary for an abdominal ultrasound, where a full bladder is part of the examination. Coffee, tea and chewing gum all break the fast, even without sugar, because they stimulate digestion and shift glucose. Smoking does the same. If you take a morning walk to the clinic, that is fine; a run is not.
Medicines and supplements
The general rule is that prescribed medicines are taken as usual, with a sip of water. Blood pressure tablets, heart medicines, thyroid replacement, antiepileptics, inhalers and blood thinners are not stopped for a check-up, and stopping them on your own initiative is far more dangerous than any effect they have on a result. What does need a conversation is diabetes medication, because taking insulin or a sulfonylurea while fasting can cause a hypo: ask the clinic in advance and bring something sugary in your bag either way. Metformin is usually held on the day if a scan with contrast is planned. Supplements are the opposite case — most are worth pausing, and one matters enough to name.
- <strong>Biotin</strong> (vitamin B7, in most hair and nail products): stop 3 days before. High doses interfere with the laboratory method used for thyroid tests and troponin, and can produce results that mimic Graves' disease or a heart attack
- <strong>Vitamin D and calcium</strong>: pause 24 hours if these levels are being measured
- <strong>Iron</strong>: pause 24 hours, or ferritin and serum iron read falsely high
- <strong>Fish oil and high-dose vitamin C</strong>: pause 24 hours
- <strong>Herbal preparations of unknown content</strong>: pause 3 days, and mention them anyway
- <strong>Never pause</strong> blood thinners, blood pressure, heart, thyroid, epilepsy, transplant or psychiatric medication without being told to
The three days before
- No heavy or unusual exercise for 48 hours — it raises muscle enzymes and creatinine, and can make kidney function look impaired
- No alcohol for 24 hours, ideally 48: it changes liver enzymes and triglycerides more than most people expect
- Eat normally otherwise. A sudden low-fat week before a cholesterol test hides the number you came to find out
- Sleep normally the night before; a night shift or a long-haul flight distorts glucose and cortisol
- Postpone by a week if you have a fever, an infection or a course of antibiotics; inflammation markers and white cells will be misleading
- If you are menstruating, urine tests and cervical screening are best moved a few days; blood tests are unaffected
Rules that belong to particular tests
| Test | What it needs from you |
|---|---|
| PSA (prostate) | No ejaculation, cycling or rectal exam for 48 hours before the blood draw |
| Abdominal ultrasound | Fasting, and a full bladder — drink water and do not empty it on arrival |
| Mammogram | No deodorant, talc or body cream on the day; book outside the week before a period if breasts are tender |
| Stool test for hidden blood | Collect before the visit; avoid it during bleeding piles or menstruation |
| CT with contrast | Kidney blood test first; tell them about metformin, iodine allergy and pregnancy |
| Colonoscopy, if included | A separate preparation with a laxative regime, usually starting the day before |
What to bring, and what to say on arrival
Bring your previous results, even old ones and even on paper. A single cholesterol value is a number; the same value next to the one from three years ago is information, and it is the comparison that decides whether anything needs doing. Bring a written list of every medicine and supplement with doses, rather than trusting memory at seven in the morning. Wear something with sleeves that roll up easily and leave jewellery at the hotel. On arrival, say out loud anything that changes interpretation: a recent infection, a new medication, a pregnancy or the possibility of one, a family history of heart disease or cancer, and any symptom you have been meaning to mention. A check-up is a screening programme, not a consultation about a specific complaint — but a symptom mentioned at reception changes which tests get added, and mentioning it after the results are printed helps nobody.
Frequently asked questions
I forgot and had coffee this morning. Do I have to come back?
Usually not the whole programme. Say so at reception rather than staying quiet: the laboratory can still run most of the panel, and only the fasting-dependent tests — glucose, insulin and the lipid profile — may need repeating on another morning. Black coffee has a smaller effect than coffee with milk or sugar, and one cup matters less than a breakfast. What causes real trouble is not mentioning it, because then a mildly raised glucose gets treated as a finding rather than as a cup of coffee.
Should I stop my blood pressure tablet on the morning of the tests?
No. Take it as usual with a sip of water. Blood pressure medication is meant to be measured while it is working, and the reading the doctor wants is the one you actually live with — a number recorded on a skipped dose describes a day you do not have. The same logic applies to heart, thyroid, epilepsy and psychiatric medicines. The short list of things sometimes held on the day is diabetes medication and, before contrast imaging, metformin — and both of those are decisions for the clinic to make, not you.
Can I do a check-up while I am on holiday and drinking a little?
You can, as long as the last 24 to 48 hours are dry. Alcohol raises liver enzymes and triglycerides quickly and clears slowly, so two evenings of holiday wine can produce a liver panel that looks like a problem and sends you home worried for nothing. If the trip is planned around the appointment, put the check-up on the first morning rather than the last: it is easier to keep two evenings clear at the start than to reorganise a holiday around a repeat blood test.
Want the preparation list for your own programme?
Send us your age, your regular medicines and any tests you already know you want. We will send back the exact preparation for that combination — the fasting window, which of your tablets to take that morning, which supplements to pause and on which day, and the appointment time that lets you eat again soonest.