Practical guide · Preparing for your blood test
Fasting sounds simple and gets fudged constantly: a coffee on the way in, a late snack, a gym session the night before. For most markers it does not matter. For a handful, it changes the result enough to change the conclusion. Here is exactly what to do, why, and what happens if you get it wrong.
Applies to all TrueVitals panels. If your GP or a specialist has given you different instructions for a specific test, follow theirs.
The short answer
That is the whole instruction for a TrueVitals test, and for most comprehensive panels. The reason is that a comprehensive panel includes the four markers that genuinely need a fasted sample, glucose, fasting insulin, triglycerides and C-peptide, alongside several that are simply more accurate in the morning. Fasting for 10 to 12 hours and testing before 10am gets all of them right in one visit.
If your test is a single marker, the answer may be different. Vitamin D, B12, ferritin, thyroid, a full blood count and HbA1c do not need fasting at all. Below is the marker-by-marker version so you know what your fast is actually protecting.
Marker by marker
| Marker | Fasting needed? | Why |
|---|---|---|
| Glucose | Yes | Rises within minutes of eating and stays up for two hours or more. A non-fasted glucose cannot be compared with the diabetes and prediabetes thresholds. |
| Fasting insulin, HOMA-IR, C-peptide | Yes | The entire point of fasting insulin is the baseline. After food it can be five times higher, which would make anyone look insulin resistant. |
| Triglycerides | Yes | Can double after a fatty meal and stay raised for eight hours. Also pushed up by alcohol the night before. |
| LDL cholesterol (calculated) | Helpful | Usually calculated from triglycerides, so a non-fasted sample can make LDL read artificially low. |
| Iron, transferrin saturation | Morning | Serum iron peaks in the morning and falls through the day. Take iron supplements after the draw, not before. |
| Cortisol, testosterone | Morning | Both peak shortly after waking. Their reference ranges assume an 8 to 10am sample; an afternoon draw reads low and is hard to interpret. |
| ApoB, Lp(a), HDL, total cholesterol | Minor | Barely affected by a meal. Fasting is still worthwhile because they are reported alongside triglycerides. |
| HbA1c, thyroid, FBC, vitamin D, B12, folate, ferritin, liver, kidney, CRP | No | Stable across the day and unaffected by food. Fasting does no harm; it simply is not why you are fasting. |
Every TrueVitals panel includes glucose, insulin and triglycerides, so the fasting instruction applies to all of them. Signature draws at Randox clinics follow the same rule.
The rules
Plain or sparkling, as much as you like, right up to the appointment. Being well hydrated plumps up your veins and makes the draw faster and more comfortable. Dehydration also concentrates the blood and can nudge haemoglobin, kidney markers and protein upward.
Even black. Caffeine raises cortisol and adrenaline, shifts glucose, and coffee's own compounds affect triglycerides. Milk or sugar breaks the fast outright. Have it as soon as the draw is done; the clinic is usually next to somewhere that sells it.
Sugar-free gum still triggers an insulin response through taste and chewing, and most contain sorbitol or xylitol. Brushing your teeth is fine as long as you do not swallow the toothpaste.
Alcohol raises triglycerides and liver enzymes for a day or more and lowers glucose the morning after. One or two drinks the night before is enough to move both, and a heavy night can put GGT out of range on its own.
A heavy gym session or long run raises creatine kinase, ALT, AST and creatinine for 24 to 72 hours, which reads like liver or kidney strain. It also elevates cortisol and inflammatory markers. A walk is fine. Deadlifts are not. Blood tests for gym-goers.
Nicotine raises glucose, cortisol and blood pressure and constricts the veins the phlebotomist is trying to find. Wait until after the draw.
Medication and supplements
Prescribed medication should be taken on your usual schedule with a sip of water unless your prescriber has told you otherwise. Stopping blood pressure, heart or diabetes medication for a blood test is never worth the risk. A few things are worth timing around the draw, because they change the reading rather than your health:
A dose taken an hour before the draw spikes Free T4 and can distort the thyroid picture. Take it straight after the appointment with water, then eat 30 minutes later as usual. Thyroid blood test guide.
High-dose biotin interferes with the laboratory method used for thyroid, vitamin D, troponin and several hormones, producing falsely high or low results. Most multivitamins are fine; dedicated biotin or beauty supplements at 5,000 micrograms or more are not.
An iron tablet before the draw sends serum iron and saturation through the roof for a few hours. Vitamin D is slower-acting and less of a problem, but for a clean baseline take supplements after your appointment. Ferritin blood test guide.
Gel applied that morning can contaminate the draw site and inflate the result. Apply after the appointment, and tell us you use it so the report interprets your level correctly.
Any corticosteroid, including inhalers and skin creams, suppresses cortisol. Keep taking them, but declare them on your quiz so a low cortisol is read as medication rather than adrenal disease. Cortisol blood test guide.
If you use insulin or medication that can cause hypoglycaemia, a 12-hour fast is not safe without a plan. Speak to your diabetes team about timing your dose, or book the earliest slot and eat immediately after.
The checklist
Eat normally. Do not try a special "healthy" day; the point is a picture of your real life. Skip the gym or keep it gentle, skip alcohol, and stop dedicated biotin supplements if you take them. Drink water through the evening.
Nothing but water. Set an alarm if you are prone to a midnight snack. Ten to twelve hours is the target; longer than about 16 hours starts to distort glucose and triglycerides in the other direction, so do not over-fast either.
A large glass of water on waking and another before you leave. Take medication as normal except the ones listed above. No coffee, no gum, no cigarette. Bring a snack for immediately after the draw, and go for the coffee.
Fasting is not advised for children under 12 or in late pregnancy without medical guidance. If you feel faint, dizzy or unwell while fasting, eat and rebook; a test is never worth a fall.
If it goes wrong
Tell the phlebotomist, or email us before your results are processed. Depending on what and when, either the affected markers can be interpreted as non-fasting (a small snack six hours earlier matters far less than a fry-up at 7am), or we rebook your draw for another morning at no extra charge. What we will not do is quietly report a non-fasted insulin or triglyceride as though it were fasted, because that turns a good test into a wrong one. Your report always states whether the sample was fasted.
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74 biomarkers · £269
Fasted glucose, HbA1c, triglycerides, ApoB, full thyroid, morning cortisol and testosterone, full iron studies, vitamins, liver and kidney.
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114 biomarkers · £349
Everything in Advanced plus fasting insulin, HOMA-IR and C-peptide, the markers a proper fast is really for, with Lp(a), complete hormones and tumour markers.
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230 biomarkers · £799
Processed by Randox. The deepest private panel in the UK, drawn fasted at a Randox clinic with physical measurements included.
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FAQs
Ten to twelve hours, water only. For a morning appointment that means nothing after 10pm the night before. Do not fast longer than about 16 hours, which can distort glucose and triglycerides in the other direction.
Yes, and you should. Plain or sparkling water in any quantity keeps you hydrated, makes the veins easier to find and prevents the mild concentration of the blood that dehydration causes.
No, including black coffee. Caffeine raises cortisol and affects glucose, and coffee itself influences triglycerides. Tea is the same. Have it immediately after the draw.
Glucose, fasting insulin, HOMA-IR, C-peptide and triglycerides need a genuine fast; calculated LDL benefits from it. Iron, cortisol and testosterone need a morning sample rather than a fast. HbA1c, thyroid, full blood count, vitamins, ferritin, liver, kidney and CRP do not need fasting. Comprehensive panels fast because they include the first group.
Yes, on your usual schedule with water, unless your prescriber has said otherwise. The exceptions worth timing after the draw are levothyroxine, iron supplements, testosterone gel and high-dose biotin (stop 72 hours before). Never stop diabetes, heart or blood pressure medication for a test.
Brush your teeth, yes. Chewing gum, no, even sugar-free: chewing and sweeteners trigger an insulin response and most gums contain sugar alcohols.
Hard exercise in the 24 to 72 hours before a draw raises creatine kinase, liver enzymes, creatinine, cortisol and inflammatory markers, which can look like organ strain. Keep the day before gentle. Regular training weeks earlier is fine and is part of the picture you want.
Tell the phlebotomist or email us. Depending on what you ate and when, the affected markers are either interpreted as non-fasting or your draw is rebooked for another morning at no extra charge. Your report always states whether the sample was fasted.
Between 8 and 10am. Cortisol, testosterone and iron all peak in the morning and their reference ranges assume a morning sample, and an early slot keeps the fast to a sensible length. TrueVitals clinics offer early appointments seven days a week.
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