Fasting for a blood test: what is and is not allowed?

Fasting means not eating or drinking anything for a number of hours before the draw, with water as the only exception. Everything you take in during that period counts: even a small biscuit, a glass of juice, a spoonful of sugar in your tea or a mint breaks the fast. After every bite or sip containing calories, digestion starts, blood sugar rises and fats appear in the blood. That is precisely the response the laboratory wants to keep out of the picture for a fasted test.

Water is the exception, and for good reason: being well hydrated makes your veins easier to find, which makes the draw easier and more comfortable.

What fasting means exactly is not described the same way everywhere. The Dutch Society for Clinical Chemistry (NVKC) notes that the duration can differ per laboratory and that the instruction of your doctor or laboratory is leading, and the European federation of laboratory specialists (EFLM) calls for that reason for one agreed definition of the duration and of what is and is not allowed. That is why you will first read below what Bloodworks asks for.

For a fasted draw, Bloodworks asks for at least 8 hours without eating or drinking, except water. That matches Thuisarts.nl, the Dutch GP patient site, which describes fasting as 8 hours or more without food or drink. Some laboratories and information sources mention longer periods, for example 10 hours for a fasting glucose or 12 to 14 hours for triglycerides, because the fats in the blood stay elevated for longer after a meal than blood sugar does.

If your lab form states a duration, that duration applies. If it only says that you must be fasted, the 8 hours of Bloodworks is the minimum; longer is never wrong. If you eat nothing after dinner and have your blood drawn early the next morning, you are automatically at 10 to 12 hours; a late snack moves that line.

Fasting longer than requested is not necessary. A whole day without food does not make the result more reliable, and arriving at the collection site feeling weak or dizzy helps no one. The hours you mostly spend asleep are enough.

The rule is simple: water only. In practice it still raises questions. This is what Bloodworks applies:

• Water: allowed, and encouraged. Sparkling water too, as long as it contains no sugar, flavouring or juice.
• Coffee and tea: not allowed, not even black or unsweetened. Some laboratories allow tea without milk and sugar; because that differs per laboratory, water is the safe choice.
• Soft drinks, juice, diet and energy drinks: not allowed, not even the sugar-free versions.
• Chewing gum and mints: not allowed, not even sugar-free. Chewing sets digestion in motion.
• Alcohol: preferably not the evening before. For triglycerides and liver values, Bloodworks, like the NVKC, advises no alcohol in the 24 hours before the draw.
• Smoking: preferably not on the morning of the draw.
• Medication: you usually take it as normal, with a sip of water; according to the NVKC, that is generally allowed unless your treating doctor has indicated otherwise. With insulin or other blood sugar-lowering medication, consult your GP or pharmacist beforehand.
• Brushing your teeth: allowed. Rinse well and do not swallow the toothpaste.
• Strenuous exercise: preferably not right before the draw; a hard workout can temporarily raise muscle values such as CK. A calm walk is no problem.

Glucose and triglycerides respond directly to what you eat. After a meal your blood sugar rises and the fats from the meal appear in the blood, where they stay elevated for hours; according to the NVKC, triglycerides can reach a multiple of the fasted value a few hours after a meal. A non-fasted measurement then mainly shows what you just ate, not your baseline. It is precisely that baseline that is needed for a sound assessment and for comparison with a later measurement.

For many other tests, eating makes little difference. HbA1c reflects your average blood sugar over the past two to three months, because glucose binds to red blood cells that live for roughly that long; one breakfast does not change it. Total cholesterol and HDL cholesterol also change only modestly after a meal. A European consensus report from 2016 concluded that for most people a lipid profile is also usable when non-fasted, and that a fasted repeat measurement is mainly useful with strongly elevated non-fasted triglycerides.

So why does Bloodworks still ask you to come fasted for a lipid profile? Because in many laboratories LDL cholesterol is calculated from total cholesterol, HDL and triglycerides. If the triglycerides are elevated by a meal, the calculated LDL comes out too low. Measuring fasted keeps that calculation clean and delivers a reliable fasting glucose in the same draw.

Bloodworks states for each test whether fasting is needed. This is the overview, from strict to not needed:

• Fasting required: fasting glucose (at least 8 hours) and fasting insulin. In the Blood sugar test, HbA1c is measured in the same draw; HbA1c itself is not sensitive to food, but you come fasted for the whole.
• Fasting advised: the lipid profile with total cholesterol, HDL, LDL and triglycerides, the liver values (at least 8 hours and no alcohol for 24 hours), the kidney values urea and creatinine, and the electrolytes and minerals, especially magnesium. All Bloodworks value packages contain glucose and triglycerides and therefore ask for 8 hours of fasting.
• Not strictly required, but fasted when combined: lipoprotein(a) and apolipoprotein B can be measured without fasting, but together with a lipid profile you come fasted for the whole. The same applies to a blood count alongside glucose or cholesterol.
• Not required, but a time of day matters: hormones such as testosterone, oestradiol, FSH and LH do not require a fasted draw, but Bloodworks advises a morning draw, for testosterone between 7 and 10 a.m., because the value is then at its highest. Serum iron fluctuates over the day; preferably have it drawn between 8 and 10 a.m. For cortisol, the morning value says the most.
• Not required: the thyroid values TSH and free T4, vitamin D, vitamin B12, folic acid, ferritin, the blood count and inflammation values such as CRP. Do choose a calm moment of the day. If you take iron tablets, mention it in the intake.

Unsure about a test that is not listed here? On the page of every test, Bloodworks answers the question 'Do I need to fast?'.

Eat a normal dinner the evening before the draw; fasting does not require a special diet. An unusually fatty or large meal is better avoided, as is alcohol. Plan the draw early in the morning: you then sleep through most of the fasting hours. Check the opening hours of the collection site beforehand; at most of them you can walk in without an appointment.

Take your medication as you normally would, with a sip of water, unless your prescribing doctor or pharmacist has said otherwise. Bring your lab form, on your phone or printed, and a bottle of water. Put your breakfast in your bag for right after the draw.

Do you work shifts or sleep during the day? Fasted is fasted, regardless of the time: what matters is the 8 hours without food, not the clock. Do mention in the intake that you are having your blood drawn at an unusual time, because for tests with a daily rhythm, such as cortisol and iron, the doctor takes that into account.

A short checklist for the morning itself:

• 1Get up calmly and take your time; rushing and stress do not make the draw any more pleasant.
• 2Drink one or two glasses of water. That makes the veins easier to find.
• 3Leave coffee, tea, juice and chewing gum until after the draw.
• 4Take your medication as agreed, with water.
• 5Wear something with loose sleeves, so your upper arm is easy to free.
• 6Bring your lab form, and breakfast for afterwards.
• 7Say so at the draw if you did eat or drink something after all, or if you have fainted before when having blood taken.

Nervous about the needle? Look away, breathe out calmly while the needle goes in and stay seated for a moment before you get up.

Did you accidentally eat or drink something after all? Then say so honestly, for example at the draw or in a message to us. That information goes to the doctor who reviews your result and prevents wrong conclusions. A raised glucose that in reality reflects your breakfast can otherwise cause unnecessary worry.

A non-fasted result is not worthless. Many tests are barely affected by a meal and remain perfectly usable: thyroid values, vitamins, iron status, blood count, HbA1c and inflammation values. Values that do respond to food, especially glucose and triglycerides, are then interpreted differently; the doctor takes into account that you were not fasted and notes this in the report. Sometimes the advice is to re-measure a single value while fasted at a later moment.

Do you notice it before you reach the collection site? Then it is wise to postpone the draw by a day and come back fasted.

You do not have to figure this out yourself. Whether you need to fast is stated in three places: in the frequently asked questions on the page of the test you order, in the confirmation email you receive from us and on your lab form. Check that instruction before you plan the draw, and follow the longest duration if you combine several tests.

Are you putting together your own test from individual markers? Then the strictest marker in your selection determines whether you come fasted. One fasting glucose next to ten markers that do not require it means: come fasted. If you deliberately choose non-fasting glucose, that requirement lapses, but you measure your blood sugar at a random moment of the day.

If you are still in doubt, come fasted. Tests that do not require it do not become less reliable because of it. The reverse does apply: if you are not fasted while it was needed, you may have to come back for some values.

From order to report, a fasted blood test at Bloodworks takes four steps.

• 1Choose your test and pay online. With every test you can see whether fasting is needed. You then complete a short intake for our doctor; that is also where you report medication and anything else relevant.
• 2Receive your lab form. After approval you receive the lab form digitally, with the instruction whether to come fasted. You choose one of the affiliated collection sites across the Netherlands yourself.
• 3Have your blood drawn. Come fasted, bring your lab form and say so if you did have something after all. The draw takes a few minutes; right afterwards you may eat and drink.
• 4Receive your report with a doctor's interpretation. The result is typically available within 2 working days. A Bloodworks doctor assesses your values, takes into account whether you were fasted, and explains any abnormalities in plain language. In the case of strongly abnormal values, we contact you.

Would you like to have a value measured again later? Then come fasted again and at a comparable time, so that the measurements are comparable.

A fasted value is a snapshot under standardised conditions. That makes it easy to compare, but not infallible. Three things help when reading your report.

The laboratory is leading for the reference values. Every laboratory sets its own reference limits, depending on measurement method, equipment and the population on which those limits are based; the same value can therefore fall just inside the limits at one laboratory and just outside them at another. Bloodworks always shows the reference of the laboratory that analysed your blood.

An abnormality is not a diagnosis. A slightly raised fasting glucose can be related to stress, a bad night, an infection, certain medicines or chance, and triglycerides fluctuate from day to day. In the case of an unexpected abnormality, the doctor therefore often first advises a repeat measurement, fasted again.

The doctor weighs the context. The Bloodworks doctor looks at your intake: medication, pregnancy, recent illness, the time of the draw and whether you were fasted. That context is part of the explanation in your report.

A fasted blood test on your own initiative is meant for insight: a periodic check, a suspicion you want to rule out or a value you want to follow. It does not replace a consultation. If you have symptoms that fit a disturbed blood sugar, such as being very thirsty, urinating often, unintended weight loss or blurred vision, your GP is the first step. The same applies to chest pain, shortness of breath or other symptoms that worry you, and if you are already being monitored for a condition.

Thuisarts.nl and the RIVM point out that testing without symptoms has advantages, such as being in control, but also disadvantages: you pay yourself, a result can cause unrest and an abnormality sometimes calls for further investigation. Bloodworks limits those disadvantages by having every result assessed by a doctor. If you are unsure whether a test is useful, use our test finder or consult your GP.