Fasting Blood Test: Which Tablets You Still Take

"Please come in fasting" — and suddenly the tablet box is a question mark. The short answer: fasting means no food and no sugary drinks. Water is allowed and in fact welcome — and the vast majority of long-term medicines are simply taken as usual. Leaving something out on your own initiative is as a rule the bigger risk than a slightly shifted laboratory value. There are a handful of genuine exceptions, though, and they are worth knowing.

1. The key message: fasting is about food, not about your tablets

The most common mistake before a blood test is not the forgotten ban on breakfast but the medicine left out as a precaution. The reasoning sounds sensible: if you are fasting, then do it properly. Except that "fasting" refers to one very specific purpose — making a small number of laboratory values comparable, the ones that rise after a meal.

A blood pressure medicine, an antidepressant or an anticoagulant has nothing to do with those values. Leaving them out gains the laboratory nothing and may earn you a blood pressure spike, an unsettled day or — with anticoagulants — a real risk.

• Allowed and encouraged: water and unsweetened tea. Drinking actually improves the conditions for taking the sample.
• Not allowed: food, sugary drinks, juices, milk in your coffee, chewing gum with sugar, sweets.
• Grey area: black coffee without sugar or milk. Some laboratories tolerate it, others do not — when in doubt, skip it and drink it afterwards.
• Smoking: better not before the sample, as it can affect individual values.

2. What "fasting" actually means

The usual requirement is eight to twelve hours without food. In practice that means: eat normally in the evening, nothing afterwards except water, and go for the sample early in the morning. Fasting for considerably longer is not better — after more than about 14 hours individual values can tip the other way, because the body switches over to burning fat.^¹

Because many blood values follow a daily rhythm, the early morning is the most sensible time anyway. Cortisol and iron are at their highest in the morning; a value taken in the late afternoon compares less well with the reference range. So if you are monitored regularly, try to go for the sample at the same time of day every time — that makes the trend more meaningful than any single number.

3. Which values really are measured fasting

The list is shorter than most people think.

Blood sugar

Fasting blood sugar is the classic case — it is used to detect and monitor diabetes. After a meal it is inevitably raised and therefore no longer comparable with the reference range. The long-term value HbA1c, by contrast, is independent of food and needs no fasting. How to measure it yourself between appointments is covered in Measuring blood sugar correctly.

Triglycerides and the lipid profile

Triglycerides rise markedly after a fatty meal — this is where fasting makes the biggest difference. With cholesterol the picture is different, and that often goes unsaid: total, LDL and HDL cholesterol vary only slightly after a meal. European professional societies therefore consider a non-fasting measurement sufficient for most questions.^²

German practices nevertheless often continue to measure fasting — for follow-up monitoring, with very high triglyceride levels, or simply out of habit. Both are defensible. What you can take from it: if you have eaten breakfast by accident, a lipid profile is not automatically worthless. Tell the staff, and it will be noted on the report. More on the subject in the guide Lowering cholesterol.

4. What does NOT have to be fasting

For the great majority of routine values, breakfast makes no difference:

• Full blood count — red and white blood cells, haemoglobin, platelets.
• Kidney values — creatinine and the eGFR calculated from it.
• Liver values — AST, ALT and gamma-GT are as a rule not relevantly changed by breakfast.
• Thyroid values — TSH, fT3 and fT4 need no fasting. A separate rule applies to taking the tablet, though, see section 6.
• Inflammatory markers and clotting — CRP, blood sedimentation rate, INR and Quick value.

If you are unsure what a sample has been ordered for, ask — the question is legitimate and takes a minute. What the individual values on your report mean is explained in the guide Understanding blood values.

5. Special cases among medicines at a glance

The table is a guide, not a ruling. Special investigations — hormone measurements or drug levels, for example — have their own requirements, which the practice will give you. The decision always rests with the practice treating you.

6. Levothyroxine: the section hardly anyone knows about

This is the practically most valuable point in this article. Levothyroxine is taken in the morning on an empty stomach for hypothyroidism — and the blood sample for monitoring is also taken in the morning, fasting. So both fall at the same moment.

The problem: after the dose, the free thyroid hormone fT4 in the blood rises noticeably over several hours before it settles again. If blood is taken shortly afterwards, the value looks higher than it actually is on average across the day. That can lead to the false conclusion that the dose is too high — and to a reduction that was never needed.^³

Consistency matters just as much: if you normally take it at six o'clock, do so in the weeks before the check as well. A single missed day changes the value less than a pattern that has been irregular for weeks. Everything about taking it properly — including the gap to coffee, calcium and iron — is in the guide How to take levothyroxine.

7. Diabetes medicines: the one genuinely critical case

This is not about distorted laboratory values but about your safety. If you inject insulin or take a sulfonylurea such as glimepiride, you are lowering your blood sugar — regardless of whether you have had breakfast. The usual morning dose without a meal afterwards can trigger hypoglycaemia.

• Call ahead — say when you book the appointment that you take insulin or a sulfonylurea. Then the instructions come with the appointment rather than as a surprise on the morning.
• Take an early appointment — a short fast means a lower risk. Meter, glucose tablets and a breakfast belong in your bag.
• Do not dose insulin on your own judgement — whether and how much you inject in the morning depends on the preparation and your regimen. That is a medical instruction.

Metformin on its own does not cause hypoglycaemia and is therefore usually continued unchanged. Combined with insulin or a sulfonylurea, however, the instruction for the more critical preparation applies. Other everyday situations — illness, fasting, exercise — are covered in the guide Diabetes medications in daily life. The basics of the condition are at Type 2 diabetes.

8. Blood pressure medicines and anticoagulants: please just keep taking them

Blood pressure medicines

Medicines such as ramipril or amlodipine do not affect the standard laboratory values. Before a blood test they are as a rule taken as usual with a sip of water. A missed day, by contrast, can lead to blood pressure spikes — what that means in detail is set out in Forgetting your blood pressure medication.

There is one genuine exception: with special hormone investigations — for instance to investigate hormonally driven high blood pressure — certain blood pressure medicines can distort the result and are temporarily switched on medical instruction. That is a planned measure with replacement medication, not a spontaneous omission.

Anticoagulants

Phenprocoumon, aspirin and the direct oral anticoagulants are not paused before a blood test. Taking a blood sample is not a procedure — the puncture site is simply pressed a little longer.

Something different applies if a procedure is planned at the same appointment — separate rules apply there, set out in Medications before surgery.

9. Biotin: the interference almost nobody has on their radar

Biotin — also known as vitamin B7 or vitamin H — is in many products for hair, skin and nails, often at a very high dose. It is considered harmless, and for the body it is. For the laboratory it is not.

The reason is technical: many modern laboratory tests use a detection method that relies on exactly this binding between biotin and a carrier protein. If extra biotin from a tablet is circulating in the blood, it disrupts that system — depending on how the test is built, values can come out falsely high or falsely low. Thyroid values are among those affected and, particularly awkwardly, troponin, which is used in the emergency department to investigate a heart attack. Regulators have therefore drawn explicit attention to this interference.^⁴

• What stands out: a thyroid result that does not fit how you feel at all — values as in an overactive thyroid, for example, with no symptoms at all.
• What you can do: mention supplements at the blood test without being asked. They are on no prescription and therefore appear in no record.
• The usual recommendation: pause high-dose biotin products a few days before the sample — the exact length depends on the dose and should be discussed with the practice.
• No reason to panic: the usual amounts in a multivitamin product are mostly uncritical. It is the high-dose hair and nail products that cause problems.

Other products can shift values too — high doses of vitamin C, for instance, or iron preparations shortly before iron levels are measured. The overview is in Supplements and medications.

10. The day before: exercise, alcohol and how much you drink

What happens the evening before often influences the result more than the tablet in the morning.

• Intense exercise pushes up the muscle value CK, sometimes considerably and over one to two days. Liver values can follow suit. A marathon or a hard weights session the day before can produce an abnormal result that indicates no illness at all. A walk yes, a personal best no — more on this in Medications and exercise.
• Alcohol affects liver values, triglycerides and blood sugar. For a meaningful check, at least 24 hours without alcohol makes sense. Details in Medications and alcohol.
• A very fatty meal the evening before can still raise triglycerides the next morning. A normal dinner is enough.
• Drink enough — one or two glasses of water the evening before and again in the morning. Well-filled veins are easier to hit, and when you are dehydrated some values appear artificially raised because the blood is more concentrated.

11. At the appointment itself: what to bring and what to say

• Your complete list of medicines — including supplements and over-the-counter products. How to build one is set out in Keeping a medication list.
• The time of your last dose — especially with thyroid hormones, anticoagulants and medicines whose levels are being measured.
• The tablets themselves and a breakfast — for straight after the sample, rather than driving home hungry.
• Earlier results — a trend says more than a single value. How to prepare for the appointment as a whole is shown in Preparing for a doctor's appointment.

And the most important rule for afterwards: if you have been told to take a medicine only after the sample — then make sure you actually take it. The missed day is a common but unnecessary side effect of laboratory appointments. The basics are in How to take medications correctly.

12. How brite helps you around a blood test

No more forgotten dose days — with brite

Medication plan, movable reminders and laboratory values in one place. Free.

Start for free