How to prepare for a blood test, and what can throw it off

What preparation actually changes

Very little, and that is the useful finding. In the vast majority of cases, an NHS hospital leaflet notes, you do not need to prepare for a blood test at all. A meal moves glucose and triglycerides for a few hours, coffee disturbs the same two, and an iron tablet distorts a serum iron. Almost everything else on a routine form is indifferent to what you did that morning.

The pages here take one variable each and say what it moves, by how much and for how long. Where the advice has changed, they say so: a cholesterol test no longer needs fasting by default in the UK, the single biggest shift in this area in a decade.

Fasting: a shorter list than you were told

Where a fast is required, the usual instruction is 8 to 12 hours with nothing but plain water. It protects one test above all: fasting glucose, which digestion pushes up within minutes. A full blood count, thyroid function tests, liver function tests, urea and electrolytes and an HbA1c do not need it.

Cholesterol changed column. NICE’s lipid guideline states that a fasting sample is not mandated for a full lipid profile: after an ordinary meal, triglycerides rise by about 0.3 mmol/L at most and LDL cholesterol falls by about 0.2 mmol/L, HDL cholesterol does not move, and non-fasting values predict heart disease just as well. A fasting repeat is kept for triglycerides between 10 and 20 mmol/L.

What fasting means, and what breaks it

Most people were never told. When researchers questioned 150 people arriving for a blood test, only 39% could define fasting correctly, 46% thought the time since the last meal did not matter, and more than half had received no instructions at all.

Plain water is allowed throughout the fast, including on the morning of the test. An NHS biochemistry leaflet rules out tea, coffee and fizzy drinks, black coffee and diet drinks included, as well as gum, smoking, vaping and nicotine patches. Black coffee is the most common mistake: in a crossover trial, healthy volunteers given caffeinated coffee before a meal showed a glucose response 147% to 216% larger than with decaffeinated coffee.

Alcohol, tobacco and tablets leave traces of their own

Alcohol works on two clocks. One evening moves triglycerides and some liver values for a day or two; regular drinking shifts GGT and the average size of red cells over weeks to months, which a dry evening before the test does not undo. Neither line is an alcohol test: a review of alcohol markers stresses that liver enzymes are not specific to alcohol.

Regular smokers show higher white cell counts, haemoglobin and haematocrit than non-smokers, and the gap grows with the number of cigarettes. For medicines, keep taking your usual tablets unless the person who requested the test says otherwise. Biotin, sold in many hair, skin and nail supplements, is the one to mention: an MHRA-agreed safety letter warns it can falsely raise or lower thyroid results, and in healthy volunteers taking 10 mg a day it distorted 9 of 23 biotin-based tests.

Where to start

If you were told to fast and want to know whether it applies to your tests, start with the fasting blood test page. If your question is narrower, go straight to it: water, coffee, alcohol, smoking or tablets.