For many blood tests, no special preparation is needed. For some, fasting, timing or medicine advice matters. I always recommend following instructions for your exact panel rather than stopping medicines or fasting on your own.
Preparation is part of a reliable blood test. Food, alcohol, supplements, exercise, posture, collection timing and some medicines can affect selected results, so a useful result begins before the blood draw [1].
At Victoria Medical, I want preparation advice to be specific to the clinical question. A blanket instruction to fast or stop medication can be unhelpful, and in some circumstances unsafe. Our team confirms the plan at booking and can arrange a GP discussion if the right test is unclear.
Written & reviewed by
Dr George XynopoulosMD, MRCP — Consultant Physician & Cardiologist
View full profileWhen Fasting May Matter
Some glucose and lipid assessments may need a period without food, while many blood tests do not. Water is usually acceptable when fasting is requested, but the exact duration and permitted drinks depend on the panel and laboratory instruction.
Do not add fasting because you think it may make a result ‘more accurate’. Follow the clinic’s instructions for the tests booked. If you have diabetes, are pregnant, feel unwell with fasting, or have an eating-disorder history, tell us before the appointment so the approach can be individualised.
| Situation | Safe next step |
|---|---|
| Fasting requested | Follow the stated interval and drink water unless you are told otherwise. |
| No fasting instruction | Eat and drink normally unless the clinic advises differently. |
| Diabetes, pregnancy or fasting difficulty | Contact the team before changing meals, insulin or other treatment. |
| Morning medicines | Ask whether they should be taken as usual; never stop prescribed treatment without clinical advice. |
Medicines, Supplements and Timing
Prescribed medicines, over-the-counter products and supplements can influence selected laboratory measurements. I advise bringing an up-to-date medicines list and asking about a particular product before changing it. This is especially important for diabetes medicines, thyroid treatment, anticoagulants, steroids and high-dose biotin supplements [1][4].
If a result is used to monitor a medicine, the timing of the dose and the sample can matter. The right instruction comes from the clinician or laboratory protocol for that test, not from a generic online checklist.
What to Bring and What Happens at the Appointment
Bring referral information if you have it, a current medication list and previous relevant results. Wear clothing that allows comfortable access to your arm. A trained clinician takes a small blood sample; if you tend to faint, feel anxious about needles or have had difficult collections, let the team know beforehand.
Afterwards, eat and drink normally unless you have been given separate medical advice. If you have a bandage, keep it on briefly and avoid strenuous use of the arm if it is sore. If fasting or a medicine change leaves you acutely unwell, fainting, confused, severely breathless, with chest pain or another emergency symptom, use urgent services and call 999 for a life-threatening emergency rather than waiting for a routine appointment.
What to Expect at Your Appointment
Clarify the clinical question
I recommend deciding what you need the test to answer before selecting a panel or preparation plan.
Follow the specific instructions
Our team confirms fasting, timing and medicine instructions for the exact tests booked; do not stop prescribed medicines unless your clinician tells you to.
Put the result in context
A result is interpreted alongside symptoms, history, medicines and, where needed, earlier results rather than in isolation.
Agree the next step
The next step may be reassurance, repeat testing, a GP review, treatment monitoring or specialist assessment depending on the clinical picture.
Frequently Asked Questions
References
- [1]Lee L et al. Preparation for blood tests: what can go wrong before the sample reaches the lab. Australian Prescriber. 2025. DOI: 10.18773/austprescr.2025.034.
- [2]Newsome PN et al. Guidelines on the management of abnormal liver blood tests. Gut. 2018;67:6-19. DOI: 10.1136/gutjnl-2017-314924.
- [3]Li A et al. Patterns of follow-up testing of abnormal eGFR and UACR for the detection of chronic kidney disease in Australian primary care. BMJ Open. 2025. DOI: 10.1136/bmjopen-2024-096730.
- [4]Soh SB, Aw TC. Laboratory testing in thyroid conditions: pitfalls and clinical utility. Annals of Laboratory Medicine. 2019;39:3-14. DOI: 10.3343/alm.2019.39.1.3.
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