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preparing for your blood test

This time of year a lot of people are checking off items on their to-do list, one of these might be getting an annual check-up. A common thing we’ll get you to do as part of this is to get a blood test. A lot of people ask: What is the best way to prepare for my blood test to get the most meaningful results? This article will discuss exactly that, so read on!

If you’re going to the trouble to get a blood test done, you want to make sure it’s done right and the results are reliable. This is especially true if you don’t love needles, not to mention the time you take out of your day to get the blood collected. Often it needs to be done first thing in the morning when you likely have a bunch of other things going on.

There are some things you can do and avoid to make sure your blood test results are reliable and we as your doctors can interpret the results and therefore, give you the best advice.

Here are 5 DO’s and DON’Ts to be aware of in preparation for your blood test:

1.Fast for the test.

Usually check-up bloods are fasting bloods because we might want to check your fasting blood sugar, insulin and lipids. Do an overnight fast for your fasting blood test, but please don’t over-fast! The ideal time to fast for is about 12 hours. If you fast for 15 hours and beyond, this can mean that some of the blood markers become unreliable. For example, sometimes we see people’s blood sugars rebound high when they’re over-fasted, due to the stress response and the body trying to mobilise fuels.

2.Make sure you’re well when you get your bloods taken.

That is, don’t have your fasting blood taken if you’ve got a head cold, sore throat or are otherwise unwell. Similarly to when you over-fast, if you have bloods taken when you’re not well, some of the results can be skewed. The White cells, inflammatory markers and liver function tests can appear elevated, and the iron panel (if we’re testing it) becomes harder to interpret. This is because ferritin (the indicator of your body’s iron stores) is also an ‘acute-phase’ reactant, meaning it can also be elevated in the context of infection. The best thing to do is to wait until you’re feeling well plus at least another couple of days to ensure the results aren’t skewed by the infection your body is trying to fight off!

As a side note, of course if we’re arranging bloods for you to investigate an acute illness you’ve come to see us about, go ahead and get the bloods done as soon as you can so we can try and work out what’s going on. These are usually not fasting bloods.


Make sure you’re well hydrated before you go for your blood test. I’d recommend drinking a big glass of water the morning of the test and even some salt, if you like. This will ensure your veins are plump and well hydrated, and hopefully make it easier for the phlebotomist to easily find a good vein to enable a smooth and non-traumatic blood collection! Do avoid drinking caffeine before the blood test though, as this also can mobilise fuel stores in the body (blood sugar and triglycerides) and cause the results to appear skewed. If you’re like me and are almost on autopilot when you have your morning coffee, I’d recommend to physically place your blood request form on the kettle or coffee machine to remind yourself not to have the coffee yet and go ahead and get the blood taken!

4.Reproductive-age females: pay attention to your cycle.

If you are a female and having hormonal bloods checked, please have the blood at the time your doctor recommends during your menstrual cycle. The fertility hormones fluctuate wildly, depending where you are in your cycle, so this is very important to pay attention to. Your doctor will give you clear instructions and normally write the window of your cycle within which to have the blood collected.

5.Avoid strenuous exercise.

Avoid strenuous exercise in the 24 hours before your blood collection. Once again, doing this can skew the blood results, specifically increasing urea, CK and sometimes making your kidney function look worse than it actually is. Strenuous exercise also causes a mild inflammatory response which can temporarily raise your blood inflammatory markers. Go for walks etc, if this is what you normally do, but no HIIT sessions of heavy weight training in the 24 hours before your blood collection, please.

So there they are, the 5 Do’s and Don’ts of getting an accurate blood test! I hope you found these helpful. If you are due for a check-up, contact our friendly reception to arrange this or book online. Looking forward to seeing you in the practice soon!

Dr Jackie Montefiore