What you didn't know about your blood test

General Health

Here are a number of interesting facts that may surprise you about blood testing!


Emily Condon
BSc (Hons) Biological Sciences

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Whether you have tested with us before or are yet to get Medichecked, here are a number of interesting blood test facts that you may be surprised to hear. 

1. You do not need to fast before taking a cholesterol test

Cardiovascular disease risk is calculated by measuring the total cholesterol, “good” HDL cholesterol, “bad” LDL cholesterol and triglycerides in a sample. In the past, it was necessary to fast before taking a cholesterol blood test as eating prior to the test may have produced a lower than true LDL cholesterol value, giving false reassurance about cardiovascular risks. Similarly, if a person had a slightly elevated triglyceride level due to having had a big breakfast, for example, this would interfere with an accurate measurement of HDL cholesterol.

Over the past 5 – 10 years laboratories have improved their cholesterol assays and are now able to accurately report HDL cholesterol even in the presence of a triglyceride concentration that is much higher than normal i.e. up to 10 mmol/L. The NICE guidelines [i] on reducing cholesterol have also been changed and the focus is now much more on “Non-HDL cholesterol” which looks at all the harmful cholesterol subtypes such as LDL cholesterol and VLDL cholesterol. This means our cholesterol checks give an accurate measure of HDL cholesterol on a non-fasting sample and can calculate the Non-HDL fraction to give an indication of how much “bad cholesterol” there is which may increase the risk of heart disease. 

Eating can still affect your triglyceride levels, especially if you've just consumed a fatty meal, so sometimes you may be asked to repeat a triglyceride test with a fasting sample. This gives us a clearer picture of whether your levels are consistently high or have just been affected by your latest meal. 

2. Your pet may affect your test results

Although rare, it is possible that your pet could affect your blood test results. To understand this a little better, we asked Medichecks’ Clinical Pathologist Dr Peter Prinsloo to explain the how this might happen.

"When we are exposed to any type of protein, our body can respond by producing antibodies which in some cases may cause a minor annoyance or irritating allergies but in other cases protect us from disease. This antibody response has been used to our benefit over many years, protecting us from harmful bacteria and viruses and creating effective vaccines.

Over the past 30-40 years laboratory technology has developed methods where antibodies are used to measure the amount of protein, hormones, virus particles, bacteria and many other molecules in blood samples - using what are called immunoassays. This is done by creating an animal antibody against the human molecule being tested - i.e. oestrogen, CRP, thyroid hormones, cancer markers and even for pregnancy testing. The most commonly used ones are goat or sheep antibodies and they are called capture antibodies.

In the majority of cases the immunoassays work very well but occasionally the person that is being tested already has antibodies in their blood (anti-goat antibodies, anti-mouse antibodies etc) that may be due to previous exposure to pharmaceutical products that also use animal antibodies e.g. x-ray dyes, drugs, vaccines and many more. These human anti-animal antibodies in the blood sample can cause interference with the antibody-based testing process.

Some individuals may have developed antibodies as a result of close contact with animals (such as pet or farm animals) and these antibodies may interfere with the antibodies used in laboratory testing. Fortunately this is very rare and laboratories have developed strategies to overcome these antibody interferences. On very rare occasions it is not possible to eliminate the interfering antibody effect and doctors will revert to a good clinical history and examination to assess whether there is an underlying issue or will use alternative tests". 

3. Biotin supplements can give false test results

Lots of people supplement vitamin B7 also known as biotin as they believe it will improve their hair, nails, and skin. But biotin may interfere with some laboratory testing, causing results of tests to be either falsely high or falsely low. Excess biotin in the blood sample can interfere with immunoassay tests as many of these use biotin as part of the testing process to bind chemicals and other substances in the blood to the test tube so they can be measured. Excess biotin from supplements can block that binding process and the substance being tested won't be measured accurately [ii]. Other supplements such as vitamin D and B12 can also affect test results if these vitamins are being tested for. If you are taking an oral supplement then we recommend not taking it for one week prior to the test to ensure an accurate measure of your levels. However, if the supplements you are taking are prescribed to you by your doctor then you should discuss this first with them before ceasing supplementation.

4. Having an acute illness may affect thyroid test results

Every cell in the body depends upon thyroid hormones for metabolism regulation as thyroid hormone status correlates with body weight and energy expenditure [iii]. During and after acute illness such as a chest or kidney infection, thyroid function tests may decrease or increase reflecting the body's changing need for these hormones [iv]. Therefore it is wise to wait up to 6 weeks after an acute illness before testing your thyroid function.

If you ever have any questions about your Medichecks test don't hesitate to contact our customer service team on 03450 600 600 available 8am-8pm Monday to Friday who will be more than happy to help you.

[i] NICE. (2018). A fasting sample is not required when taking samples before starting lipid modification therapy for the primary prevention of CVD. Take at least 1 lipid sample to measure a full lipid profile.. [online] Available at: https://www.nice.org.uk/donotdo/a-fasting-sample-is-not-required-when-taking-samples-before-starting-lipid-modification-therapy-for-the-primary-prevention-of-cvd-take-at-least-1-lipid-sample-to-measure-a-full-lipid-profile [Accessed 17 Oct. 2018].

[ii] Labtestsonline.org. (2018). FDA Warns that Biotin May Affect Some Lab Test Results | Lab Tests Online. [online] Available at: https://labtestsonline.org/news/fda-warns-biotin-may-affect-some-lab-test-results [Accessed 17 Oct. 2018].

[iii] Iwen, K., Schröder, E. and Brabant, G. (2013). Thyroid Hormones and the Metabolic Syndrome. European Thyroid Journal, 2(2), pp.83-92.

[iv] Premawardhana, L. (2017). Thyroid testing in acutely ill patients may be an expensive distraction. Biochemia Medica, 27(300), pp.300-307.

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