What can affect your blood test results?

Updated by Dr Francesca Haarer BSc, PG Cert (Med Ed), BMBS, MRCS and Dr Kate Smith BSc, MBChB January 2025

There’s a lot that doctors or other trained healthcare professionals can tell from a blood test. Some factors affect the accuracy of your results.

In this article, we’ll go through 7 things that can impact your blood test results.

If you’re about to take a test, this will help you plan ahead and avoid anything that can interfere with your results.

1. Not cleaning your finger properly

Unclean hands may affect the accuracy of your finger-prick blood test results.¹

Before your test, wash your hands with warm water and soap, making sure your hands are completely dry.

2. Time of day

Some blood tests are affected by the time of day you take your sample.

For example, testosterone levels peak in the morning, so you need to take your sample between 7 a.m. and 10 a.m. or your results will seem too low.

Some blood tests are best done first thing in the morning. Make sure to check the instructions of the test you are taking.

3. Medications, vitamins or supplements

If you’re taking any regular medication or supplements, some of these can affect your results.

For example, some labs use a chemical that interacts with biotin (vitamin B7) when they analyse your blood sample for certain tests. This interaction can skew your results.²

Chat to ya healthcare professional before you do your test if you have any concerns – and don’t stop taking any prescription medicine without speaking to your doctor.

4. Exercise

A very strenuous fitness session may increase a type of protein called creatine kinase in your blood.³,⁴

To avoid any potential of this happening, you can avoid doing blood tests that include creatinine kinase straight after the gym!

5. Squeezing your finger too hard

Squeezing your finger with too much pressure when taking a finger prick blood test can cause your sample to haemolyse.⁵

This means the red blood cells in your blood sample have ruptured, so the sample might be more difficult to test or can’t be analysed and will need to be repeated.⁶

Instead, massage your finger gently in a milking motion to allow the droplets to fall gently into the tube. You can watch our lab partner's "how to collect a blood sample" video here.

6. Clotting

If collecting your sample takes a while, your blood can clot. To stop this from happening, ensure you’re ready with everything set up before you start.

Have a spare lancet ready, and make sure you’ve read the instructions a few times.

If you’ve exhausted all blood from one finger, get another lancet and try the next finger.

After filling each tube, replace the lid and gently tip each sample back and forth to mix it well and stop it from clotting. 

7. Amount of blood

We ask for one or two small tubes of blood for a reason!

Having more blood to work with improves the chance that your sample will be analysed successfully.

The final word

Remember all this when you’re prepping for your at-home blood test. It’ll give you the best chance of getting accurate results.

And, as a rule of thumb, do your at-home blood test first thing in the morning, with clean hands, and when you’re feeling relaxed – so, not straight after your weekly exercise! Remember to send your sample the same day you take it.

References:

1. Olamoyegun MA, Oloyede T, Adewoye OG, Abdulkarim SO, Adeleke AA. Pseudohyperglycemia: Effects of Unwashed Hand after Fruit Peeling or Handling on Fingertips Blood Glucose Monitoring Results. Annals of medical and health sciences research [Internet]. 2016;6(6):362–6. Available from: https://www.ncbi.nlm.nih.gov/pubmed/28540104 

2. Avery G. Biotin interference in immunoassay: a review for the laboratory scientist. Annals of Clinical Biochemistry: International Journal of Laboratory Medicine [Internet]. 2019 Apr 25;56(4):424–30. Available from: https://journals.sagepub.com/doi/abs/10.1177/0004563219842231 

3. Kindermann W. Creatine Kinase Levels After Exercise. Deutsches Aerzteblatt Online. 2016 May 13;113(19).

4. Thomas P. BLOOD SCIENCES DEPARTMENT OF CLINICAL BIOCHEMISTRY Title of Document: Investigation of elevated Creatine Kinase Q Pulse Reference N o : BS/CB/DCB/PROTOCOLS/32 Version N Investigation of elevated Creatine Kinase BLOOD SCIENCES DEPARTMENT OF CLINICAL BIOCHEMISTRY Title of Document: Investigation of elevated Creatine Kinase Q Pulse Reference N o : BS/CB/DCB/PROTOCOLS/32 Version N [Internet]. Available from: https://www.nbt.nhs.uk/sites/default/files/document/Investigation%20of%20elevated%20Creatine%20Kinase.pdf 

5. The technology | Helge for detecting haemolysis | Advice | NICE [Internet]. Nice.org.uk. NICE; 2020 [cited 2025 Feb 6]. Available from: https://www.nice.org.uk/advice/mib225/chapter/The-technology 

6‌. London Medical Laboratory FAQs [Internet]. Londonmedicallaboratory.com. 2025 [cited 2025 Feb 6]. Available from: https://www.londonmedicallaboratory.com/faq#category-content-21eb8863-1ae3-4eab-833a-bdc440a410a0