Why do blood samples clot? What it means and how to avoid it

If you’ve been told your blood sample couldn’t be tested because it clotted, you’re not alone. Clotted blood samples are one of the most common reasons a test needs to be repeated.

In this article, we’ll explain what it means when your blood clots after being drawn, why it happens, and how you can avoid it. We’ll also look at other sample issues like haemolysis and low volume, and answer a common question: are finger-prick tests accurate?

Whether it’s your first time testing or you’ve had a result rejected, this guide will help you feel more confident next time you collect your sample.

• What is a clotted blood sample?
• Why does my blood sample clot?
• Can a clotted blood sample be used?
• What happens if my blood test fails?
• What else causes blood sample errors?
• Are finger-prick blood tests accurate?
• Glossary: common blood test problems explained

What is a clotted blood sample?

A clotted blood sample is one where your blood has started to thicken and form clots before reaching the lab. This happens when the blood begins its natural clotting process after being collected, making it unusable for testing.

Clotting is your body’s way of preventing too much blood loss. But once it happens in a sample tube, it prevents the lab from separating the blood cells from the serum (the liquid part of your blood used for most tests) [1].

Why does my blood sample clot?

If your blood sample clots before it reaches the lab, it can’t be analysed properly, which means you may need to take the test again. But clotting is usually easy to prevent with the right technique.

Common reasons a blood sample might clot:

• The blood wasn’t mixed gently after collection
• The sample took too long to collect

To reduce the risk of clotting:

• Gently invert the tube several times as instructed, as soon as your sample is collected
• Don’t shake the sample, as this can cause haemolysis (red blood cell breakdown)
• Collect your blood within 10–15 minutes of pricking your finger

Taking a little extra care can help ensure your blood sample arrives in good condition, so you get fast, accurate results without delay.

Can a clotted blood sample be used?

No. Unfortunately, if your blood sample has clotted, the lab won’t be able to test it. You’ll need to collect a new sample.

Clotting interferes with the separation process and prevents accurate analysis of most biomarkers, especially hormones, vitamins, and blood chemistry results [2].

What happens if my blood test fails?

If your sample clots, haemolyses, or is too small, we’ll email you as soon as we get the lab results. There’s no need to worry. This can happen, especially for first-time finger-prick users. Many customers find that their second try goes more smoothly with a few adjustments.

You can:

• Request a free replacement test from our customer care team
• Upgrade to a clinic visit or home nurse visit if you prefer a professional collection
• Watch our handy video on how to take a finger-prick test before collecting your sample.

What else causes blood sample errors?

There are three common types of blood sample errors. Here’s what they are and how to prevent them.

1. Haemolysed blood sample

   Haemolysis happens when red blood cells burst and release haemoglobin into the surrounding blood. This can interfere with many test results, including potassium, liver function, and hormone levels [3].

   Why does it happen?

   • You squeezed your finger too hard to get the blood flowing
   • You scraped your finger against the tube instead of letting drops fall in
   • The sample was shaken too vigorously

   How to avoid it:

   • Warm your hands and let gravity help
   • Let the blood drip in naturally
   • Invert the tube gently, don’t shake

2. Insufficient blood sample

   This happens when there’s not enough blood in the tube to be tested. Once your blood is processed, it’s spun to separate the serum, and if there isn’t enough volume, the lab can’t run your tests [4].

   How to avoid it:

   • Always aim to fill to the line shown in the instructions
   • Follow our top tips for finger-prick testing
   • Keep warm, hydrated, and relaxed before you start
   • If you stop bleeding, try pricking another finger using the additional lancets

Are finger-prick blood tests accurate?

Yes, finger-prick blood tests are just as accurate as venous (needle) tests when collected correctly. They’re used widely in healthcare for monitoring and screening, and are especially useful for routine checks [5][6].

The difference is that you're collecting the sample yourself, which can increase the chance of collection errors. But if you follow the instructions carefully, the results are reliable and clinically valid.

Glossary: common blood test problems explained

Clotted sample

Your blood thickened before it could be tested, usually because the sample wasn’t mixed properly or was left too long during collection.

Haemolysed sample

Red blood cells broke down, releasing their contents and contaminating the sample, often due to squeezing the finger or shaking the tube.

Insufficient sample

There wasn’t enough blood to test, which means the lab couldn’t get the serum it needed to analyse your results.

Failed blood test

This means your test couldn’t be completed due to one of the above issues. We’ll always let you know and help you with the next steps.

References

1. American Society of Hematology, 2022. Blood Clots. [online] Available at: https://www.hematology.org/education/patients/blood-clots [Accessed 5 June 2025].
2. Stowell, C. P., 2016. Clotted specimens: Causes and prevention. Laboratory Medicine, 47(3), pp.241–244. Available at: https://academic.oup.com/labmed/article/47/3/241/2460062 [Accessed 5 June 2025].
3. Wan Azman, W., Omar, J., Koon, T. and Tuan Ismail, T., 2019. Hemolyzed specimens: Major challenge for identifying and rejecting specimens in clinical laboratories. Oman Medical Journal, 34(2), pp.94–98. Available at: https://doi.org/10.5001/omj.2019.18 [Accessed 5 June 2025].
4. Zhou, X. et al., 2021. Preanalytical errors in laboratory medicine: A review of the literature. Journal of Clinical Laboratory Analysis, 35(5), e23738. Available at: https://doi.org/10.1002/jcla.23738 [Accessed 5 June 2025].
5. Ong, R. and Chan, E., 2021. Capillary blood sampling for laboratory tests: a systematic review. BMJ Open, 11(5), e044656. Available at: https://bmjopen.bmj.com/content/11/5/e044656 [Accessed 5 June 2025].
6. Spooner, R., Oreskovic, N., & Grassin-Delyle, T., 2023. Comparability of capillary and venous blood for laboratory testing. Journal of Clinical Pathology. Available at: https://doi.org/10.1136/jclinpath-2022-208709 [Accessed 5 June 2025].