Why one blood test isn't the whole picture

Testing once gives you a snapshot. Testing regularly gives you a story.

dr-joshua-smith

Author: Dr Joshua Smith - Clinical Insights Lead, MBBS, BMedSci, PGCertMedEd

Reviewed by: Dr Tina Ghela – MBBS, MRCGP, BSc (Hons)

Published on: 29 April 2025

You took a blood test. Everything came back normal. Job done, right?

Not quite.

Blood tests are one of the most powerful ways to check in on your health. But relying on a single result is a bit like thinking one MOT means your car is roadworthy forever.

In this article, we reveal just how much your numbers can change and when’s a good time to check-in again.

In this article, we look at:


Your body changes, and so do your results


Just like your mood or energy levels, your blood markers fluctuate. This is known as biological variation — the normal ebb and flow in your results, even when you’re healthy.

Some blood markers, known as biomarkers, fluctuate more than others. For example, a 30% change in your free testosterone level might be normal, whereas a 5% change in your HbA1c could be more significant.

Want to know if your results likely reflect a true change? Use the tool below. Just make sure you’re comparing like with like — test at the same time of day, phase of your cycle, and season, wherever possible.
 

Has my result significantly changed?

 


Coefficient of variation values taken from: https://biologicalvariation.eu/
 

What else affects your blood test results?


If your result has changed more than expected, it could be down to something else. There are lots of possible reasons for this.

Why your blood test result might change:

  • Biological factors — Your age, sex, and menstrual status (e.g. menopause) can affect hormones like AMH, DHEAS, and testosterone.
  • Natural rhythms — Some markers change throughout the day (like cortisol and testosterone), while others shift with the seasons (like vitamin D or TSH).
  • Health status and stress — Illness, infection, or stress (both physical and mental) can impact inflammatory markers and white cell count.
  • Pre-testing factors — Hydration affects kidney markers. A fatty meal can raise triglycerides. Even recent sex or poor sleep can influence hormones like prolactin and testosterone.
  • Supplements and medications — Hormonal contraceptives, for example, can affect many results. Nutritional supplements can skew vitamin and mineral levels.
  • Laboratory factors — Differences in equipment or sample handling can cause small shifts, but accredited labs (like the ones we use) minimise this.
     

How lifestyle changes can make a big difference

  • HbA1c can improve by 0.1% per kg lost if you have diabetes [2]
  • Losing just 2–5% body weight can improve triglyceride levels [3]
  • Regular meditation may reduce cortisol levels by up to 20% [4]

 

When should you re-check your levels?


One result is useful, but two or more is powerful.

It’s worth re-testing whenever something significant changes — just give it two to three months for your bloodwork to catch up. But if you’re experiencing new or worsening symptoms, don’t wait. It’s best to take a test sooner and check in with your doctor.

Consider another blood test after you’ve:

  • Made big changes to your diet or started a new exercise routine
  • Developed new symptoms
  • Started taking supplements or medications
  • Been sleeping much better, or worse
  • Noticed a big shift in your stress levels
  • Lost or gained a significant amount of weight
  • Had a baby — the body undergoes massive changes during pregnancy and postpartum
  • Gone through a major biological shift, like menopause

If nothing’s changed, we generally recommend testing once a year. Even without any major lifestyle changes, your results will likely have shifted.

Find out more about recommended testing intervals.


The power of routine testing and tracking


If you’ve only tested once, you haven’t yet seen your full health picture. Repeating your test allows you to:

  • Spot trends and patterns
  • Measure the impact of lifestyle changes
  • Distinguish true changes from noise

Here’s what it looks like to track your health over time. Spot dips, peaks, and plateaus to make sure you’re a step ahead.
 

Medichecks blood test tracker - track your results


Your next step


If it’s been a year or your lifestyle has changed, it might be time for another blood test. Your blood is always changing, and that’s a good thing. Tracking your markers over time gives you the insight to take action, feel better, and stay ahead of your health.

Find your next test with our test finder.
 


References

  1. EFLM Biological Variation. [cited 17 Apr 2025]. Available: https://biologicalvariation.eu/
  2. Gummesson A, Nyman E, Knutsson M, Karpefors M. Effect of weight reduction on glycated haemoglobin in weight loss trials in patients with type 2 diabetes. Diabetes Obes Metab. 2017;19: 1295–1305. doi:10.1111/dom.12971
  3. Ryan DH, Yockey SR. Weight Loss and Improvement in Comorbidity: Differences at 5%, 10%, 15%, and Over. Curr Obes Rep. 2017;6: 187–194. doi:10.1007/s13679-017-0262-y
  4. Koncz A, Demetrovics ,Zsolt, and Takacs ZK. Meditation interventions efficiently reduce cortisol levels of at-risk samples: a meta-analysis. Health Psychology Review. 2021;15: 56–84. doi:10.1080/17437199.2020.1760727