Common Health Screening Mistakes That Can Affect Your Blood Test Results
Discover the most common health screening mistakes that can affect your blood test results and learn how to prepare properly for accurate readings.
If you have ever received blood test results that seemed unexpected — a cholesterol reading higher than anticipated, an unusually elevated glucose level, or an iron marker that did not match how you were feeling — the explanation may not lie in your health at all. It may lie in how you prepared for the test. Whether you are booking a private screening or attending an NHS check, proper preparation is essential.
The Most Common Mistakes That Affect Blood Test Accuracy
1. Not Fasting When Required
Many blood panels — particularly those measuring glucose, triglycerides, and cholesterol — require a fasting period of 8 to 12 hours beforehand. Consuming food or calorie-containing beverages during this window can cause temporary spikes in blood sugar and lipid levels.
2. Forgetting Supplements and Medications
Biotin (vitamin B7), commonly found in hair, skin, and nail supplements, can interfere with certain laboratory tests. High-dose biotin intake may affect thyroid function results, cardiac biomarkers, and hormone panels. Similarly, iron supplements can artificially elevate serum iron readings.
3. Intense Exercise Shortly Before the Test
Strenuous physical activity can temporarily raise creatine kinase (CK), liver enzymes (ALT and AST), and inflammatory markers such as C-reactive protein (CRP). Avoiding vigorous exercise for 24 to 48 hours before a blood test can help ensure results are not influenced by post-exercise inflammation.
4. Dehydration on the Day of the Test
Arriving dehydrated can concentrate certain biomarkers in the blood. Kidney function markers such as creatinine and urea, as well as haematocrit levels, may appear elevated in a dehydrated sample.
5. Alcohol Consumption Before Testing
Even moderate alcohol consumption 24 to 48 hours before a blood draw may influence liver enzymes (GGT, ALT, AST), triglyceride levels, blood glucose readings, and uric acid levels.
6. Stress and Poor Sleep
Significant stress, anxiety, or a poor night's sleep can elevate cortisol levels beyond typical ranges, which may in turn affect blood glucose, white blood cell counts, and inflammatory markers.
7. Testing at the Wrong Time of Day
Certain biomarkers fluctuate naturally throughout the day. Testosterone levels, for example, are typically highest in the early morning and may decline by up to 30% by the afternoon. Morning appointments are often recommended for hormone panels.
How Common Mistakes Affect Key Biomarkers
| Mistake | Biomarkers Affected | Likely Impact |
|---|---|---|
| Not fasting | Glucose, triglycerides, cholesterol | May appear elevated |
| Biotin supplements | Thyroid hormones, troponin | Falsely high or low |
| Intense exercise | CK, ALT, AST, CRP | May appear elevated |
| Dehydration | Creatinine, urea, haematocrit | May appear elevated |
| Alcohol (within 48hrs) | GGT, ALT, triglycerides, glucose | May appear elevated |
| Stress/sleep deprivation | Cortisol, glucose, WBC | May appear elevated |
| Afternoon testing | Testosterone, cortisol, TSH | May appear lower |
Once you receive your results, our guide on what your screening results mean can help you understand each marker in context.
Blood Test Preparation: A Quick Checklist
- Confirm whether fasting is required and for how long
- Drink plenty of water in the hours before your appointment
- Avoid alcohol for at least 48 hours beforehand
- Skip vigorous exercise for 24–48 hours prior
- Note down all supplements and medications you are taking
- Try to get a reasonable night's sleep
- Book a morning appointment where possible, especially for hormone panels
For a more complete walkthrough, read our guide on how to prepare for a private health assessment in London.
Taking a Proactive Approach to Your Wellbeing
Understanding the common mistakes that can affect your blood test results is a simple but powerful step towards making your health screening as informative as possible. By preparing properly, you give yourself the best chance of receiving results that genuinely reflect your current health. If you are unsure which screening level is best for you, explore the differences between basic, advanced, and platinum health checks.
Explore our available health screening options or book an appointment at a time that suits you. If you are still deciding on the right test, our guide to choosing the right health screening package can help.
The information provided in this blog is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional regarding any health concerns.
