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Why Do Blood Test Results Vary?

Getting different results from the same blood test? It happens, and here's why.

Scott Fotheringham, PhD

Medically reviewed by

Stefano Guandalini, MD


Curious what your blood has to say about your current health? Try a test today.

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Curious what your blood has to say about your current health? Try a test today.
Curious what your blood has to say about your current health? Try a test today.

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Key takeaways
  • It’s not unusual for blood test results to vary from time to time or lab to lab, but even blood taken at the same time and measured by the same lab can generate slightly different results.
  • Some factors, such as differences in lab equipment and processes, are out of your hands. 
  • Following instructions and taking tests at the correct time can help prevent unnecessary variations.

It’s fairly common to have the same blood test performed at different times from different labs. Doctors might order lab tests to monitor an ongoing health condition, to assess the effects of a treatment or to rule out false-positive or false-negative results. 

Ideally, if the samples were collected in a similar way and analyzed using the same technique, the results should be consistent. However, blood tests can give different values depending on the lab, the time of day the sample was taken and many other factors. 

This happens because each lab has a unique process for collecting, storing and analyzing samples, and because labs often use different equipment. Even when different labs use similar equipment and techniques, the test results can still vary a little.

But what if the results differ significantly? Why does this happen and what can you do about it? Learn more here.

Understanding your target range

Your results will be accompanied by a target range, also known as a reference range. This is the range of values that are considered normal. It comes from test results taken from a very large group of healthy people (and, when necessary, divided into age groups). Your healthcare professional uses this range as a basis of comparison to interpret test results. If your results fall outside this range it could mean that you have a health problem needing to be addressed. For example, the normal range for the magnesium blood test is 1.7 to 2.2 mg/dL.¹ 

Each laboratory will have its own target range, and it may differ between labs depending on the demographics of the control group. For example, the HbA1C test can be unreliable for those of African, Mediterranean or Southeast Asian descent, because people in these groups may have a type of hemoglobin that can interfere with an appropriate reading of some of these tests.²

Even when a healthy person has a result that falls outside the target range, it might not indicate the presence of a disease. It is important to consult with your healthcare professional to assess what your results mean.

Factors that can impact blood test results

Biomarkers are substances measured in the blood, like vitamin D, cortisol, glucose and others. They’re what you’re looking at when you take a blood test, and their levels can vary for a variety of reasons. Here are some common factors that can impact the results of routine blood tests.

Time of day, diet and hydration

Blood levels of biomarkers can vary during the day. Thyroid stimulating hormone (TSH) is a good example of a biomarker that fluctuates throughout the day and in response to diet, with higher levels occurring after fasting.³

What you eat and drink can also affect blood levels of various biomarkers. A good example is the HbA1C blood test for diabetes, since blood glucose levels can vary depending on diet and exercise. Similarly, dehydration tends to cause higher levels of blood urea nitrogen (BUN), an indicator of kidney health. 

If you’re taking a blood test, it’s important to follow all instructions, especially as they relate to time of day, diet or fasting and hydration.

Sickness and stress

Some conditions can alter biomarker levels temporarily. For example, blood glucose can be impacted by sickness or stress, which can in turn skew HbA1C results.

Changes in cyclical hormone levels

The levels of some biomarkers can fluctuate during the menstrual cycle, including iron, lipid levels (cholesterol), vitamin D and hsCRP. TSH levels are lower during menstruation, pregnancy and after menopause, sometimes falling below the normal range.¹⁰


Medications can impact blood test results. Among the drugs that lower TSH levels are glucocorticoids¹¹ and chemotherapy meds.¹² Other medications interfere with the accurate testing of TSH even though they don’t alter levels of the hormone, including NSAIDs, heparin and the B vitamin complex component biotin.¹³ Creatinine test results can be affected by some antibiotics and other drugs; if indicated, your healthcare professional may get you to stop taking these briefly prior to a test.¹⁴

It’s important that your healthcare provider knows about any medications, supplements or drugs you’re taking, so they can interpret your results accurately.

Sample collection, handling and storage

The same blood sample, tested more than once in the same lab, can give different results due to even slight changes in temperature, handling and equipment.

Sample collection

The way you collect your blood sample may differ from sample to sample. Variation in test result levels can even exist from drop to drop when collected from the same person at the same time.¹⁵ This is why imaware instructs patients to multiple drops for each blood test — not only does it ensure a good sample volume, it also allows for a more accurate average from all the collected drops.

Sample handling and storage

Time is a factor in sample results, which can become less accurate the longer a sample spends waiting between collection and analysis. Handling errors, like placing the sample in the wrong container or at the wrong temperature, can also affect results. Blood drop sample collection, during which drops of blood are deposited, dried and then shipped, is a stable way of collecting samples and unlikely to cause significant differences in results.

Lab-to-lab process differences

There can be differences in the equipment and analytical process between labs, including instrument calibration and sample normalization, a technique used to compare results and establish a baseline or context for how to measure them. 

What you can do to help ensure an accurate reading

If subsequent blood test results are much different than previous results, it could be because they've been processed at a different lab or it could indicate a potential health concern. Your healthcare professional may recommend repeating the test to confirm results. In the meantime, here’s what you can to do get the most accurate results:

  1. Take blood test results from the same lab or same type of at-home test kit each time.
  2. Follow your healthcare professional’s advice and be sure to read and follow the instructions in your at-home test kit before taking it.
  3. Update your healthcare provider on changes to your lifestyle, health and medications since your last test.


Repeated blood tests for the same biomarker can give different results. It is important to know whether this is due to changes in your health profile or because of differences in sample collection or lab techniques. Using the same lab for subsequent tests and communicating with your healthcare professional can help reduce preventable variations in blood test results.

Updated on
January 17, 2024
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1.  UCSF Health. Medical tests: Magnesium blood test. Accessed January 10, 2022.

2.  National Institute of Diabetes and Digestive and Kidney Diseases. The A1C Test & Diabetes. Accessed January 10, 2022.

3.  Indian Journal of Endocrinology and Metabolism. Does fasting or postprandial state affect thyroid function testing? Accessed February 11, 2022. 

4.  National Institute of Diabetes and Digestive and Kidney Diseases. The A1C Test & Diabetes. Accessed January 10, 2022.

5.  CDC. Diabetes: 10 surprising things that can spike your blood sugar. Accessed February 11, 2022.

6.  The American Journal of Clinical Nutrition. Variations in iron-status measures during the menstrual cycle. Accessed February 11, 2022.

7.  Journal of Clinical Lipidology. Variations in lipid levels according to menstrual cycle phase: clinical implications. Accessed February 11, 2022.

8.  Scientific Reports. Menstrual cycle rhythmicity: metabolic patterns in healthy women. Accessed February 11, 2022.

9.  Epidemiologic Reviews. Failure to consider the menstrual cycle phase may cause misinterpretation of clinical and research findings of cardiometabolic biomarkers in premenopausal women. Accessed February 11, 2022.

10.  MedicineNet. What Is a Normal TSH Level in a Woman? Accessed January 11, 2022. 

11.  Best Practice & Research: Clinical Endocrinology & Metabolism. Pitfalls in the measurement and interpretation of thyroid function tests. Accessed February 11, 2022. 

12.  Journal of the Nationall Cancer Institute. Thyroid dysfunction from antineoplastic agents. Accessed February 11, 2022. 

13.  Thyroid. Misdiagnosis of Graves' disease with apparent severe hyperthyroidism in a patient taking biotin megadoses. Accessed February 11, 2022. 

14.  UCSF Health. Medical tests: Creatinine blood test. Accessed January 11, 2022.

15. American Journal of Clinical Pathology. Drop-to-Drop Variation in the Cellular Components of Fingerprick Blood: Implications for Point-of-Care Diagnostic Development. Accessed February 11, 2022.

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