Lab Testing FAQs > Factors That Can Affect Lab Test Results

Lab Testing Results: 5 Factors to Know

Has your physician ever instructed you to fast or stop taking medications before having your blood drawn? The reason doctors sometimes ask patients to modify their behavior in specific ways before a lab test is because certain factors, including antibiotics and diet, can impact the accuracy of test results.

Usually, a skewed result can be rectified by performing the test again on a different day. But if the mistake goes unnoticed, the patient could receive an inaccurate diagnosis, be prescribed the wrong medication, or be subjected to unnecessary medical procedures.

Below, we discuss the five most common factors that can skew blood work and other laboratory test results.


Certain prescription and over-the-counter medications can impact the results of your blood test. Examples of medicine that could skew your lab test results include:

  • Vitamins (for example, Biotin)
  • Antibiotics
  • Antidepressants
  • Steroids (like Prednisone)
  • Acetaminophen
  • NSAIDs

However, keep in mind that this does not necessarily mean that you should stop taking your medications before lab testing, especially if they are prescribed by a healthcare provider. Rather, you should share your list of medications and supplements with your physician before testing so they can take them into consideration when interpreting your results.

If you are unsure about whether you should stop taking medications before laboratory testing or not, be sure to ask your doctor for clarification.

Diet & Nutrition

What and when a patient eats before having their lab sample collected can have an effect on lab test results. Certain lab tests—such as cholesterol tests and triglyceride level tests—may require the patient to fast for up to 12 hours before the test. This is because the nutrients in some foods can cause chemical changes to occur in the body, potentially skewing test results.

Diets that exclude certain categories of foods (for example, vegan or low-carb) may also produce an unexpected laboratory result. In these cases, it’s important for the healthcare provider to interpret the results within the scope of the patient’s overall lifestyle. 

Likewise, dehydration may affect the testing of lipids (cholesterol tests), urea, creatinine, and other substances. It can also be the cause of sodium and potassium imbalances indicated by test results.


Vigorous exercise puts stress on the body, which can trigger certain hormonal and other changes. For instance, high-intensity exercise has been found to cause an increase in glucose production that lasts for up to several hours after the activity ends. 

Intense exercise can also elevate creatine kinase (CK) levels, inflammatory markers, and red blood cell count. When testing occurs shortly after exercise, these and other indicators may appear skewed in the results.


Caffeine, alcohol, and tobacco are all examples of substances that could affect lab test results, depending on the amount consumed and the type of test. Caffeinated beverages such as coffee and black tea are considered diuretics and can affect the speed of digestion. As such, they should usually be avoided before lab testing.

Likewise, alcoholic beverages can elevate liver enzymes and blood sugar levels, thereby skewing the results of blood tests that require fasting. Tobacco usage has also been linked to elevated blood sugar, triglycerides, and cortisol, and should be avoided before testing.

Biological Factors

In addition to the lifestyle factors listed above, there are certain non-modifiable biological factors that may affect lab test results. These factors cannot be controlled or mitigated by the patient; therefore, they must be taken into consideration when interpreting the results.

Examples of biological factors that can impact test results include puberty, pregnancy, menopause, menstruation, age, and gender. For instance, adults older than 65 are more likely to have results that fall outside of the normal reference range compared to younger adults. Some of the most common lab tests include different reference ranges based on age, gender, etc. to help offset these variations.

Normal biological rhythms can also affect the accuracy of some blood tests. For example, hormone production and cardiac function follow a 24-hour circadian rhythm. Testing an individual in the morning may produce a different result compared to testing in the evening. These factors can be controlled for by testing at the recommended time of day.

Keep in mind that not every type of laboratory test requires behavioral changes like fasting. Your physician will let you know how to properly prepare for your specific test based on your unique medical history and the type of test being prescribed.

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