Blood tests can be very helpful in guiding your health and targeting areas where you may need improvement—as long as your results are accurate.

While you may be given specific instructions around your particular test (i.e., don't eat or drink before, make sure you have blood drawn in the morning, and so on), several factors can skew your results, which may be overlooked.

In this article, we'll cover what you can do to best prepare yourself for your blood work and explore some of the most common mistakes people make that throw off their results.

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How To Prepare For A Blood Test

There are a few universal factors to remember when preparing for blood work (regardless of which markers you're testing).

First, you must pay attention to whether or not you need to be fasted prior to your blood draw. Your results can become significantly skewed if you go in for a fasted blood draw having consumed food or caloric beverages. It should state clearly on your requisition form whether or not the blood draw needs to be done in a fasted state, so always look at that information ahead of time so you can plan.

The second thing to keep in mind is your hydration. When you're dehydrated, it can be more challenging for your phlebotomist to find a good vein to puncture. Furthermore, if your water balance is off, the concentration of certain compounds in your blood could appear higher. This will throw off your test results.

Third, if you're on any form of medication—be sure to take it as directed. Certain meds can impact blood test results, and if you miss a dose or take it at the wrong time, it could skew your blood markers.

And finally, getting adequate sleep and rest is important for maintaining homeostasis in your body. You have a natural circadian rhythm that your body likes to follow, so the night before your blood test, try to go to bed at a regular time, avoiding too much late-night activity.

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6 Factors That Can Affect Your Test Results

Let's dive deeper into some factors that may influence your results. Depending on the type markers you're looking at, some of these factors may be irrelevant, but some may be highly influential.

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Meal timing

The timing of your last meal before your test is crucial if you're testing blood glucose or lipid panels. Recent food intake can significantly impact these markers and throw off your results.

For example, blood glucose markers will be much more stable after an overnight fast. If you have breakfast before your blood draw, it could insinuate that your glucose levels are high when they are perfectly normal[1].

Your food intake can also directly impact the results of a lipid panel, as your cholesterol and triglyceride markers will fluctuate depending on what you eat[2].

This goes back to reviewing your lab requisition form and making sure that you're fasted if the markers you're checking require you to be in a fasted state.

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Timing of the test

Both the date and time of your best may make a difference in your results.

For women, choosing the correct time during their menstrual cycle is crucial for accurate hormone test results. Most physicians request blood draws somewhere between the 19th and 21st day of a woman's cycle so they can assess whether levels of estrogen and progesterone are where they should be during the luteal phase.

For other markers, the time of day in which you test can have an influence due to the fluctuations of certain markers throughout the 24-hour cycle.

Cortisol is a great example. As a hormone, cortisol rises and falls with your circadian rhythm. It's naturally highest in the morning (when it's in balance), so most cortisol tests are scheduled for first thing in the morning[3].

Several other hormones follow specific biological rhythms, such as thyroid stimulating hormone (TSH) and melatonin. Depending on which hormones you're hoping to test, you'll want to schedule your blood draw to work within their optimal assessment window.

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Physical activity

Physical activity is often overlooked as a culprit for invalid test results for several reasons.

First, depending on how active you are, you may throw off your electrolyte balance with physical activity before a blood test if you sweat a lot. This jeopardizes your test results for electrolytes (including calcium, potassium, magnesium, and sodium) and could skew kidney function results. Furthermore, as we already covered, dehydration may make certain markers appear in higher concentrations than they actually are[4].

Exercise can also impact the way your body uses blood glucose. If you test directly after a workout, blood glucose may appear high as your body was just liberating nutrients to use as energy. On the other hand, intense exercise may lower blood glucose levels and potentially lead to a false result showing low blood sugar[5][6].

The hormones cortisol and adrenaline are also at the whims of exercise, pumping them up directly after a workout and potentially lowering them an hour or so later[7][8].

It's also possible physical activity could impact your white blood cell count as part of your immune and inflammatory response. If your activity is intense enough, the number of white blood cells could temporarily increase, giving you a false positive for high white blood cells[9].

Either way, if you're getting blood drawn (especially for glucose or hormones), it may be a good idea to skip the morning workout.

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Alcohol

Drinking alcohol the night before a blood test could potentially influence several markers.

First, due to its impact on your nervous system, alcohol consumption may shift levels of cortisol the day after drinking. Insulin, an important metabolic marker, is also affected by alcohol consumption, which could have downstream effects on any test that relates to endocrine function[10][11].

Blood glucose levels may also appear higher or lower the day after drinking alcohol. This means that even if you're fasting for your blood glucose draw your levels may not be accurate as a result of the previous day's alcohol intake[12].

And finally, due to its diuretic nature, drinking alcohol the night before a blood draw could lead to dehydration—influencing tests that involve electrolytes or kidney function (creatinine and blood urea nitrogen).

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Medications

Prescription medications can influence blood markers in several ways, depending on the medication and the specific markers you're looking at.

Some common examples include:

1. Anticoagulants like warfarin and heparin (among others) may impact blood clotting tests like INR (International Normalized Ratio) or PT (Prothrombin Time)[13].
2. Diuretics may influence electrolyte levels due to their potentially dehydrating effect, impacting both electrolyte and kidney lab tests.
3. Statin drugs may interfere with liver enzyme tests[14].
4. Antidepressant drugs (SSRIs) may influence the results of thyroid tests[15].
5. Steroid drugs like prednisone can alter several tests including liver markers, inflammation, and glucose levels[16].
6. Diabetic medications can influence blood glucose levels and, therefore, interfere with tests that measure blood glucose or insulin.
7. Hormonal medications, including hormone replacement therapy or birth control pills, not only affect hormone levels but could also influence lipid profiles and liver function markers[17].

The list goes on, but what's key to understanding is that if you're taking any type of prescription drug, you should consult with your doctor on how it should be taken in the 24 hours prior to your test. In some cases, they may advise that you take it as regularly directed (particularly if they want to determine how well that drug is working for you.) In other cases, however, they may suggest holding off until after your test if your medication could potentially alter other results. Always discuss the protocol with your prescribing doctor before making any changes to your medication schedule.

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Supplements

Many people don't realize that in addition to medications, over-the-counter supplements can also significantly influence blood work.

Here are some prime examples:

• High levels of folic acid may mask a vitamin B12 deficiency in bloodwork[18].
• Biotin (vitamin B7) may interfere with thyroid tests as well as other hormones, including testosterone and estrogen. It could also potentially influence troponin tests, a marker used to confirm a heart attack[19].
• Vitamin D may interfere with calcium testing as this nutrient causes calcium levels in the blood to rise, potentially showing a false result related to serum calcium levels [20].
• Taking creatine may influence kidney function tests by temporarily elevating creatinine levels[21].
• Ginseng can lower blood glucose levels and when taken before a lab test may interfere with the results by altering the true fasting glucose status of the individual[22].

Again, if you're working with a health professional, always refer to their guidance and share your current supplements. If you're ordering labs independently, cross-check between your supplements and the labs you're having performed to ensure there won't be any interference.

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Takeaway

While blood tests can provide incredibly valuable information about the state of your health, it's crucial that the results you receive are accurate. If your bloodwork comes back and you're surprised with your results, you may want to re-check and ensure you followed the steps outlined in this article.

In addition to following the instructions on your requisition form (and those given by your healthcare practitioner), make sure that you get adequate rest and hydration prior to your blood draw to set yourself up for success and optimize your body for accurate results.

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References

1. Takahashi, Masaki, et al. "Effects of meal timing on postprandial glucose metabolism and blood metabolites in healthy adults." Nutrients 10.11 (2018): 1763.
2. Grant, Leilah K., et al. "Time-of-day and meal size effects on clinical lipid markers." The Journal of Clinical Endocrinology & Metabolism 106.3 (2021): e1373-e1379.‍
3. ‍Cortisol Test: What It Is, Types, Purpose & Results
4. Baker, Lindsay B. "Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability." Sports Medicine 47 (2017): 111-128.
5. Understanding Blood Glucose and Exercise | ADA
6. ‍Why Does Exercise Sometimes Raise Blood Glucose | ADA
7. Hill, E. E., et al. "Exercise and circulating cortisol levels: the intensity threshold effect." Journal of endocrinological investigation 31 (2008): 587-591.
8. Hackney, Anthony C., and Elizabeth A. Walz. "Hormonal adaptation and the stress of exercise training: the role of glucocorticoids." Trends in sport sciences 20.4 (2013): 165.
9. Neves, Pedro Rogério Da Silva, et al. "Acute effects of high-and low-intensity exercise bouts on leukocyte counts." Journal of exercise science & fitness 13.1 (2015): 24-28.
10. Badrick, Ellena, et al. "The relationship between alcohol consumption and cortisol secretion in an aging cohort." The Journal of Clinical Endocrinology & Metabolism 93.3 (2008): 750-757.
11. Rachdaoui, Nadia, and Dipak K. Sarkar. "Effects of alcohol on the endocrine system." Endocrinology and Metabolism Clinics 42.3 (2013): 593-615.
12. Emanuele, Nicholas V., Terrence F. Swade, and Mary Ann Emanuele. "Consequences of alcohol use in diabetics." Alcohol health and research world 22.3 (1998): 211.
13. Zaidi, Syed Rafay H., and Preeti Rout. "Interpretation of Blood Clotting Studies and Values (PT, PTT, aPTT, INR, Anti-Factor Xa, D-Dimer)." StatPearls [Internet]. StatPearls Publishing, 2024.
14. Ashraf, Jibran, et al. "Statins and Abnormal Liver Function Tests: Is There a Correlation?." Cureus 12.8 (2020).
15. Liao, Huijuan, David S. Rosenthal, and Salini C. Kumar. "Abnormal thyroid function laboratory results caused by selective serotonin reuptake inhibitor (SSRI) antidepressant treatment." Case Reports in Psychiatry 2023.1 (2023): 7170564.
16. Kauh, Eunkyung, et al. "Prednisone affects inflammation, glucose tolerance, and bone turnover within hours of treatment in healthy individuals." European journal of endocrinology 166.3 (2012): 459-467.
17. Godsland, Ian F., et al. "The effects of different formulations of oral contraceptive agents on lipid and carbohydrate metabolism." New England Journal of Medicine 323.20 (1990): 1375-1381.
18. Cuskelly, Geraldine J., Kathleen M. Mooney, and Ian S. Young. "Folate and vitamin B12: friendly or enemy nutrients for the elderly*: Symposium on ‘Micronutrients through the life cycle’." Proceedings of the Nutrition Society 66.4 (2007): 548-558.
19. Rosner, Ilana, et al. "Clinically significant lab errors due to vitamin B7 (Biotin) supplementation: a case report following a recent FDA warning." Cureus 11.8 (2019).
20. Tebben, Peter J., Ravinder J. Singh, and Rajiv Kumar. "Vitamin D-mediated hypercalcemia: mechanisms, diagnosis, and treatment." Endocrine reviews 37.5 (2016): 521-547.
21. Yoshizumi, Wyndie M., and Candy Tsourounis. "Effects of creatine supplementation on renal function." Journal of herbal pharmacotherapy 4.1 (2004): 1-7.

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