How reliable are blood marker tests?

Your results may not tell the whole story. Discover what affects the accuracy of readings, and how to discuss your numbers with your doctor.

WRITTEN BY

Dan Wagener

Updated: 12/10/2025|8 min read

Blood tests seem straightforward: a simple needle stick that reveals the inner workings of your body. But a web of factors—from your supplement regime and stress levels to when you last worked out and how hydrated you are—can influence your test results, sometimes leading to readings that don’t indicate your true health status. Because of this, blood tests are most useful when analyzed alongside other elements of clinical assessment, such as related tests and any symptoms or health conditions you have.

Though it’s always best to consult your medical provider about any results, knowing what can affect these tests can help you better gauge what may be at play and have a constructive conversation with your doctor. Here’s what to keep in mind and when you might consider retesting.

This information does not constitute medical advice. This is compiled expert opinion for educational purposes. Your doctor knows you best. Talk to your doctor for specific questions about your personal lab results.

Individual factors that affect blood test results

Some of the factors that can impact blood tests—such as that cold you’re getting over or medications—might seem obvious. But your last workout or too many late nights can also skew your lab work.

Below are some of the most important factors to be aware of and which tests they can influence.

Your last meal

When your body digests food and beverages, sugars, fats, and other nutrients enter your bloodstream and can affect certain markers. You may need to avoid eating or drinking anything (including coffee and gum) for eight to 12 hours before certain blood tests, including:

• Blood glucose
• Cholesterol (total, HDL, LDL)
• Triglycerides
• Basic metabolic panel

Medications and supplements

Certain prescription and over-the-counter medications may have effects on the body beyond their primary purpose, and some of these can show up on blood tests.

• Diuretics, steroids, and insulin can affect a comprehensive metabolic panel. Diuretics promote water and electrolyte excretion, while steroids cause water retention and increase the excretion of potassium and calcium. Both of these medications can alter electrolyte levels. Additionally, steroids increase blood glucose levels, while insulin lowers them.
• Tylenol can increase liver function test results when taken in high doses (3,000 milligrams a day, according to one study).
• Antibiotics can lower blood glucose, either when taken alone or with sulfonylureas or meglitinides.
• Chronic use of opiates and high doses of aspirin can falsely increase hemoglobin A1c (HbA1c), a test used to diagnose Type 2 diabetes and monitor blood sugar levels in diabetes patients.
• Selective serotonin reuptake inhibitors (SSRIs), a type of antidepressant, can increase red blood cell counts, decrease white blood cell counts, and affect other blood markers such as mean corpuscular volume (average size of red blood cells) and hemoglobin (a protein in red blood cells that carries oxygen throughout the body).

Vitamins, minerals, and herbal supplements can also temporarily raise or lower certain blood markers. For example, high doses of biotin (vitamin B7, which is often taken for hair, nail, and skin health) can lead to inaccurately low results for troponin, a blood test used to diagnose heart attacks, and interfere with thyroid hormone tests. The reason? Many labs attach biotin to antibodies to help detect certain proteins and hormones in the blood. But high levels of biotin in a sample can compete with the biotin on the antibodies, skewing the results.

Before taking a test, tell your doctor about all the medications and supplements you’re taking. If you don’t need to stop taking them before your blood work, your doctor will take your medication use into account when interpreting your results.

Time of day

You might want to schedule certain tests in the morning. Some hormones, such as thyroid and testosterone, follow a circadian rhythm, so many clinicians prefer morning testing (roughly 7–10 a.m.) for consistency. If you can’t make an early appointment and later retest these hormones, try to retest at the same time for an accurate comparison.

Phase of your cycle

Levels of estradiol, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and progesterone fluctuate during the menstrual cycle. Many providers recommend testing these on day three of your cycle.

Health conditions

Short-term illnesses and long-term conditions can create challenges for interpreting bloodwork.

Acute ailments such as a cold and the flu trigger the immune response, resulting in inflammation, which shows up as higher white blood cell counts and C-reactive protein (CRP) levels.

Most chronic health conditions also modify test outcomes. For example, in addition to raising A1C (blood glucose) levels, diabetes can affect how your body manages cholesterol, leading to high levels of low-density lipoprotein (LDL) and triglycerides. And since rheumatoid arthritis is an inflammatory condition, it can increase levels of inflammatory markers such as erythrocyte sedimentation rate (ESR) and CRP.

Stress

Chronic physical and emotional stress can raise blood sugar in response to the release of hormones such as adrenaline and cortisol. Stress can also weaken your immune system, lowering white blood cell counts and increasing inflammatory markers.

Lifestyle factors

Daily habits—from exercise to water intake to sleep—can affect test results.

Intense workouts (such as high-intensity interval training or heavy weight training) within 24 hours of testing can alter blood markers, causing:

• Decreased blood glucose
• Decreased triglycerides
• Increased HDL
• Increased liver markers
• Increased white blood cell count

In addition, dehydration concentrates your blood, which can elevate the levels of some markers, such as red blood cell count and hematocrit (the proportion of red blood cells in your blood). On the other hand, overhydration can lower red blood cell count.

Both short-term and chronic sleep deprivation can throw your tests out of whack, too, boosting white blood cells, CRP, and blood sugar levels due to disrupted glucose metabolism.

How lab processing affects your results

Another thing to consider when doing any testing is where your blood sample is processed, because “normal” range test results can vary from lab to lab.

To determine their “normal,” each lab collects data from a large group of healthy people matched by age, gender, and ethnicity. They identify the central 95 percent of results, and make that the normal range. That means 5 percent of healthy people will have abnormal results. Plus, labs may use different populations to establish their normal ranges, so what's considered normal at one lab might be abnormal at another.

What’s more, different labs may use different machines and calibration standards, which can lead to slightly different results from the same blood sample.

Additionally, mistakes can happen during blood sample collection and processing. Samples may be improperly collected, damaged, contaminated, or lost. They might also be stored at the wrong temperature or not processed quickly enough, which can cause an inaccurate reading. However, these errors aren’t common (they’re estimated to occur in 0.01 percent to 0.6 percent of all lab tests), and most errors are caught before results are sent to patients.

Several of the factors covered above (such as meal timing, medications, and illnesses) can also cause false positives (a result outside the normal range when you’re actually healthy) and false negatives (a healthy result when you’re actually outside the normal range). That’s why it’s important to tell your provider about all the medications and supplements you’re taking before testing, and to follow their pre-test instructions. Then, if any results don’t align with your symptoms or medical history and you suspect a false positive or negative, talk to your provider.

Best practices for interpreting blood test results

A single test doesn’t typically equal a diagnosis. If a particular marker comes back out of range, your provider will likely want to retest. They may also order related tests (such as a cholesterol panel and inflammatory markers for heart disease, for example) and compare those results to your symptoms, medical history, and physical exam findings.

Multiple tests over time may reveal a pattern, such as that you tend to fall on the high or low end of the normal range. But if you’re normally on the low end of a range and suddenly test on the high end, that may be cause for concern.

How to talk to your provider about your results

If a test result comes back out of range, talk to your doctor. You may want to ask:

• This test was flagged as high/low. Does it indicate a problem?
• What could have caused this result? For example, could my weight-training session or meal the day before the test have affected the test?
• What does this result mean in the context of other tests, my symptoms, and my physical exams?
• What does this result mean for my treatment plan?
• Do I need to retest or schedule a follow-up visit?
• Should I do anything differently for the retest to get more accurate results?

Preparing for your next trip to the lab

Though blood tests are fundamental for screening for diseases and assessing overall health, test preparation, physiology, health conditions, and lifestyle can affect the results. How the lab processes your sample can also affect the results.

So retest to confirm any abnormal result, and be sure your provider evaluates those results in the context of related tests, your symptoms, and your health history.