Many things besides disease can cause abnormal blood test results. What you ate, when you exercised, which supplements you take, and even how the blood sample was handled can all shift your numbers outside the normal range. Understanding these factors can save you from unnecessary worry and help you get more accurate results next time.
Eating Before a Fasting Test
Fasting blood tests exist for a reason. Eating before one can temporarily raise your blood sugar and lipid levels enough to flag as abnormal. Triglycerides are especially sensitive: after a normal meal, they can rise by about 26 mg/dL within one to four hours. That increase alone could push someone from the “normal” category into the “borderline high” range on a lipid panel. Blood glucose similarly spikes after eating, which is why fasting glucose and fasting lipid panels require 8 to 12 hours without food.
Even small snacks, sugary drinks, or coffee with cream can affect results. If you accidentally ate before a fasting test, mention it to the person drawing your blood. In many cases, the lab can note it on your results or your doctor can interpret them with that context in mind.
Dehydration
Not drinking enough water before a blood draw concentrates your blood. With less plasma volume, the proportion of red blood cells rises, which inflates your hematocrit (the percentage of red blood cells in your blood). Kidney markers shift too: BUN tends to rise more than creatinine when you’re dehydrated, creating an elevated ratio that can mimic kidney problems. Hemoglobin, protein levels, and electrolytes can all read higher than they truly are simply because there’s less fluid diluting them.
Mild dehydration from skipping water overnight during a fast is common and usually minor, but if you’ve been sweating heavily, vomiting, or restricting fluids, the effect can be large enough to trigger a retest.
Medications and Supplements
Dozens of common medications can cause liver enzyme elevations that look alarming on paper but don’t reflect actual liver damage. Statins, NSAIDs, acetaminophen, certain antidepressants (fluoxetine, sertraline, trazodone), blood pressure medications like lisinopril and losartan, acid reflux drugs like omeprazole, and even herbal products like green tea extract and kava can all raise liver enzymes. These elevations are usually mild, produce no symptoms, and resolve after stopping the medication. Clinically significant liver injury from these drugs is rare.
Biotin: A Widely Underestimated Problem
Biotin supplements deserve special attention because they interfere with the testing technology itself, not just the biology being measured. High-dose biotin (often found in hair, skin, and nail supplements at 5,000 to 10,000 mcg per pill) can skew results on a wide range of immunoassay-based tests.
The interference goes in both directions depending on the test. Biotin can falsely lower results for thyroid hormones (free T4, free T3, TSH-related markers), troponin (a cardiac damage marker), ferritin, vitamin B12, folate, parathyroid hormone, insulin, and even pregnancy tests. At the same time, it can falsely raise cortisol, testosterone, estradiol, progesterone, and vitamin D levels. A falsely low troponin result during a heart attack, or a falsely normal thyroid panel when someone actually has thyroid disease, could have serious consequences.
If you take biotin supplements, stop them at least 48 to 72 hours before any blood work. This is one of the simplest ways to prevent a misleading result.
Intense Exercise
A hard workout in the day or two before a blood draw can significantly alter your results. The most dramatic effect is on creatine kinase (CK), an enzyme released when muscle fibers break down. Triathletes showed a 12-fold average increase in CK levels 24 hours after a race. In one study, 55% of participants still had CK levels above 2,000 IU/L four days after performing intense arm exercises. For reference, the normal upper limit is typically around 200 IU/L.
AST, a liver enzyme, is also found in muscle tissue. A tough gym session can raise AST enough to suggest liver problems when your liver is perfectly fine. Even a long run or an unusually strenuous hike can do this. If possible, avoid heavy exercise for 48 to 72 hours before blood work, especially if your doctor is checking muscle or liver enzymes.
Time of Day
Your body doesn’t produce hormones and other substances at a constant rate. Cortisol, for example, follows a pronounced daily cycle. It surges 50 to 150% above baseline within the first 30 to 60 minutes after waking, then gradually declines through the afternoon and evening. A cortisol test drawn at 8 a.m. and one drawn at 4 p.m. on the same person can produce very different numbers, both of which are “normal” for that time of day but could look abnormal without context.
Iron levels also fluctuate throughout the day, typically peaking in the morning and dropping by afternoon. Testosterone follows a similar morning-high pattern. This is why doctors order certain tests for specific time windows: an early morning draw gives the most standardized, comparable result.
Acute Illness and Infection
Even a mild cold can change your blood count. Your white blood cell (WBC) differential, which breaks down the types of immune cells in your blood, shifts depending on what your body is fighting. Viral infections like the flu or a common cold typically cause lymphocytosis, an increase in lymphocytes. Bacterial infections tend to raise neutrophils instead. These shifts are your immune system working correctly, not a sign of a blood disorder.
Inflammation from any cause, whether an infection, injury, or autoimmune flare, can also raise markers like C-reactive protein, ferritin, and platelet counts. If you were feeling under the weather when your blood was drawn, that context matters when interpreting the results.
Sample Handling and Lab Errors
Sometimes the problem isn’t you at all. Hemolysis, when red blood cells rupture during or after the blood draw, is one of the most common reasons labs reject or flag samples. When red blood cells break open, they release their contents into the serum, and potassium is the biggest concern. Potassium rises roughly 0.003 mmol/L for each unit increase in the hemolysis index, which can push levels into the “high” range and falsely suggest a dangerous electrolyte imbalance.
Hemolysis can happen if the needle is too small, if the tourniquet is left on too long, if the sample is shaken roughly, or if it sits at the wrong temperature. Other collection issues, like underfilling a tube or mixing up sample labels, can also produce errors. If a single result looks wildly out of place compared to your other values, a redraw is a reasonable next step.
Stress, Sleep, and Lifestyle Factors
Acute stress raises cortisol and blood sugar, sometimes enough to produce abnormal readings on a single test. Poor sleep has a similar effect: even one night of significant sleep deprivation can alter insulin sensitivity, inflammatory markers, and hormone levels. Smoking raises your white blood cell count chronically, and heavy alcohol use can elevate liver enzymes, mean corpuscular volume (a measure of red blood cell size), and triglycerides.
These lifestyle factors are worth mentioning to your doctor when reviewing results, especially if a test comes back unexpectedly abnormal. A single abnormal value often means less than a pattern of abnormal values across multiple tests taken under consistent conditions.