Blood tests are powerful, but they only tell the truth if the sample reflects your usual baseline. The hours and days before a draw can change results enough to flag a healthy person as sick, or hide a real problem. Below are nine common factors that interfere with blood work, and how to prepare for each.
Alcohol
Even a single evening of moderate drinking shifts several markers. Alcohol is processed by the liver, and that processing pushes AST and ALT up. A few drinks the night before a blood draw can make a healthy liver look injured on paper.
Alcohol also messes with blood sugar. There's an initial glucose spike, then reactive hypoglycemia hours later as the liver prioritizes alcohol metabolism over glycogen release. So your "fasting" glucose the morning after drinking can read artificially low.
Triglycerides rise sharply after alcohol, sometimes staying up for 24 to 72 hours. Chronic heavy drinking adds more distortion: macrocytic anemia (high MCV on a CBC) from alcohol's effect on red cell production, and low magnesium and potassium from its diuretic effect.
Preparation tip: skip alcohol for at least 24 to 48 hours before any blood test. For a liver or lipid panel, 72 hours is safer.
Diet and nutrition
What you eat in the days before a test (not just the night before) shapes the result. A diet high in saturated and trans fats raises LDL and triglycerides, sometimes enough to push a borderline lipid panel into the abnormal range. The flip side: a sudden week of clean eating before a test can make chronic dyslipidemia look better than it actually is.
High sugar intake causes glucose spikes that can read as prediabetes on a fasting glucose or HbA1c. A heavy protein meal the night before can lift blood urea nitrogen (BUN) and some liver enzymes, since the liver and kidneys are working harder to process the load.
Dehydration is the most underrated confounder. When plasma volume drops, blood markers concentrate. Hemoglobin, hematocrit, electrolytes, and creatinine all read higher than they should, which can make kidney function look worse than it is.
Long-term diet patterns matter too. Vegetarian and vegan diets are associated with lower B12 and iron, which may flag as deficiencies even in otherwise healthy people. The Heart and Stroke Foundation of Canada recommends limiting sodium to 2,000 mg per day. Regularly exceeding that can shift electrolyte panels.
Preparation tip: fast for 8 to 12 hours before testing (water is fine and encouraged). Eat normally in the days prior. Don't crash-diet or binge before a test.
Intermittent fasting
If you practise intermittent fasting (IF) regularly, your body has shifted its metabolic baseline. During longer fasting windows, the body moves from glucose metabolism to fat oxidation and starts producing ketone bodies. That's normal for a regular faster but can surprise a clinician who isn't told about it.
Regular IF practitioners often show lower fasting glucose and better insulin sensitivity. Triglycerides are usually lower too, since fat stores are being used efficiently.
IF can also bump liver enzymes (mild AST/ALT elevation) as the liver ramps up gluconeogenesis and ketone production. Electrolyte shifts, mostly lower sodium and potassium, are common during longer fasts if hydration and minerals aren't managed.
Preparation tip: stick to your usual fasting schedule before bloods. Don't break your routine to "normalize" the numbers. Your clinician needs to see the real baseline. Tell them you practise IF so they can read the results in context.
Sleep
A bad night of sleep doesn't just leave you tired. It distorts blood chemistry. Sleep deprivation activates the HPA axis and pushes up cortisol. Higher cortisol triggers gluconeogenesis in the liver, which raises fasting glucose and can mimic early insulin resistance on a metabolic panel.
Inflammation markers move too. Even one night of restricted sleep (under 5 hours) raises C-reactive protein (CRP) and interleukin-6 (IL-6). A single rough night can make a healthy person look like they have an active inflammatory process.
Sleep loss also shifts the lipid profile: triglycerides go up, HDL goes down. White blood cell counts rise as the immune system responds to the physiological stress.
Preparation tip: aim for 7 to 9 hours the night before bloods. If you had a brutal night, reschedule if you can. The Canadian Mental Health Association recommends consistent sleep habits as a baseline for overall health.
Stress
Acute and chronic stress activate the sympathetic nervous system, flooding the bloodstream with cortisol and adrenaline. Both have direct, measurable effects on blood chemistry.
Cortisol triggers gluconeogenesis: the liver converts stored glycogen and amino acids into glucose. That raises fasting blood sugar even when you haven't eaten. In a stressed patient this can look identical to early type 2 diabetes on a glucose test. Long-term stress compounds the effect and contributes to real insulin resistance over time.
Stress hormones also push up LDL cholesterol by promoting VLDL production in the liver. White blood cell counts rise as the body prepares for potential injury or infection, and CRP climbs with it. For patients getting tested to investigate fatigue, weight gain, or mood changes, stress-driven distortions can send the diagnostic process down the wrong path.
Preparation tip: deep breathing, a short walk, or any calming activity before your appointment helps. If you're in a particularly stressful stretch of life, mention it to your provider. Context matters for interpretation.
Medications and supplements
Prescription drugs, OTC drugs, and supplements are among the biggest confounders, and patients regularly forget to disclose them.
Blood thinners (warfarin, heparin, DOACs) directly affect coagulation panels and INR. Even small dosing changes shift the result. Diuretics alter electrolytes, often causing low potassium and sodium that can be misread as a metabolic disorder. Statins work well for cholesterol but can raise AST and ALT, sometimes prompting unnecessary worry about the liver.
NSAIDs like ibuprofen and naproxen affect kidney and liver markers with regular use. SSRIs can shift thyroid hormones and occasionally produce numbers that look like hypothyroidism. Herbal supplements aren't harmless either: St. John's Wort induces liver enzymes that speed up metabolism of other drugs, changing both liver function tests and blood levels of any co-administered medications.
Biotin (vitamin B7), often taken for hair and nails, is a well-known interferent in immunoassays. It can throw off thyroid, troponin, and hormone results.
Preparation tip: bring a complete list of all medications, supplements, and OTCs to every blood draw. Don't stop prescribed medications without checking with your doctor.
Smoking
Smoking dumps a complex mix of chemicals into the bloodstream that distorts several panels at once. Chronic smoke-driven inflammation elevates white blood cell counts. Smokers' WBC readings are consistently higher than non-smokers', which can mask or mimic an infection.
Carbon monoxide from smoke binds to hemoglobin and forms carboxyhemoglobin. The body compensates by making more red cells, which shows up as falsely elevated hemoglobin and hematocrit. The polycythemia-like picture can either hide a real anemia or trigger an unnecessary workup for a blood disorder.
Smoking worsens the lipid profile: LDL and triglycerides up, HDL down. Liver enzymes (especially GGT) rise as the liver works to clear smoke chemicals. Nicotine itself raises blood glucose via adrenaline release and adds to insulin resistance.
Preparation tip: hold off on smoking for 12 to 24 hours before a blood test. Be honest with your provider about your smoking status: it directly affects how the results are read.
Climate and altitude
Environmental conditions shift blood composition in ways that are easy to miss, especially for Canadians moving between regions or testing across seasons.
Cold weather increases blood viscosity as peripheral vessels constrict to conserve heat. Hematocrit and hemoglobin concentrate, which can be misread as polycythemia. Hot weather and heavy sweating cause dehydration, which also concentrates electrolytes, creatinine, and hemoglobin.
High altitude has a well-documented effect. Lower oxygen drives the kidneys to produce more erythropoietin, which drives red cell production. People living above 1,500 metres or Canadians visiting mountain towns in BC or Alberta will show elevated hemoglobin and hematocrit that are physiologically appropriate but can look abnormal against sea-level reference ranges.
Seasonal allergies can raise eosinophil counts on a CBC, a finding that might otherwise prompt a workup for parasitic infection.
Preparation tip: if you recently travelled to a high-altitude area or had your blood drawn during extreme weather, tell your provider. Labs in mountain regions often use altitude-adjusted reference ranges.
Antioxidant supplements
Antioxidants like vitamins C and E, selenium, beta-carotene, and coenzyme Q10 are widely used, and their effects on blood work are routinely underestimated.
Vitamin E supplementation may modestly improve lipid panels by reducing LDL oxidation, making cholesterol look better than it might otherwise. Selenium can lower certain liver enzymes. Antioxidants in general can reduce CRP and other inflammatory markers, which can mask real chronic inflammation a clinician is trying to detect.
They aren't uniformly beneficial either. High-dose beta-carotene can paradoxically raise CRP and has been linked to increased lung cancer risk in current and former smokers (CARET and ATBC trials). High-dose vitamin E can interfere with warfarin and other anticoagulants, raising INR and bleeding risk.
High-dose vitamin C can interfere with glucose assays, producing falsely high or low readings depending on the method the lab uses.
Preparation tip: pause antioxidant supplements for 48 to 72 hours before blood testing, unless your provider says otherwise. Always disclose what you're taking.
Quick reference: which tests are affected
| Blood Test | Factors That Affect It |
|---|---|
| CBC: WBC | Smoking, sleep deprivation, stress, seasonal allergies |
| CBC: Hemoglobin/Hematocrit | Smoking, dehydration, altitude, cold weather |
| CBC: MCV | Chronic alcohol use (macrocytic anemia) |
| CBC: Platelets | Alcohol, smoking, stress |
| Liver Function (AST/ALT) | Alcohol, medications (statins, NSAIDs), smoking, intermittent fasting, high-protein diet, St. John's Wort |
| Lipid Profile (LDL, HDL, Triglycerides) | Diet, alcohol, smoking, sleep deprivation, stress, vitamin E, intermittent fasting |
| Blood Glucose | Diet, stress, sleep deprivation, smoking, alcohol, intermittent fasting, medications (diuretics), vitamin C |
| Electrolytes (Na, K, Mg) | Alcohol, dehydration, diuretics, intermittent fasting, heat/cold exposure |
| Kidney Function (BUN, Creatinine) | High-protein diet, dehydration, NSAIDs |
| Inflammatory Markers (CRP, IL-6) | Sleep deprivation, stress, smoking, antioxidant supplements, seasonal allergies |
| Cortisol | Stress, sleep deprivation |
| Thyroid Panel (TSH, T3, T4) | SSRIs, biotin supplements, St. John's Wort |
| Coagulation (INR, PT) | Blood thinners (warfarin), vitamin E, vitamin K intake |
How to prep for accurate results
Reliable blood work comes down to a short list of habits in the days before. A consolidated checklist:
- Fast for 8 to 12 hours before testing if required. Water is always fine and encouraged.
- Sleep 7 to 9 hours the night before your blood draw.
- Skip alcohol for at least 24 to 48 hours before testing.
- Skip smoking for 12 to 24 hours before your appointment.
- Pause supplements, especially antioxidants and biotin, for 48 to 72 hours before testing, unless directed otherwise.
- Bring a complete medication list: prescriptions, OTCs, and supplements.
- Disclose lifestyle factors: fasting routines, recent travel, altitude exposure, stress, and smoking all help your clinician read the result.
- Stay hydrated: drink water normally before and on the day. Dehydration concentrates markers and makes venipuncture harder.
When in doubt, call your lab or provider ahead of time to confirm prep instructions for the specific tests ordered.
TeleTest offers online blood testing in Ontario and BC. Order tests, get a requisition, walk into the lab nearest you.
Disclaimer: This blog post is intended for educational purposes only and should not be taken as medical advice. Always consult your healthcare provider for personal health concerns.