Several common habits, from your morning coffee to a tough workout, can throw off the results of a routine physical exam. The biggest mistakes happen in the 8 to 24 hours before your appointment, and most are easy to avoid once you know what to watch for. Here’s what to skip and why it matters.
Eating or Drinking Before Blood Work
If your physical includes bloodwork, you’ll typically need to fast for 8 to 12 hours beforehand. Blood sugar tests and cholesterol panels (lipid panels) are the most common tests that require fasting. During that window, you should avoid all food and beverages except plain water. That means no juice, coffee, soda, or flavored sparkling water, even if it seems harmless. Sugars and artificial sweeteners can enter your bloodstream and skew results, potentially making your glucose or cholesterol numbers look worse than they actually are.
Your doctor’s office will usually tell you in advance whether fasting is required. If you’re unsure, call ahead. A good rule of thumb: schedule your appointment for the morning so most of your fasting window falls while you’re asleep.
Drinking Coffee or Caffeine
Even if your exam doesn’t involve fasting bloodwork, caffeine can still cause problems. It temporarily raises blood pressure, sometimes noticeably, within 30 to 120 minutes of drinking it. If your doctor checks your blood pressure during a physical (and they will), that spike could push you into a range that triggers unnecessary concern or follow-up testing.
Caffeine also causes a mild increase in certain stress-related markers in your blood. Skip coffee, energy drinks, tea, and caffeinated soda for at least 30 minutes before your appointment. If your physical is first thing in the morning, hold off on the coffee until afterward.
Intense Exercise the Day Before
A hard workout in the 24 hours before your exam can temporarily change dozens of blood values. Strenuous exercise raises markers of muscle damage, kidney function indicators like creatinine and urea, liver enzymes, and even cholesterol and triglyceride levels. Some of these changes come from actual tissue stress during the workout. Others happen because you lose fluid through sweat, which concentrates everything in your blood and makes levels appear higher than they truly are.
Muscle damage markers are especially sensitive. In men, levels can stay elevated well beyond the immediate post-workout period. If your doctor sees unexpectedly high liver enzymes or kidney markers, you could end up with unnecessary follow-up tests for a problem that doesn’t exist. Light walking or gentle stretching the day before is fine. Save the long run or heavy lifting for after your appointment.
Taking Certain Supplements and OTC Medications
Both prescription and over-the-counter drugs can cause lab results to come back falsely high or low. A few common culprits deserve special attention:
- Biotin (vitamin B7): Found in many multivitamins, B-complex supplements, and hair/skin/nail formulas. Biotin interferes with a wide range of hormone and disease-marker tests, including thyroid function, vitamin D, iron storage, and even cardiac markers. It can make thyroid hormone levels appear normal when they aren’t, or make a PSA screening look reassuringly low when it shouldn’t be.
- Herbal supplements: Ginseng, kava-kava, and St. John’s Wort all affect specific blood tests. Kava-kava can falsely elevate liver enzymes, which could lead your doctor to suspect liver disease.
- Nicotine: Smoking or using nicotine products raises cortisol, fatty acids, and certain tumor markers in blood tests.
Tell your doctor about every supplement and medication you take, including things that seem minor like a daily multivitamin. They can advise whether to pause anything before your appointment or simply factor it into how they read your results.
Not Drinking Enough Water
Showing up dehydrated affects your exam in two ways. First, dehydration concentrates your blood, which can artificially inflate values across multiple tests. Second, if your physical includes a urine sample, dehydration produces highly concentrated urine with a high specific gravity. This can make it harder to get a clean reading and may obscure or exaggerate certain findings.
Drink water normally in the hours leading up to your appointment. Even if you’re fasting for bloodwork, plain water is always allowed and encouraged. It also makes your veins easier to find for a blood draw.
Wearing Lotions, Makeup, or Heavy Products
If your physical includes a skin check or any imaging, personal care products can get in the way. Many lotions, cosmetics, and hair products contain metallic compounds that create visual distortions on MRI and other imaging scans. In one study, 16 out of 22 personal care products tested caused artifacts on MRI images, ranging from mild to severe. When those artifacts overlap with the area being examined, they can obscure real findings or mimic problems that aren’t there.
For a standard physical with a skin exam, thick moisturizers, body glitter, or heavy makeup can also make it harder for your doctor to evaluate moles, rashes, or skin changes. Come with clean, product-free skin when possible, especially on areas you want examined.
Rushing In Stressed and Hurried
Arriving flustered, anxious, or out of breath directly affects your vital signs. Blood pressure readings taken when you’re stressed or rushing tend to run higher than your true baseline. This is the same mechanism behind white coat syndrome, where anxiety about the medical setting itself inflates numbers.
Arrive a few minutes early so you can sit and breathe. Avoid smoking, drinking coffee, or exercising for at least 30 minutes before your appointment. If you know you tend to get anxious at the doctor’s office, mention it. Your provider can take your blood pressure again later in the visit after you’ve settled in, or ask you to monitor it at home for a more accurate picture.
Scheduling During Your Period
If your physical includes a urine sample, menstruation can contaminate the specimen with blood and throw off the results. Red blood cell counts in urine rise significantly during the menstrual phase compared to other times in the cycle. This can trigger a false-positive result for conditions like urinary tract infections or kidney problems, leading to unnecessary follow-up.
When possible, schedule your physical for a time when you’re not actively menstruating. If you can’t reschedule, let your doctor know so they can interpret your urinalysis with that context in mind. A Pap smear, if it’s part of your visit, is also generally easier to read when collected outside of heavy menstrual flow.