Laboratory preparation tips

Small things change blood results. These expert tips help you get accurate numbers and avoid unnecessary worry.

• Fasting

  Most lipid and glucose tests need 8–12 hours with only water. UK guidance now often accepts non-fasting lipids — confirm with your GP. Never fast for a thyroid or kidney test unless instructed.

• Biotin (Vitamin B7)

  High-dose biotin supplements (commonly in 'hair, skin and nails' formulas) can falsely lower or raise thyroid, hormone and troponin results. Stop at least 48 hours — ideally 72 hours — before testing.

• Aspirin and NSAIDs

  Affect platelet function and can prolong bleeding time. Tell the lab if you take them daily. Do not stop prescribed aspirin without medical advice.

• Coffee, smoking and alcohol

  Coffee raises cortisol and adrenaline. Smoking acutely changes red and white cell counts. Alcohol raises liver enzymes (GGT) for up to 48 hours. Avoid all three the morning of the test where possible.

• Exercise

  Heavy exercise in the 24 hours before testing raises CK, AST, urea and white cells. Schedule rest days before a routine blood test.

• Best time of day

  Cortisol and testosterone should be taken in the morning (7–10am). Iron is best fasting in the morning. Glucose tolerance tests must start in the morning.

• Hydration

  Drink water normally before the test. Dehydration concentrates the blood and falsely raises sodium, haematocrit, urea and many proteins.

• Medications to mention

  Warfarin, heparin, steroids, thyroxine, metformin, statins, hormonal contraception, and any new antibiotic. They all change specific results.

• UK vs US differences

  UK reports use SI units (mmol/L, µmol/L). US reports use mass units (mg/dL). HbA1c uses mmol/mol (UK) or % (US). The unit converter in this app handles all of these.