fasting from water as well as food
Many people treat a fasting blood test like a Ramadan fast and skip water too. That dehydrates you, makes the vein harder to find and concentrates the sample enough to distort results. Plain water is allowed and encouraged right up to the draw — only food, caffeine and sugary drinks break the fast.
trying to do the visa medical at home
No home provider can perform the residence visa medical fitness test, whatever a booking page implies. The HIV test and TB chest X-ray happen only at government-approved centres with in-person identity checks. Paying for a home panel expecting it to count toward your visa wastes money and time.
switching laboratories between follow-up tests
Reference ranges and assay methods differ between labs, so a borderline thyroid or cholesterol value retested somewhere new may move for reasons that have nothing to do with your health. For trend-watching, stick with one laboratory — rebooking the same vendor keeps your numbers comparable year to year.