For some nutrients a blood test genuinely changes what you should do. For others it tells you very little that matters. Knowing which is which is the difference between a test budget well spent and money on a panel that looks thorough and changes nothing. The short version: test when the right dose depends on how low you are starting from, or when too much carries its own risk.
Iron is the clearest case for testing first
Iron is the one supplement where guessing can do real harm. Too little leaves a deficiency uncorrected; too much is genuinely toxic and builds up because the body has no easy way to get rid of it. The marker to ask for is ferritin, which reflects your iron stores. A ferritin below around 50 is associated with fatigue, low mood and poor exercise recovery even when a standard blood count looks normal, and most GPs will not run ferritin without a separate trigger. Starting iron without that number is taking a real risk for no good reason, which is why it is one supplement that should always follow a test, with a retest a few months later.
Vitamin D: test because the dose depends on the gap
The right vitamin D dose depends on how low you are, and through a UK winter many people are low. A 25-hydroxyvitamin D test gives you a starting point, and it is worth interpreting against the season: a "fine" reading in late summer can still mean a shortfall by February. It is one of the few tests that genuinely shapes the dose rather than just confirming a hunch.
B12: test, and test the right second marker
If you have neurological symptoms like tingling or unexplained fatigue, a B12 level is worth checking, and where the result is borderline a second marker called methylmalonic acid (MMA) sorts out a genuine deficiency from a number that only looks low. This matters more if you eat little or no animal food, or take a long-term acid-reducing or diabetes medicine, both of which lower B12 over time.
Where a test usually is not needed
For a lot of everyday supplements, testing first adds cost without changing the plan. Magnesium, vitamin C, and most general support nutrients do not have a cheap, informative blood test that would alter a sensible dose, and the safety margin is wide enough that careful supplementation is reasonable without one. The honest rule is to test where the result changes the dose or flags a real risk, and to skip it where it would just produce a number you were going to ignore.
If you do supplement, it is still worth knowing the interaction patterns with any medicines you take, and choosing the form that actually absorbs.